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{{short description|Percent of variation in IQ scores in a given population associated with genetic variation}}
Research on the [[heritability]] of [[intelligence quotient]] (IQ) inquires into the degree of variation in IQ within a [[population]] that is associated with [[genetic variation]] between individuals in that population. There has been significant controversy in the academic community about the heritability of IQ since research on the issue began in the late nineteenth century.<ref name="devlin">{{cite journal |last1=Devlin |first1=B. |last2=Daniels |first2=Michael |last3=Roeder |first3=Kathryn |author3-link=Kathryn Roeder |year=1997 |title=The heritability of IQ |journal=Nature |volume=388 |issue=6641 |pages=468–71 |bibcode=1997Natur.388..468D |doi=10.1038/41319 |pmid=9242404 |s2cid=4313884 |doi-access=free}}</ref><ref name="rose">{{cite journal |last1=Rose |first1=Steven P R |date=June 2006 |title=Commentary: Heritability estimates—long past their sell-by date |journal=International Journal of Epidemiology |volume=35 |issue=3 |pages=525–527 |doi=10.1093/ije/dyl064 |pmid=16645027 |doi-access=free}}</ref> [[Intelligence]] in the normal range is a [[Quantitative trait locus#Quantitative traits|polygenic trait]], meaning that it is influenced by more than one gene,<ref>Alice Marcus. 2010. Human Genetics: An Overview. Alpha Science section 14.5</ref><ref name=Davies2011>{{cite journal | last1 = Davies | first1 = G. | last2 = Tenesa | first2 = A. | last3 = Payton | first3 = A. | last4 = Yang | first4 = J. | last5 = Harris | first5 = S. E. | last6 = Liewald | first6 = D. | last7 = Deary | first7 = I. J. | year = 2011 | title = Genome-wide association studies establish that human intelligence is highly heritable and polygenic | journal = Molecular Psychiatry | volume = 16 | issue = 10| pages = 996–1005 | doi = 10.1038/mp.2011.85 | pmid=21826061 | pmc=3182557}}</ref> and in the case of intelligence at least 500 genes.<ref>{{cite web|url=https://www.newscientist.com/article/2163484-found-more-than-500-genes-that-are-linked-to-intelligence/|title=Found: more than 500 genes that are linked to intelligence|first=New Scientist staff and Press|last=Association|website=New Scientist|access-date=2018-11-29|archive-date=2019-12-13|archive-url=https://web.archive.org/web/20191213201937/https://www.newscientist.com/article/2163484-found-more-than-500-genes-that-are-linked-to-intelligence/|url-status=live}}</ref> Further, explaining the similarity in IQ of closely related persons requires careful study because environmental factors may be correlated with genetic factors. Outside the normal range, certain single gene [[genetic disorder]]s, such as [[phenylketonuria]], can negatively affect intelligence.<ref name="general">{{cite book |author1=Robert J. Sternberg |url=https://archive.org/details/generalfactorint00ster_484 |title=The general factor of intelligence |author2=Elena Grigorenko |publisher=Lawrence Erlbaum Associates |year=2002 |isbn=978-0-8058-3675-2 |pages=[https://archive.org/details/generalfactorint00ster_484/page/n272 260]–261 |url-access=limited}}{{page needed|date=April 2013}}</ref>

Estimates in the academic research of the heritability of IQ vary significantly by study and by study design. The general figure for heritability of IQ from behavioral genetic studies is about 0.5 across multiple studies in varying populations.<ref name=":4">{{cite book |last1=Plomin |first1=Robert |url=https://books.google.com/books?id=OytMMAEACAAJ |title=Behavioral Genetics |last2=DeFries |first2=John C. |last3=Knopik |first3=Valerie S. |last4=Neiderhiser |first4=Jenae M. |date=24 September 2012 |publisher=Worth Publishers |isbn=978-1-4292-4215-8 |pages=195–196 |quote= |access-date=4 September 2013}}</ref>{{Reference page|page=172}} However, alternative study designs using path analysis that explicitly account for cultural transmission have produced estimates around 0.3.<ref name=":14" /> The relationship between heritability and age is uncertain, though most researchers believe that there is an increase in heritability over the course of the lifespan and that this increase reflects the importance of gene-environment correlations. Recent genetic research has come to more equivocal results, with estimates of heritability lower than those derived from twin studies, causing what is known as the "[[missing heritability problem]]". 

Although IQ differences between individuals have been shown to have a hereditary component, it does not follow that disparities in IQ between groups have a genetic basis.<ref name=":10">{{Cite journal |last1=Visscher |first1=Peter M. |last2=Hill |first2=William G. |last3=Wray |first3=Naomi R. |date=2008 |title=Heritability in the genomics era – concepts and misconceptions |url=https://www.nature.com/articles/nrg2322 |journal=Nature Reviews Genetics |volume=9 |issue=4 |pages=255–266|doi=10.1038/nrg2322 |pmid=18319743 |s2cid=690431 |url-access=subscription }}</ref><ref name=":9" /><ref name=":1">{{Cite journal|last1=Nisbett|first1=Richard E.|last2=Aronson|first2=Joshua|last3=Blair|first3=Clancy|last4=Dickens|first4=William|last5=Flynn|first5=James|last6=Halpern|first6=Diane F.|last7=Turkheimer|first7=Eric|date=2012|title=Intelligence: New findings and theoretical developments.|journal=American Psychologist|language=en|volume=67|issue=2|pages=130–159|doi=10.1037/a0026699|issn=1935-990X|pmid=22233090}}</ref><ref name=":0">{{Cite news|last=Mitchell|first=Kevin|date=2 May 2018|title=Why genetic IQ differences between 'races' are unlikely: The idea that intelligence can differ between populations has made headlines again, but the rules of evolution make it implausible|work=The Guardian|url=https://www.theguardian.com/science/blog/2018/may/02/why-genetic-iq-differences-between-races-are-unlikely|access-date=13 June 2020|archive-date=29 June 2020|archive-url=https://web.archive.org/web/20200629045033/https://www.theguardian.com/science/blog/2018/may/02/why-genetic-iq-differences-between-races-are-unlikely|url-status=live}}</ref> The scientific consensus is that genetics does not explain [[Race and intelligence|average differences in IQ test performance between racial groups.]]<ref>{{Cite journal|last1=Panofsky|first1=Aaron|last2=Dasgupta|first2=Kushan|last3=Iturriaga|first3=Nicole|date=28 September 2020|title=How White nationalists mobilize genetics: From genetic ancestry and human biodiversity to counterscience and metapolitics|journal=American Journal of Physical Anthropology|volume=175|issue=2|pages=387–398|doi=10.1002/ajpa.24150|pmid=32986847|pmc=9909835 |quote=[T]he claims that genetics defines racial groups and makes them different, that IQ and cultural differences among racial groups are caused by genes, and that racial inequalities within and between nations are the inevitable outcome of long evolutionary processes are neither new nor supported by science (either old or new).|doi-access=free}}</ref><ref name=":02">{{cite journal|last1=Nisbett|first1=Richard E.|last2=Aronson|first2=Joshua|last3=Blair|first3=Clancy|last4=Dickens|first4=William|last5=Flynn|first5=James|author-link5=Jim Flynn (academic)|last6=Halpern|first6=Diane F.|author-link6=Diane F. Halpern|last7=Turkheimer|first7=Eric|date=2012|title=Group differences in IQ are best understood as environmental in origin|url=http://people.virginia.edu/~ent3c/papers2/Articles%20for%20Online%20CV/Nisbett%20(2012)%20Group.pdf|journal=American Psychologist|volume=67|pages=503–504|doi=10.1037/a0029772|issn=0003-066X|pmid=22963427|access-date=22 July 2013|number=6|author-link1=Richard E. Nisbett|archive-date=23 January 2015|archive-url=https://web.archive.org/web/20150123114230/http://people.virginia.edu/~ent3c/papers2/Articles%20for%20Online%20CV/Nisbett%20(2012)%20Group.pdf|url-status=live}}</ref><ref>{{Cite web |last=Nisbett |first=Eric Turkheimer, Kathryn Paige Harden, and Richard E. |date=2017-05-18 |title=Charles Murray is once again peddling junk science about race and IQ |url=https://www.vox.com/the-big-idea/2017/5/18/15655638/charles-murray-race-iq-sam-harris-science-free-speech |access-date=2025-10-30 |website=Vox |language=en-US |quote=All three of us are academic psychologists who have studied human intelligence, and it is our contention that Murray's views do not represent the consensus in our field.}}</ref><ref>{{Cite journal |last=Bird |first=Kevin A. |date=2021 |title=No support for the hereditarian hypothesis of the Black–White achievement gap using polygenic scores and tests for divergent selection |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.24216 |journal=American Journal of Physical Anthropology |language=en |volume=175 |issue=2 |pages=465–476 |doi=10.1002/ajpa.24216 |pmid=33529393 |bibcode=2021AJPA..175..465B |issn=1096-8644}}</ref>

== Heritability and caveats ==
{{Main|Heritability}}

Heritability is a [[statistic]] used in the fields of [[Animal husbandry|breeding]] and [[genetics]] that estimates the degree of ''variation'' in a [[phenotypic trait]] in a population is explained by genetic variation between individuals in that population.<ref name="scitable-estimating">{{cite journal |last1=Wray |first1=Naomi |last2=Visscher |first2=Peter |name-list-style=vanc |date=2008 |title=Estimating Trait Heritability |url=http://www.nature.com/scitable/topicpage/estimating-trait-heritability-46889 |url-status=live |journal=Nature Education |volume=1 |issue=1 |page=29 |archive-url=https://web.archive.org/web/20150802043009/http://www.nature.com/scitable/topicpage/estimating-trait-heritability-46889 |archive-date=2 August 2015 |access-date=24 July 2015}}</ref> The concept of heritability can be expressed in the form of the following question: "What is the proportion of the variation in a given trait within a population that is ''not'' explained by the environment or random chance?"<ref>{{cite book |last1=Gazzaniga |first1=Michael S |title=Psychological science |last2=Heatherton |first2=Todd F |last3=Halpern |first3=Diane F |date=February 2015 |isbn=978-0-393-26313-8 |edition=5th |location=New York |publisher=W.W. Norton |oclc=908409996 |name-list-style=vanc}}</ref> 

Estimates of heritability take values ranging from 0 to 1; a heritability estimate of 1 indicates that all variation in the trait in question is genetic and a heritability estimate of 0 indicates that ''none'' of the variation is genetic.

There are a number of caveats to consider when interpreting heritability:
* Heritability measures the proportion of ''variation'' in a trait that can be attributed to genes, and not the proportion of a trait caused by genes.<ref name=":40">{{Cite journal |last1=Moore |first1=David S. |last2=Shenk |first2=David |date=2017 |title=The heritability fallacy |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/wcs.1400 |journal=WIREs Cognitive Science |language=en |volume=8 |issue=1–2 |article-number=e1400 |doi=10.1002/wcs.1400 |pmid=27906501 |issn=1939-5086}}</ref> Thus, if the environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability (because the variation or differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.<ref name="unknown">{{cite journal |doi=10.1037/0003-066X.51.2.77 |title=Intelligence: Knowns and unknowns |year=1996 |last1=Neisser |first1=Ulric |last2=Boodoo |first2=Gwyneth |last3=Bouchard |first3=Thomas J. Jr. |last4=Boykin |first4=A. Wade |last5=Brody |first5=Nathan |last6=Ceci |first6=Stephen J. |last7=Halpern |first7=Diane F. |last8=Loehlin |first8=John C. |last9=Perloff |first9=Robert |last10=Sternberg |first10=Robert J. |last11=Urbina |first11=Susana |journal=American Psychologist |volume=51 |issue=2 |pages=77–101|display-authors=8 }}</ref> Thus, even in developed nations, a high heritability of a trait does not necessarily mean that average group differences are due to genes.<ref name="unknown"/><ref>{{cite journal |first1=Jeanne |last1=Brooks-Gunn |first2=Pamela K. |last2=Klebanov |first3=Greg J. |last3=Duncan |year=1996 |title=Ethnic Differences in Children's Intelligence Test Scores: Role of Economic Deprivation, Home Environment, and Maternal Characteristics |journal=Child Development |volume=67 |issue=2 |pages=396–408 |jstor=1131822 |doi=10.2307/1131822 |pmid=8625720}}</ref> Some have gone further, and used height as an example in order to argue that "even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability."<ref name="JohnsonBouchard2009">{{cite journal |doi=10.1111/j.1467-8721.2009.01639.x |title=Beyond Heritability: Twin Studies in Behavioral Research |year=2009 |last1=Johnson |first1=Wendy |last2=Turkheimer |first2=Eric |last3=Gottesman |first3=Irving I. |last4=Bouchard Jr. |first4=Thomas J. |journal=Current Directions in Psychological Science |volume=18 |issue=4 |pages=217–20 |pmid=20625474 |pmc=2899491}}</ref>
* A common error is to assume that a heritability figure is necessarily unchangeable. The value of heritability can change if the impact of environment (or of genes) in the population is substantially altered.<ref name="unknown"/> If the environmental variation encountered by different individuals increases, then the heritability figure would decrease. On the other hand, if everyone had the same environment, then heritability would be 100%. The population in developing nations often has more diverse environments than in developed nations. This would mean that heritability figures would be lower in developing nations. Another example is [[phenylketonuria]] which previously caused [[Intellectual disability|intellectual disabilities]] in everyone who had this genetic disorder and thus had a heritability of 100%. Today, this can be prevented by following a modified diet, resulting in a lowered heritability.<ref>{{Cite journal |last1=Haworth |first1=Claire |last2=Davis |first2=Oliver |date=2014 |title=From observational to dynamic genetics |journal=Frontiers in Genetics |volume=5 |page=6 |doi=10.3389/fgene.2014.00006 |issn=1664-8021 |pmc=3896969 |pmid=24478793 |doi-access=free }}</ref>
* A high heritability of a trait does not mean that  effects such as learning are not involved. Vocabulary size, for example, is substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high.<ref name="unknown"/>
* Contrary to popular belief, two parents of higher IQ will not necessarily produce offspring of equal or higher intelligence. Polygenic traits often appear less heritable at the extremes. A heritable trait is definitionally more likely to appear in the offspring of two parents high in that trait than in the offspring of two randomly selected parents. However, the more extreme the expression of the trait in the parents, the less likely the child is to display the same extreme as the parents. In fact, parents whose IQ is at either extreme are more likely to produce offspring with IQ closer to the mean (or average) than they are to produce offspring with high IQ. At the same time, the more extreme the expression of the trait in the parents, the more likely the child is to express the trait at all. For example, the child of two extremely tall parents is likely to be taller than the average person (displaying the trait), but unlikely to be taller than the two parents (displaying the trait at the same extreme). See also [[regression toward the mean]].<ref name="HMG4">{{cite book|last1=Strachan|first1=Tom|last2=Read|first2=Andrew|title=Human Molecular Genetics, Fourth Edition|url=https://archive.org/details/humanmolecularge00stra_254|url-access=limited|publisher=Garland Science|location=New York| year=2011|isbn=978-0-8153-4149-9|pages=[https://archive.org/details/humanmolecularge00stra_254/page/n93 80]–81}}</ref><ref name="Humphreys1978">{{cite journal |last1=Humphreys |first1=Lloyd G. |title=To understand regression from parent to offspring, think statistically |journal=Psychological Bulletin |date=1978 |volume=85 |issue=6 |pages=1317–1322 |doi=10.1037/0033-2909.85.6.1317 |pmid=734015 }}</ref><ref>Schacter, Daniel, Daniel Gilbert, and Daniel Wegner. "Intelligence." Psychology. 2 ed. New York: Worth Publishers, 2010. 405-406. Print.</ref>
== Methodology ==
{{See also|Behavioural genetics|Quantitative genetics}}
There are multiple types of studies that are designed to estimate heritability and other variance components, stemming from the field of [[Biometrics|biometrical]] genetics.<ref>{{Cite journal |last1=Evans |first1=David M. |last2=Gillespie |first2=N. A. |last3=Martin |first3=N. G. |date=2002-10-01 |title=Biometrical genetics |url=https://www.sciencedirect.com/science/article/pii/S0301051102000510 |journal=Biological Psychology |volume=61 |issue=1 |pages=33–51 |doi=10.1016/S0301-0511(02)00051-0 |pmid=12385668 |issn=0301-0511}}</ref> The [[variance]], or more simply the differences in a trait within a population, can be partitioned into specific variance components that are based on particular decomposition models for the [[phenotype]]. First pointed out by [[Ronald Fisher]] and [[Sewall Wright]], different sources of variance differ in their contribution to the resemblance between types of relatives.<ref name=":11">{{Cite book |last=Lynch |first=Michael |title=Genetics and Analysis of Quantitative Traits |publisher=Sinauer Associates |year=1998 |isbn=978-0-87893-481-2}}</ref>{{Reference page|page=131}}By making certain assumptions about the relationship between genes and environment, the field of [[quantitative genetics]] has developed equations that describe the expected correlation between relatives in terms of specific variance components (shared environment, additive genetic, etc.).<ref name=":11" />{{Reference page|pages=163-167}}
[[File:Graham Coop textbook diagram.png|thumb|Correlation between relatives on a purely additive genetic model of phenotypic determination, from Coop 2020<ref>{{Cite book |last=Coop |first=Graham |url=https://github.com/cooplab/popgen-notes/releases/tag/v1.2 |title=Population and Quantitative Genetics |date=21 September 2020 |edition=3rd}}</ref>]]
One simple case is presented by twin studies. By making assumptions about genetic additivity and sources of environmental variation, researchers can estimate the heritability of a phenotype by comparing the resemblance of twins.<ref name=":11" />{{Reference page|page=581}} Another method used to estimate heritability is adoption studies. Because adoptive parents and their adoptive children share a common environment, but not genetics,<ref name=":4" />{{Reference page|pages=80-82}} researchers in behavioral genetics have used data from adoptive families to estimate the heritability of IQ.<ref name=":16">{{Cite journal |last1=Richardson |first1=Ken |last2=Norgate |first2=Sarah H. |date=2006 |title=A Critical Analysis of IQ Studies of Adopted Children |url=https://karger.com/article/doi/10.1159/000096531 |journal=Human Development |language=en |volume=49 |issue=6 |pages=319–335 |doi=10.1159/000096531 |issn=0018-716X}}</ref><ref name=":15" /> Based on quantitative genetic theory, some researchers have taken a broader approach to analyzing familial resemblance. These researchers typically use [[Path analysis (statistics)|path analysis]]{{Efn|For the full series of papers developing the method, see <ref>{{Cite journal |last1=Rice |first1=J. |last2=Cloninger |first2=C. R. |last3=Reich |first3=T. |date=November 1978 |title=Multifactorial inheritance with cultural transmission and assortative mating. I. Description and basic properties of the unitary models |journal=American Journal of Human Genetics |volume=30 |issue=6 |pages=618–643 |issn=0002-9297 |pmc=1685878 |pmid=747189}}</ref><ref>{{Cite journal |last1=Cloninger |first1=C. R. |last2=Rice |first2=J. |last3=Reich |first3=T. |date=March 1979 |title=Multifactorial inheritance with cultural transmission and assortative mating. II. a general model of combined polygenic and cultural inheritance |journal=American Journal of Human Genetics |volume=31 |issue=2 |pages=176–198 |issn=0002-9297 |pmc=1685756 |pmid=453202}}</ref><ref>{{Cite journal |last1=Cloninger |first1=C. R. |last2=Rice |first2=J. |last3=Reich |first3=T. |date=May 1979 |title=Multifactorial inheritance with cultural transmission and assortative mating. III. Family structure and the analysis of separation experiments |journal=American Journal of Human Genetics |volume=31 |issue=3 |pages=366–388 |issn=0002-9297 |pmc=1685778 |pmid=572636}}</ref>}} to fit models using larger sets of familiar correlations, incorporating cultural transmission, adoption, other complex living arrangements, and generalized assortative mating.<ref>{{Cite journal |last1=Rao |first1=D. C. |last2=Morton |first2=N. E. |last3=Cloninger |first3=C. R. |date=April 1979 |title=Path analysis under generalized assortative mating: I. Theory |url=https://www.cambridge.org/core/journals/genetics-research/article/path-analysis-under-generalized-assortative-mating/F1784B9BD698D00E3D202CD61066C8B0 |journal=Genetics Research |language=en |volume=33 |issue=2 |pages=175–188 |doi=10.1017/S0016672300018310 |pmid=478296 |issn=1469-5073}}</ref><ref name=":14">{{Cite journal |last1=Feldman |first1=Marcus W. |last2=Ramachandran |first2=Sohini |date=2018-02-12 |title=Missing compared to what? Revisiting heritability, genes and culture |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=373 |issue=1743 |article-number=20170064 |doi=10.1098/rstb.2017.0064 |pmc=5812976 |pmid=29440529}}</ref> However, with the transition into the [[postgenomic era]], issues with controversial assumptions in previous models analyzing familial resemblance may be superseded by genetic analysis.<ref name=":10" /> Methods have developed in genetic analysis to estimate the total heritability of a trait using large datasets with millions of genetic variants.<ref>{{Cite journal |last1=Zaitlen |first1=Noah |last2=Kraft |first2=Peter |date=2012-10-01 |title=Heritability in the genome-wide association era |journal=Human Genetics |language=en |volume=131 |issue=10 |pages=1655–1664 |doi=10.1007/s00439-012-1199-6 |issn=1432-1203 |pmc=3432754 |pmid=22821350}}</ref><ref name=":20">{{Cite journal |last1=Tang |first1=Mingsheng |last2=Wang |first2=Tong |last3=Zhang |first3=Xuefen |date=2022-03-14 |title=A review of SNP heritability estimation methods |url=https://academic.oup.com/bib/article/23/3/bbac067/6548385 |journal=Briefings in Bioinformatics |language=en |volume=23 |issue=3 |article-number=bbac067 |doi=10.1093/bib/bbac067 |pmid=35289357 |issn=1467-5463 |archive-url=http://web.archive.org/web/20250826140022/https://academic.oup.com/bib/article/23/3/bbac067/6548385 |archive-date=2025-08-26}}</ref> 

In the case of IQ, heritability estimates based on direct observation of molecular genetics have been significantly lower (around 10%) than those employing traditional methods (40–80%).<ref name=":13" /><ref name=":12" /> This discrepancy has been termed the "missing heritability problem."<ref name=":13" /> While researchers such as [[Robert Plomin]] and [[Sophie von Stumm]] contend that this gap will likely disappear with the acquisition of more genetic data, [[Eric Turkheimer]] and Lucas J. Matthews argue that it reveals a deeper set of methodological problem inherent in the concept of heritability itself, which will not be easy to overcome.<ref name=":13" />

{| class="wikitable"
|+ Heritability Estimation Methods
|-
! Method
! Gene-Environment Assumptions
! Method Type
! Originating Author
|-
| Classical Twin Design
| Assumes equal environment between MZ and DZ twins, lack of GxE (gene-environment interactions), no nonadditive genetic variance (e.g., no dominance or epistasis); can be biased by assortative mating.
| BG (Biometric Genetic)
| 
|-
| Adoption Studies
| Assumes genetic/environmental separation, and that the adoptive environment is not systematically different from the general population; can be biased by assortative mating.
| BG (Biometric Genetic)
| 
|-
| GREML-SNP
| Assumes additive genetic effects, lack of genetic nurture, and that environmental relatedness does not scale with genetic relatedness; SNP effect sizes follow a normal distribution; misses rare variants; can be biased by assortative mating.
| SNP-based
| Yang et al. (2010)<ref>{{Cite journal |last1=Yang |first1=Jian |last2=Benyamin |first2=Beben |last3=McEvoy |first3=Brian P. |last4=Gordon |first4=Scott |last5=Henders |first5=Anjali K. |last6=Nyholt |first6=Dale R. |last7=Madden |first7=Pamela A. |last8=Heath |first8=Andrew C. |last9=Martin |first9=Nicholas G. |last10=Montgomery |first10=Grant W. |last11=Goddard |first11=Michael E. |last12=Visscher |first12=Peter M. |date=July 2010 |title=Common SNPs explain a large proportion of the heritability for human height |journal=Nature Genetics |language=en |volume=42 |issue=7 |pages=565–569 |doi=10.1038/ng.608 |pmid=20562875 |pmc=3232052 |issn=1546-1718}}</ref>
|-
| Within-Family GWAS
| Isolates genetic effects within families; can still be biased by latent assortative mating, sibling indirect effects, ascertainment bias; may miss rare variants or low-frequency variants.
| SNP-based
| Possibly Benyamin et al. (2009)<ref>{{Cite journal |last1=Benyamin |first1=Beben |last2=Visscher |first2=Peter M. |last3=McRae |first3=Allan F. |date=February 2009 |title=Family-based genome-wide association studies |journal=Pharmacogenomics |volume=10 |issue=2 |pages=181–190 |doi=10.2217/14622416.10.2.181 |issn=1744-8042 |pmid=19207019}}</ref>
|-
| KINSHIP
| Assumes fixed genetic effects across kin (relatives) and that environmental relatedness does not scale with genetic relatedness; can be biased by genetic nurture and assortative mating.
| IBD-based
| Zaitlen et al. (2013)<ref>{{Cite journal |last1=Zaitlen |first1=Noah |last2=Kraft |first2=Peter |last3=Patterson |first3=Nick |last4=Pasaniuc |first4=Bogdan |last5=Bhatia |first5=Gaurav |last6=Pollack |first6=Samuela |last7=Price |first7=Alkes L. |date=May 2013 |title=Using extended genealogy to estimate components of heritability for 23 quantitative and dichotomous traits |journal=PLOS Genetics |volume=9 |issue=5 |article-number=e1003520 |doi=10.1371/journal.pgen.1003520 |doi-access=free |issn=1553-7404 |pmc=3667752 |pmid=23737753}}</ref>
|-
| SibReg
| Does not assume anything about the environment; is biased by GxE (gene-environment interactions); can be biased by assortative mating.
| IBD-based
| Visscher et al. (2006)<ref name=":3" />
|-
| RDR
| Does not assume any specific gene-environment relationship; misses ultra-rare variants; can be biased by assortative mating.
| IBD-based
| Young et al. (2017)<ref name=":34" />
|}

== Estimates ==
Although individual family studies that estimate the heritability of IQ vary greatly{{Efn|For example, individual studies have estimated values as low as "not significantly different from zero" <ref>{{Cite journal |last1=Adams |first1=B. |last2=Ghodsian |first2=M. |last3=Richardson |first3=K. |date=September 1976 |title=Evidence for a low upper limit of heritability of mental test performance in a national sample of twins |url=https://www.nature.com/articles/263314a0 |journal=Nature |language=en |volume=263 |issue=5575 |pages=314–316 |doi=10.1038/263314a0 |pmid=986563 |bibcode=1976Natur.263..314A |issn=1476-4687}}</ref> to as high as over .8.}}, most family studies estimate the heritability of IQ in the range from 0.4 to 0.8<ref name=":1" /> and reviews of the literature typically summarize classical family design research with an estimate of 0.5.<ref name=":13">{{Cite journal |last1=Matthews |first1=Lucas J. |last2=Turkheimer |first2=Eric |date=2022-06-01 |title=Three legs of the missing heritability problem |journal=Studies in History and Philosophy of Science |volume=93 |pages=183–191 |doi=10.1016/j.shpsa.2022.04.004 |pmid=35533541 |pmc=9172633 |issn=0039-3681}}</ref><ref name=":12">{{Cite journal |last1=Plomin |first1=Robert |last2=von Stumm |first2=Sophie |date=8 January 2018 |title=The new genetics of intelligence |journal=Nature Reviews. Genetics |volume=19 |issue=3 |pages=148–159 |doi=10.1038/nrg.2017.104 |issn=1471-0064 |pmc=5985927 |pmid=29335645}}</ref> Issues with the methodology of family studies have long plagued the research field,<ref>{{Cite book |last=Panofsky |first=Aaron |title=Misbehaving Science: Controversy and the Development of Behavior Genetics |date=2014 |publisher=University of Chicago Press |isbn=978-0-226-05845-0 |language=en-US}}</ref><ref>{{Cite journal |last1=Meyer |first1=Michelle N |last2=Appelbaum |first2=Paul S. |last3=Benjamin |first3=Daniel J. |last4=Callier |first4=Shawneequa |last5=Comfort |first5=Nathaniel |last6=Conley |first6=Dalton |last7=Freese |first7=Jeremy |last8=Garrison |first8=Nanibaa' A |last9=Hammonds |first9=Evelyn M. |last10=Harden |first10=K. Paige |last11=Lee |first11=Sandra Soo-Jin |last12=Martin |first12=Alicia R. |last13=Martschenko |first13=Daphne Oluwaseun |last14=Neale |first14=Benjamin M. |last15=et. |first15=al |date=20 April 2023 |title=Wrestling with Social and Behavioral Genomics: Risks, Potential Benefits, and Ethical Responsibility |journal=The Hastings Center Report |volume=53 |issue=S1 |pages=S2–S49 |doi=10.1002/hast.1477 |pmid=37078667 |pmc=10433733 }}</ref> but many researchers now believe that genome-wide association studies can provide less biased estimates of heritability.<ref>{{Cite journal |last1=Yang |first1=Jian |last2=Zeng |first2=Jian |last3=Goddard |first3=Michael E. |last4=Wray |first4=Naomi R. |last5=Visscher |first5=Peter M. |date=1 September 2017 |title=Concepts, estimation and interpretation of SNP-based heritability |url=https://www.nature.com/articles/ng.3941 |journal=Nature Genetics |language=en |volume=49 |issue=9 |pages=1304–1310 |doi=10.1038/ng.3941 |pmid=28854176 |issn=1546-1718}}</ref> These genomic studies provide estimates of heritability that are much lower than those derived from twin studies, leading to what is denoted as the "missing heritability problem".<ref name=":21">{{Cite journal |last=Young |first=Alexander I. |date=2019-06-24 |title=Solving the missing heritability problem |journal=PLOS Genetics |language=en |volume=15 |issue=6 |doi=10.1371/journal.pgen.1008222 |issn=1553-7404 |pmc=6611648 |pmid=31233496 |doi-access=free |article-number=e1008222}}</ref> The latest results from the best genomic methods are between 0.10 to 0.20, and researchers have proposed various theories to explain the gap.<ref name=":27" /><ref>{{Cite book |last1=Rizzi |first1=Thais S. |title=The Oxford handbook of cognitive psychology |last2=Posthuma |first2=Danielle |date=2013 |publisher=Oxford University Press |isbn=978-0-19-997830-4 |editor-last=Reisberg |editor-first=Daniel |series=Oxford library of psychology |location=New York Oxford |pages=834–836 |chapter=Genes and Intelligence}}</ref>

=== Twin and family research ===
{{See also|Twin study|Adoption study}}

==== Twin studies ====
Twin studies are the most common method used to estimate the heritability of most traits.<ref>{{Cite journal |last1=Polderman |first1=Tinca J. C. |last2=Benyamin |first2=Beben |last3=de Leeuw |first3=Christiaan A. |last4=Sullivan |first4=Patrick F. |last5=van Bochoven |first5=Arjen |last6=Visscher |first6=Peter M. |last7=Posthuma |first7=Danielle |date=July 2015 |title=Meta-analysis of the heritability of human traits based on fifty years of twin studies |url=https://www.nature.com/articles/ng.3285 |journal=Nature Genetics |language=en |volume=47 |issue=7 |pages=702–709 |doi=10.1038/ng.3285 |pmid=25985137 |issn=1546-1718}}</ref> Because [[monozygotic]] twins derive from one fertilized egg, they are largely genetically identical{{Efn|Strictly speaking, monozygotic twins are not purely genetically identical due to somatic mosaicism and mutations.<ref>{{Cite journal |last=Charney |first=Evan |date=2012-10-24 |title=Behavior genetics and postgenomics |url=https://www.cambridge.org/core/journals/behavioral-and-brain-sciences/article/abs/behavior-genetics-and-postgenomics/AEDDF1A7D830480B80BF5A5BD7BD5BD0 |journal=Behavioral and Brain Sciences |language=en |volume=35 |issue=5 |pages=331–358 |doi=10.1017/S0140525X11002226 |pmid=23095378 |issn=0140-525X}}</ref>}}, while [[dizygotic]] twins are expected to share 50% of their DNA.<ref name=":4" />{{Reference page|pages=85-86}} Twin studies compare the resemblance of monozygotic twins to the resemblance of dizygotic twins to estimate the heritability of trait using various models.<ref>{{Citation |last1=Verweij |first1=Karin J. H. |title=Estimating Heritability from Twin Studies |date=2012 |work=Statistical Human Genetics: Methods and Protocols |pages=151–170 |editor-last=Elston |editor-first=Robert C.  |place=Totowa, NJ |publisher=Humana Press |language=en |doi=10.1007/978-1-61779-555-8_9 |isbn=978-1-61779-555-8 |last2=Mosing |first2=Miriam A. |last3=Zietsch |first3=Brendan P. |last4=Medland |first4=Sarah E. |volume=850 |pmid=22307698 |editor2-last=Satagopan |editor2-first=Jaya M. |editor3-last=Sun |editor3-first=Shuying}}</ref>

Twin studies have shown greater resemblance between monozygotic twins than dizygotic twins.<ref name=":4" /> This has lead most researchers in behavioral genetics to conclude that the heritability of IQ is in a range between 0.4 to 0.8.<ref name=":1" />{{Reference page|page=132|quote=Most studies estimate that the heritability of IQ is somewhere between .4 and .8 (and generally less for children), but it really makes no sense to talk about a single value for the heritability of intelligence.}}<ref name=":19">{{Cite book |last=Ceci |first=Stephen |title=On Intelligence: A Biological Treatise on Intellectual Development |publisher=Harvard University Press |isbn=978-0-674-63456-5 |edition=1st |publication-date=1 September 1996}}</ref>{{Rp|page=143}} However, it is not clear if all of the assumptions in twin studies hold true.<ref>{{Cite journal |last=Stenberg |first=Anders |date=2013-03-01 |title=Interpreting estimates of heritability – A note on the twin decomposition |url=https://www.sciencedirect.com/science/article/pii/S1570677X12000731 |journal=Economics & Human Biology |volume=11 |issue=2 |pages=201–205 |doi=10.1016/j.ehb.2012.05.002 |pmid=22676967 |issn=1570-677X}}</ref> Critics typically focus on the equal environment assumption, which is the assumption that environmentally caused similarity is the same for both monozygotic and dizygotic twins.<ref name=":4" />{{Reference page|page=86}}<ref>{{Cite journal |last1=Richardson |first1=Ken |last2=Norgate |first2=Sarah |date=2005 |title=The equal environments assumption of classical twin studies may not hold |url=https://onlinelibrary.wiley.com/doi/abs/10.1348/000709904X24690 |journal=British Journal of Educational Psychology |language=en |volume=75 |issue=3 |pages=339–350 |doi=10.1348/000709904X24690 |pmid=16238870 |issn=2044-8279}}</ref> Genetic modeling has found that even in the complete absence of genetic factors for a trait, environmental similarity between monozygotic twins will cause heritability estimators to produce large estimates of genetic heritability.<ref>{{Cite journal |last=Guo |first=Sun-Wei |date=1999 |title=The Behaviors of Some Heritability Estimators in the Complete Absence of Genetic Factors |url=https://karger.com/article/doi/10.1159/000022878 |journal=Human Heredity |language=en |volume=49 |issue=4 |pages=215–228 |doi=10.1159/000022878 |pmid=10436384 |issn=0001-5652}}</ref><ref>{{Cite journal |last=Guo |first=Sun-Wei |date=2001 |title=Does Higher Concordance in Monozygotic Twins Than in Dizygotic Twins Suggest a Genetic Component? |url=https://karger.com/article/doi/10.1159/000053333 |journal=Human Heredity |language=en |volume=51 |issue=3 |pages=121–132 |doi=10.1159/000053333 |pmid=11173963 |issn=0001-5652}}</ref> Researchers have produced different analyses of the equal environment assumption, some suggesting that the assumption is untenable for related traits like educational attainment.<ref>{{Cite report |url=https://liser.elsevierpure.com/en/publications/on-the-origins-of-socioeconomic-inequalities-evidence-from-twin-f |title=On the Origins of Socioeconomic Inequalities: Evidence from Twin Families |last1=Bingley |first1=Paul |last2=Cappellari |first2=Lorenzo |last3=Tatsiramos |first3=Konstantinos |date=5 Jul 2024 |issue=LISER Working Paper Series |access-date=13 October 2025 |url-status=live |archive-url=https://web.archive.org/web/20250708100608/https://liser.elsevierpure.com/en/publications/on-the-origins-of-socioeconomic-inequalities-evidence-from-twin-f |archive-date=8 July 2025}}</ref>

Twins reared apart have also been the subject of significant debate.<ref name=":17">{{Cite book |last=Bouchard |first=Thomas |title=Intelligence, heredity, and environment |date=1999 |publisher=Cambridge Univ. Press |isbn=978-0-521-46904-3 |editor-last=Sternberg |editor-first=Robert J. |edition=Repr |location=Cambridge |chapter=IQ similarity in twins reared apart: Findings and responses to critics}}</ref><ref>{{Cite book |last=Segal |first=Nancy L. |title=Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart |date=2021 |publisher=Rowman & Littlefield Publishing Group |isbn=978-1-5381-3285-2 |location= |language=en-US}}</ref><ref>{{Cite book |last=Joseph |first=Jay |url=https://www.taylorfrancis.com/books/9781317605911 |title=The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences |date=2014-11-20 |publisher=Routledge |isbn=978-1-315-74838-2 |edition=1 |language=en |doi=10.4324/9781315748382}}</ref><ref>{{Cite book |last=Farber |first=Susan L. |title=Identical twins reared apart: a reanalysis |date=1981 |publisher=Basic Books |isbn=978-0-465-03228-0 |location=New York}}</ref> The 2006 edition of ''Assessing adolescent and adult intelligence'' by [[Alan S. Kaufman]] and Elizabeth O. Lichtenberger reports correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings.<ref name="KaufmanLichtenberger2006">{{cite book |last1=Kaufman |first1=Alan S. |author-link1=Alan S. Kaufman |title=Assessing Adolescent and Adult Intelligence |last2=Lichtenberger |first2=Elizabeth |publisher=Wiley |year=2006 |isbn=978-0-471-73553-3 |edition=3rd |location=Hoboken (NJ)}}{{page needed|date=April 2013}}</ref> Theoretically, the correlation between twins reared apart is a direct estimate of heritability, as the only common factor between the twins would be the genetic variance.<ref>{{Cite journal |last1=Pedersen |first1=N. L. |last2=Lichtenstein |first2=P. |last3=Plomin |first3=R. |last4=DeFaire |first4=U. |last5=McClearn |first5=G. E. |last6=Matthews |first6=K. A. |date=1989 |title=Genetic and environmental influences for type A-like measures and related traits: a study of twins reared apart and twins reared together |journal=Psychosomatic Medicine |volume=51 |issue=4 |pages=428–440 |doi=10.1097/00006842-198907000-00006 |issn=0033-3174 |pmid=2772107}}</ref>{{Reference page|page=431|quote=in the absence of correlated environments, the intraclass correlation for identical twins reared apart provides a direct estimate}} However, this makes an assumption of uncorrelated environments between twins, which may not be the case.<ref>{{Cite journal |last=Joseph |first=Jay |date=2022 |title=A Reevaluation of the 1990 "Minnesota Study of Twins Reared Apart" IQ Study |url=https://karger.com/article/doi/10.1159/000521922 |journal=Human Development |language=en |volume=66 |issue=1 |pages=48–65 |doi=10.1159/000521922 |issn=0018-716X}}</ref><ref>{{Cite book |last=Nisbett |first=Richard E. |title=Intelligence and How to Get It: Why Schools and Cultures Count |title-link=Intelligence and How to Get It |publisher=W. W. Norton & Company |year=2009 |isbn=978-0-393-07141-2}}</ref>{{Reference page|page=26}}<ref>{{Cite journal |last=Kempthorne |first=Oscar |date=1978 |title=A Biometrics Invited Paper: Logical, Epistemological and Statistical Aspects of Nature-Nurture Data Interpretation |journal=Biometrics |volume=34 |issue=1 |pages=1–23 |doi=10.2307/2529584 |jstor=2529584 |issn=0006-341X}}</ref>{{Reference page|page=18}} For example, some researchers argue that twins experience similar education and this causes increased similarity in IQ.<ref>{{Cite book |title=Making human beings human: bioecological perspectives on human development |date=2005 |publisher=Sage Publications |isbn=978-0-7619-2711-2 |editor-last=Bronfenbrenner |editor-first=Urie |series=The Sage program on applied developmental science |location=Thousand Oaks}}</ref>{{Reference page|pages=236-237}}<ref>{{Cite book |last=Kamin |first=Leon |title=The Science and Politics of I.Q. |title-link=The Science and Politics of I.Q. |date=2012 |publisher=Taylor and Francis |isbn=978-0-89859-129-3 |location=Hoboken}}</ref>{{Reference page|page=67}}<ref>{{Cite journal |last1=Horvath |first1=Jared C. |last2=Fabricant |first2=Katie |date=2025-07-01 |title=IQ differences of identical twins reared apart are significantly influenced by educational differences |url=https://www.sciencedirect.com/science/article/pii/S0001691825003853 |journal=Acta Psychologica |volume=257 |article-number=105072 |doi=10.1016/j.actpsy.2025.105072 |pmid=40403412 |issn=0001-6918}}</ref> [[Thomas J. Bouchard Jr.|Thomas Bouchard]] argues that some previous analyses of the data on twins reared apart are an abuse of statistical theory that he calls "psuedoanalysis".<ref>{{Cite journal |last=Bouchard |first=Thomas J. |date=March 1982 |title=Identical Twins Reared Apart: Reanalysis or Pseudo-analysis? |url=http://access.portico.org/stable?au=phzkfk9w1 |journal=Contemporary Psychology: A Journal of Reviews |volume=27 |issue=3 |pages=190–191 |doi=10.1037/021001}}</ref><ref name=":17" />{{Reference page|pages=146-147}}

==== Adoption studies ====
Another study design researchers have used is adoption studies. In theory, "adoption creates sets of genetically related individuals who do not share a common family environment because they were adopted apart".<ref name=":4" />{{Reference page|page=80}} Adoption studies have broadly found that the IQs of adoptees are more similar to their biological parents than that of their adoptive parents,<ref>{{Cite journal |last=Turkheimer |first=Eric |date=November 1991 |title=Individual and group differences in adoption studies of IQ. |url=https://psycnet.apa.org/doi/10.1037/0033-2909.110.3.392 |journal=Psychological Bulletin |language=en |volume=110 |issue=3 |pages=392–405 |doi=10.1037/0033-2909.110.3.392 |issn=1939-1455 |archive-url=http://web.archive.org/web/20250710083832/https://psycnet.apa.org/doi/10.1037/0033-2909.110.3.392 |archive-date=2025-07-10}}</ref> in tandem with findings that adoption greatly increases IQ.<ref name=":38">{{Cite journal |last1=van IJzendoorn |first1=Marinus H. |last2=Juffer |first2=Femmie |last3=Poelhuis |first3=Caroline W. Klein |date=2005 |title=Adoption and Cognitive Development: A Meta-Analytic Comparison of Adopted and Nonadopted Children's IQ and School Performance. |url=https://psycnet.apa.org/doi/10.1037/0033-2909.131.2.301 |journal=Psychological Bulletin |language=en |volume=131 |issue=2 |pages=301–316 |doi=10.1037/0033-2909.131.2.301 |pmid=15740423 |issn=1939-1455 |archive-url=http://web.archive.org/web/20250916131512/https://psycnet.apa.org/doi/10.1037/0033-2909.131.2.301 |archive-date=2025-09-16}}</ref><ref>{{Cite journal |last1=Kendler |first1=Kenneth S. |last2=Turkheimer |first2=Eric |last3=Ohlsson |first3=Henrik |last4=Sundquist |first4=Jan |last5=Sundquist |first5=Kristina |date=2015-04-14 |title=Family environment and the malleability of cognitive ability: A Swedish national home-reared and adopted-away cosibling control study |journal=Proceedings of the National Academy of Sciences |volume=112 |issue=15 |pages=4612–4617 |doi=10.1073/pnas.1417106112 |doi-access=free |pmc=4403216 |pmid=25831538 |bibcode=2015PNAS..112.4612K }}</ref> For example, one analysis of the Texas Adoption Project estimated heritability at 0.78.<ref name=":15">{{Cite book |last1=Loehlin |first1=John C. |author-link1=Thomas J. Bouchard Jr. |title=Intelligence, heredity, and environment |last2=Horn |first2=Joseph M. |last3=Willerman |first3=Lee |date=1999 |publisher=Cambridge Univ. Press |isbn=978-0-521-46904-3 |editor-last=Sternberg |editor-first=Robert J. |edition=Repr |location=Cambridge |pages=126–160 |chapter=Heredity, environment, and IQ in the Texas Adoption Project}}</ref>{{Reference page|page=123}} However, one recent adoption study estimate heritability for cognitive ability at 0.33.<ref>{{Cite report |url=https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4491976 |title=Nature-Nurture Interplay: Evidence from Molecular Genetic and Pedigree Data in Korean American Adoptees |last1=Beauchamp |first1=Jonathan |last2=Schmitz |first2=Lauren |last3=McGue |first3=Matt |author-link3=Matt McGue |last4=Lee |first4=James |date=30 November 2023 |publisher=SSRN |doi=10.2139/ssrn.4491976 |ssrn=4491976 |access-date=30 October 2025}}</ref> Some critics point to assortative mating, range restriction<ref>{{Cite journal |last=Stoolmiller |first=Mike |date=1998-11-01 |title=Correcting Estimates of Shared Environmental Variance for Range Restriction in Adoption Studies Using a Truncated Multivariate Normal Model |journal=Behavior Genetics |language=en |volume=28 |issue=6 |pages=429–441 |doi=10.1023/A:1021685211674 |issn=1573-3297 |pmid=9926612}}</ref><ref>{{Cite journal |last=Stoolmiller |first=Mike |date=1999 |title=Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior–genetic adoption studies. |url=https://doi.apa.org/doi/10.1037/0033-2909.125.4.392 |journal=Psychological Bulletin |language=en |volume=125 |issue=4 |pages=392–409 |doi=10.1037/0033-2909.125.4.392 |pmid=10414224 |issn=1939-1455}}</ref> and other complex family and social processes as providing issues in the interpretation of data from adoption studies.<ref name=":16" /><ref>{{Cite journal |last=Kaplan |first=Jack S. |date=2012 |title=The effects of shared environment on adult intelligence: A critical review of adoption, twin, and MZA studies. |url=https://doi.apa.org/doi/10.1037/a0028133 |journal=Developmental Psychology |language=en |volume=48 |issue=5 |pages=1292–1298 |doi=10.1037/a0028133 |pmid=22746221 |issn=1939-0599}}</ref><ref>{{Cite journal |last1=Burt |first1=S. Alexandra |last2=O'Keefe |first2=Patrick |last3=Johnson |first3=Wendy |last4=Thaler |first4=Daniel |last5=Leve |first5=Leslie D. |last6=Natsuaki |first6=Misaki N. |last7=Reiss |first7=David |last8=Shaw |first8=Daniel S. |last9=Ganiban |first9=Jody M. |last10=Neiderhiser |first10=Jenae M. |date=May 2024 |title=The Detection of Environmental Influences on Academic Achievement Appears to Depend on the Analytic Approach |journal=Behavior Genetics |volume=54 |issue=3 |pages=252–267 |doi=10.1007/s10519-024-10179-w |issn=1573-3297 |pmid=38587720}}</ref> Recent studies employing [[Polygenic score|polygenic scores]] for [[educational attainment]] in adoptees have found a more complicated interaction between genes and environments.<ref>{{Cite journal |last1=Cheesman |first1=Rosa |last2=Hunjan |first2=Avina |last3=Coleman |first3=Jonathan R. I. |last4=Ahmadzadeh |first4=Yasmin |last5=Plomin |first5=Robert |last6=McAdams |first6=Tom A. |last7=Eley |first7=Thalia C. |last8=Breen |first8=Gerome |date=2020-05-01 |title=Comparison of Adopted and Nonadopted Individuals Reveals Gene–Environment Interplay for Education in the UK Biobank |journal=Psychological Science |language=EN |volume=31 |issue=5 |pages=582–591 |doi=10.1177/0956797620904450 |issn=0956-7976 |pmc=7238511 |pmid=32302253}}</ref>

==== Other models ====
More broadly, researchers have analyzed the resemblance between various familial classes, such as parent-child correlations. In 1982, Bouchard and McGue reviewed such correlations reported in 111 original studies in the United States. The mean correlation of IQ scores between monozygotic twins was 0.86, between siblings 0.47, between half-siblings 0.31, and between cousins 0.15.<ref name="pmid7195071">{{cite journal |last1=Bouchard |first1=Thomas J. |last2=McGue |first2=Matthew |year=1981 |title=Familial Studies of Intelligence: A Review |journal=Science |volume=212 |issue=4498 |pages=1055–9 |bibcode=1981Sci...212.1055B |doi=10.1126/science.7195071 |pmid=7195071}}</ref> Some more basic model fitting studies are found in the behavioral genetics literature. Loehlin fit a model to the Bouchard and McGue data which estimated broad heritability at .58 from 'direct' methods and .47 from 'indirect' methods.<ref>{{Cite journal |last=Loehlin |first=John C. |date=1989 |title=Partitioning environmental and genetic contributions to behavioral development. |url=https://doi.apa.org/doi/10.1037/0003-066X.44.10.1285 |journal=American Psychologist |language=en |volume=44 |issue=10 |pages=1285–1292 |doi=10.1037/0003-066X.44.10.1285 |pmid=2679255 |issn=1935-990X}}</ref><ref>{{Cite journal |last1=Plomin |first1=Robert |last2=Loehlin |first2=John C. |date=1989-05-01 |title=Direct and indirect IQ heritability estimates: A puzzle |journal=Behavior Genetics |language=en |volume=19 |issue=3 |pages=331–342 |doi=10.1007/BF01066162 |pmid=2667514 |issn=1573-3297}}</ref> Chipuer et al. analyzed this data using a LISREL model and estimated broad sense heritability as .51.<ref>{{Cite journal |last1=Chipuer |first1=Heather M. |last2=Rovine |first2=Michael J. |last3=Plomin |first3=Robert |date=1990-01-01 |title=LISREL modeling: Genetic and environmental influences on IQ revisited |journal=Intelligence |volume=14 |issue=1 |pages=11–29 |doi=10.1016/0160-2896(90)90011-H |issn=0160-2896}}</ref> Other models successively add more components. In 1997, Devlin et al. also used these correlations to fit and compare different models and estimated that heritability was less than 50% in their best fitting models, finding that [[Maternal effect|maternal effects]] were particularly important in their analysis.<ref name="devlin" /> Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different from previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports.<ref name="pmid12486697">{{cite journal |last1=Bouchard |first1=Thomas J. |last2=McGue |first2=Matt |date=January 2003 |title=Genetic and environmental influences on human psychological differences |journal=Journal of Neurobiology |volume=54 |issue=1 |pages=4–45 |doi=10.1002/neu.10160 |pmid=12486697 |doi-access=free}}</ref> 

Genetics researchers observed that cultural heritability and genetic heritability can produce similar patterns of observations between relatives in the 1970s.<ref>{{Cite journal |last1=Shen |first1=Hao |last2=Feldman |first2=Marcus W. |date=2021-10-16 |title=Cultural versus biological inheritance: A retrospective view of Cavalli-Sforza and Feldman (1973) |url=https://www.pivotscipub.com/hpgg/1/1/0003/html |journal=Human Population Genetics and Genomics |language=en |volume=1 |issue=1 |pages=1–18 |article-number=0003 |doi=10.47248/hpgg2101010003 |issn=2770-5005}}</ref><ref>{{cite bioRxiv |last1=Benning |first1=John W. |title=Confounding Fuels Misinterpretation in Human Genetics |date=2025-07-11 |biorxiv=10.1101/2023.11.01.565061 |last2=Carlson |first2=Jedidiah |last3=Smith |first3=Olivia S. |last4=Shaw |first4=Ruth G. |last5=Harpak |first5=Arbel}}</ref> As a result, in a series of papers using models based on path analysis, a group of researchers from the University of Hawaii analyzed data collected on American families and estimated genetic heritability ranging from 0.30 to 0.34.<ref name=":14" /><ref name=":30">{{Cite journal |last1=Rao |first1=D. C. |last2=Morton |first2=N. E. |last3=Lalouel |first3=J. M. |last4=Lew |first4=R. |date=April 1982 |title=Path analysis under generalized assortative mating: II. American I.Q. |url=https://www.cambridge.org/core/journals/genetics-research/article/path-analysis-under-generalized-assortative-mating-ii-american-iq/48A730B45DFA736F0F1A35A7C4E93D1B |journal=Genetics Research |language=en |volume=39 |issue=2 |pages=187–198 |doi=10.1017/S0016672300020875 |pmid=7084667 |issn=1469-5073}}</ref><ref>{{Cite journal |last1=Rice |first1=J. |last2=Cloninger |first2=C. R. |last3=Reich |first3=T. |date=January 1980 |title=Analysis of behavioral traits in the presence of cultural transmission and assortative mating: Applications to IQ and SES |journal=Behavior Genetics |volume=10 |issue=1 |pages=73–92 |doi=10.1007/BF01067320 |issn=0001-8244 |pmid=7425997}}</ref><ref>{{Cite report |url=https://krichard12.com/docs/Goldberger1978.pdf |title=Models and Methods in the IQ Debate: Part I |last=Goldberger |first=Arthur |date=February 1978 |access-date=17 October 2025 |url-status=live |archive-url=https://web.archive.org/web/20240523090514/https://krichard12.com/docs/Goldberger1978.pdf |archive-date=23 May 2024}}</ref>{{Reference page|pages=56-95}} A later study by Otto et al. applied the path analysis method to the data collected by Bouchard and McGue and estimated heritability ranging from 0.29 to 0.42.<ref name=":14" /><ref>{{Cite report |url=https://morrisoninstitute.stanford.edu/publications/genetic-and-cultural-inheritance-continuous-traits |title=Genetic and Cultural Inheritance of Continuous Traits |last1=Otto |first1=Sarah P |last2=Feldman |first2=Marcus W. |last3=Christiansen |first3=Freddy B. |date=1995 |access-date=13 October 2025}}</ref> Path analysis models have been subject to criticism for further issues with modeling assumptions and statistical procedures,<ref>{{Cite journal |last1=Karlin |first1=S. |last2=Cameron |first2=E. C. |last3=Chakraborty |first3=R. |date=July 1983 |title=Path analysis in genetic epidemiology: a critique |journal=American Journal of Human Genetics |volume=35 |issue=4 |pages=695–732 |issn=0002-9297 |pmc=1685747 |pmid=6349335}}</ref><ref>{{Cite journal |last=Wright |first=S. |date=July 1983 |title=On "Path analysis in genetic epidemiology: a critique" |journal=American Journal of Human Genetics |volume=35 |issue=4 |pages=757–768 |issn=0002-9297 |pmc=1685743 |pmid=6881145}}</ref><ref>{{Cite journal |last1=Cloninger |first1=C. R. |last2=Rao |first2=D. C. |last3=Rice |first3=J. |last4=Reich |first4=T. |last5=Morton |first5=N. E. |date=July 1983 |title=A defense of path analysis in genetic epidemiology |journal=American Journal of Human Genetics |volume=35 |issue=4 |pages=733–756 |issn=0002-9297 |pmc=1685729 |pmid=6349336}}</ref> reflecting broader criticisms of structural equation modeling.<ref>{{Citation |last=Goldberger |first=Arthur S. |title=Structural Equation Models in Human Behavior Genetics |date=2005 |work=Identification and Inference for Econometric Models: Essays in Honor of Thomas Rothenberg |pages=11–26 |editor-last=Andrews |editor-first=Donald W. K. |url=https://www.cambridge.org/core/books/identification-and-inference-for-econometric-models/structural-equation-models-in-human-behavior-genetics/3AFA5E62B6D2BE85B9B0E31827AF01AB |access-date=2025-10-14 |place=Cambridge |publisher=Cambridge University Press |doi=10.1017/cbo9780511614491.003 |isbn=978-0-521-84441-3 |editor2-last=Stock |editor2-first=James H.}}</ref><ref>{{Cite journal |last=Gifford |first=Fred |date=1989-06-01 |title=Complex genetic causation of human disease: Critiques of and rationales for heritability and path analysis |journal=Theoretical Medicine |language=en |volume=10 |issue=2 |pages=107–122 |doi=10.1007/BF00539877 |pmid=2675372 |issn=1573-1200}}</ref>

=== Molecular genetics ===
{{See also|Genome-wide association study|Polygenic score|Missing heritability problem}}

These findings are compatible with the view that a large number of genes, each with only a small effect, contribute to differences in intelligence.<ref name="Chabris">{{cite journal |last1=Chabris |first1=C. F. |last2=Hebert |first2=B. M. |last3=Benjamin |first3=D. J. |last4=Beauchamp |first4=J. |last5=Cesarini |first5=D. |last6=Van Der Loos |first6=M. |last7=Johannesson |first7=M. |last8=Magnusson |first8=P. K. E. |last9=Lichtenstein |first9=P. |last10=Atwood |first10=C. S. |last11=Freese |first11=J. |last12=Hauser |first12=T. S. |last13=Hauser |first13=R. M. |last14=Christakis |first14=N. |last15=Laibson |first15=D. |year=2012 |title=Most Reported Genetic Associations with General Intelligence Are Probably False Positives |journal=Psychological Science |volume=23 |issue=11 |pages=1314–23 |doi=10.1177/0956797611435528 |pmc=3498585 |pmid=23012269}}</ref>
[[File:Kemper et al diagram.webp|thumb|A diagram showing the relationship between genomic relatedness and phenotypic correlation for a hypothetical trait, a method used to estimate heritability. From Kemper et al. 2021<ref name=":25" />]]
Molecular genetic studies have been central to several major scientific advances relevant to heritability. The first is the advent of polygenic scores, which take the thousands of [[Single-nucleotide polymorphism|single-nucleotide polymorphisms]] (SNPs) that influence a given trait to form an estimated "genetic value" or "genetic propensity".<ref>{{Cite journal |last1=Rosenberg |first1=Noah A |last2=Edge |first2=Michael D |last3=Pritchard |first3=Jonathan K |last4=Feldman |first4=Marcus W |date=2018-12-27 |title=Interpreting polygenic scores, polygenic adaptation, and human phenotypic differences |url=https://academic.oup.com/emph/article/2019/1/26/5262222 |journal=Evolution, Medicine, and Public Health |language=en |volume=2019 |issue=1 |pages=26–34 |doi=10.1093/emph/eoy036 |pmid=30838127 |pmc=6393779 |issn=2050-6201 |archive-url=http://web.archive.org/web/20250727113251/https://academic.oup.com/emph/article/2019/1/26/5262222 |archive-date=2025-07-27}}</ref> Most genome-wide association studies (GWAS) estimate a polygenic score (PGS) and describe its overall statistical contribution to variance for the population in question, i.e. providing a R{{Sup|2}} ([[coefficient of determination]]) for the polygenic score.<ref name=":28">{{Cite book |last1=Mills |first1=Melinda |title=An introduction to statistical genetic data analysis |last2=Barban |first2=Nicola |last3=Tropf |first3=Felix C. |date=2020 |publisher=The MIT Press |isbn=978-0-262-53838-1 |location=Cambridge, Massachusetts London, England}}</ref>{{Reference page|page=251}}<ref name=":26">{{Cite journal |last1=Plomin |first1=Robert |last2=von Stumm |first2=Sophie |date=January 2022 |title=Polygenic scores: prediction versus explanation |journal=Molecular Psychiatry |language=en |volume=27 |issue=1 |pages=49–52 |doi=10.1038/s41380-021-01348-y |pmid=34686768 |pmc=8960389 |issn=1476-5578}}</ref> This measures only the predictive power of the genetic variants in the study in question, a future study with larger sample size or more genetic variants tagged may produce a larger amount.<ref>{{Cite journal |last1=Cesarini |first1=David |last2=Visscher |first2=Peter M. |date=2017-02-01 |title=Genetics and educational attainment |journal=npj Science of Learning |language=en |volume=2 |issue=1 |page=4 |doi=10.1038/s41539-017-0005-6 |pmid=30631451 |pmc=6220209 |bibcode=2017npjSL...2....4C |issn=2056-7936}}</ref> Consequently, a given polygenic score only captures "a fraction of SNP-heritability".<ref>{{Cite journal |last1=Pingault |first1=Jean-Baptiste |last2=Allegrini |first2=Andrea G. |last3=Odigie |first3=Tracy |last4=Frach |first4=Leonard |last5=Baldwin |first5=Jessie R. |last6=Rijsdijk |first6=Frühling |last7=Dudbridge |first7=Frank |date=2022 |title=Research Review: How to interpret associations between polygenic scores, environmental risks, and phenotypes |journal=Journal of Child Psychology and Psychiatry |language=en |volume=63 |issue=10 |pages=1125–1139 |doi=10.1111/jcpp.13607 |issn=1469-7610 |pmc=9790749 |pmid=35347715}}</ref> The second is the concept of SNP heritability, which is the total proportion of phenotypic variance explained by SNPs.<ref name=":20" /><ref>{{Cite journal |last1=Speed |first1=Doug |last2=Cai |first2=Na |last3=Johnson |first3=Michael R. |last4=Nejentsev |first4=Sergey |last5=Balding |first5=David J. |date=July 2017 |title=Reevaluation of SNP heritability in complex human traits |journal=Nature Genetics |language=en |volume=49 |issue=7 |pages=986–992 |doi=10.1038/ng.3865 |pmid=28530675 |pmc=5493198 |issn=1546-1718}}</ref><ref>{{Cite journal |last1=Huang |first1=Jinguo |last2=Kleman |first2=Nicole |last3=Basu |first3=Saonli |last4=Shriver |first4=Mark D |last5=Zaidi |first5=Arslan A |date=2025-08-06 |editor-last=Li |editor-first=Y |title=Interpreting SNP heritability in admixed populations |url=https://academic.oup.com/genetics/article/doi/10.1093/genetics/iyaf100/8140958 |journal=Genetics |language=en |volume=230 |issue=4 |article-number=iyaf100 |doi=10.1093/genetics/iyaf100 |issn=1943-2631 |pmc=12273224 |pmid=40404285}}</ref>

Genomic studies have found significant correlations between IQ and various polygenic scores,<ref name=":22">{{Cite journal |last1=Oxley |first1=Florence A. R. |last2=Wilding |first2=Kirsty |last3=von Stumm |first3=Sophie |date=2024-11-01 |title=DNA and IQ: Big deal or much ado about nothing? – A meta-analysis |url=https://www.sciencedirect.com/science/article/pii/S0160289624000655 |journal=Intelligence |volume=107 |doi=10.1016/j.intell.2024.101871 |issn=0160-2896 |article-number=101871}}</ref> but the true size of this association is unclear. The largest published analysis for genetic contributions to IQ found that the polygenic score could explain 9.9% of the variance between families,<ref name=":22" /><ref name=":33">{{Cite journal |last1=Savage |first1=Jeanne E. |last2=Jansen |first2=Philip R. |last3=Stringer |first3=Sven |last4=Watanabe |first4=Kyoko |last5=Bryois |first5=Julien |last6=de Leeuw |first6=Christiaan A. |last7=Nagel |first7=Mats |last8=Awasthi |first8=Swapnil |last9=Barr |first9=Peter B. |last10=Coleman |first10=Jonathan R. I. |last11=Grasby |first11=Katrina L. |last12=Hammerschlag |first12=Anke R. |last13=Kaminski |first13=Jakob A. |last14=Karlsson |first14=Robert |last15=Krapohl |first15=Eva |date=July 2018 |title=Genome-wide association meta-analysis in 269,867 individuals identifies new genetic and functional links to intelligence |journal=Nature Genetics |language=en |volume=50 |issue=7 |pages=912–919 |doi=10.1038/s41588-018-0152-6 |issn=1546-1718 |pmc=6411041 |pmid=29942086}}</ref> while some later analyses estimated that polygenic scores can predict about 11% of the variance.<ref>{{Cite journal |last1=Procopio |first1=Francesca |last2=Liao |first2=Wangjingyi |last3=Rimfeld |first3=Kaili |last4=Malanchini |first4=Margherita |last5=von Stumm |first5=Sophie |last6=Allegrini |first6=Andrea G. |last7=Plomin |first7=Robert |date=February 2025 |title=Multi-polygenic score prediction of mathematics, reading, and language abilities independent of general cognitive ability |journal=Molecular Psychiatry |language=en |volume=30 |issue=2 |pages=414–422 |doi=10.1038/s41380-024-02671-w |pmid=39085392 |pmc=11746139 |issn=1476-5578}}</ref> However, these analyses rely on differences in genetic variants between families which introduce biases from [[Population structure (genetics)|population stratification]], [[assortative mating]], and [[indirect genetic effects]] such as dynastic effects<ref>{{Cite journal |last1=Nivard |first1=Michel G. |last2=Belsky |first2=Daniel W. |last3=Harden |first3=K. Paige |last4=Baier |first4=Tina |last5=Andreassen |first5=Ole A. |last6=Ystrøm |first6=Eivind |last7=van Bergen |first7=Elsje |last8=Lyngstad |first8=Torkild H. |date=2024-01-15 |title=More than nature and nurture, indirect genetic effects on children's academic achievement are consequences of dynastic social processes |journal=Nature Human Behaviour |volume=8 |issue=4 |pages=771–778 |doi=10.1038/s41562-023-01796-2 |pmid=38225408 |pmc=11569812 |issn=2397-3374}}</ref> or genetic nurture.<ref name=":23">{{Cite journal |last1=Howe |first1=Laurence J. |last2=Nivard |first2=Michel G. |last3=Morris |first3=Tim T. |last4=Hansen |first4=Ailin F. |last5=Rasheed |first5=Humaira |last6=Cho |first6=Yoonsu |last7=Chittoor |first7=Geetha |last8=Ahlskog |first8=Rafael |last9=Lind |first9=Penelope A. |last10=Palviainen |first10=Teemu |last11=van der Zee |first11=Matthijs D. |last12=Cheesman |first12=Rosa |last13=Mangino |first13=Massimo |last14=Wang |first14=Yunzhang |last15=Li |first15=Shuai |date=May 2022 |title=Within-sibship genome-wide association analyses decrease bias in estimates of direct genetic effects |journal=Nature Genetics |language=en |volume=54 |issue=5 |pages=581–592 |doi=10.1038/s41588-022-01062-7 |pmid=35534559 |pmc=9110300 |issn=1546-1718}}</ref><ref name=":24">{{cite medrxiv |last1=Tan |first1=Tammy |title=Family-GWAS reveals effects of environment and mating on genetic associations |date=2024-10-04 |language=en |medrxiv=10.1101/2024.10.01.24314703 |last2=Jayashankar |first2=Hariharan |last3=Guan |first3=Junming |last4=Nehzati |first4=Seyed Moeen |last5=Mir |first5=Mahdi |last6=Bennett |first6=Michael |last7=Agerbo |first7=Esben |last8=Ahlskog |first8=Rafael |last9=Anapaz |first9=Ville Pinto de Andrade}}</ref><ref>{{cite bioRxiv |last1=Border |first1=Richard |title=Simple models of non-random mating and environmental transmission bias standard human genetics statistical methods |date=2024-10-18  |biorxiv=10.1101/2024.10.16.618755 |last2=Wang |first2=Jeremy |last3=Caggiano |first3=Christa |last4=Sankararaman |first4=Sriram |last5=Schork |first5=Andrew J. |last6=Turley |first6=Patrick |last7=Young |first7=Alexander S. |last8=Benjamin |first8=Daniel J. |last9=Dahl |first9=Andy W.}}</ref> One study found that polygenic score prediction for cognitive traits was 60% greater between families than within families.<ref>{{Cite journal |last1=Selzam |first1=Saskia |last2=Ritchie |first2=Stuart J. |last3=Pingault |first3=Jean-Baptiste |last4=Reynolds |first4=Chandra A. |last5=O'Reilly |first5=Paul F. |last6=Plomin |first6=Robert |date=2019-08-01 |title=Comparing Within- and Between-Family Polygenic Score Prediction |journal=The American Journal of Human Genetics |language=English |volume=105 |issue=2 |pages=351–363 |doi=10.1016/j.ajhg.2019.06.006 |issn=0002-9297 |pmid=31303263 |pmc=6698881 }}</ref> 

Genomic studies have also estimated values of SNP heritability for IQ. Very early studies used a method called genome-wide complex trait analysis (GCTA) which calculates the overall genetic similarity (as indexed by the cumulative effects of all genotyped [[single nucleotide polymorphism]]s) between all pairs of individuals in a sample of unrelated individuals and then correlates this genetic similarity with phenotypic similarity across all the pairs.<ref>{{Cite journal |last1=Yang |first1=Jian |last2=Lee |first2=S. Hong |last3=Goddard |first3=Michael E. |last4=Visscher |first4=Peter M. |date=2011-01-07 |title=GCTA: A Tool for Genome-wide Complex Trait Analysis |journal=The American Journal of Human Genetics |language=English |volume=88 |issue=1 |pages=76–82 |doi=10.1016/j.ajhg.2010.11.011 |issn=0002-9297 |pmid=21167468 |pmc=3014363 }}</ref> Studies using this method reported relatively large estimates of heritability, ranging from 40% to 51%.<ref name="Davies2011" /> A later study by the same group estimated SNP heritability for different measures of cognitive ability at about 31%.<ref>{{Cite journal |last1=Davies |first1=G. |last2=Marioni |first2=R. E. |last3=Liewald |first3=D. C. |last4=Hill |first4=W. D. |last5=Hagenaars |first5=S. P. |last6=Harris |first6=S. E. |last7=Ritchie |first7=S. J. |last8=Luciano |first8=M. |last9=Fawns-Ritchie |first9=C. |last10=Lyall |first10=D. |last11=Cullen |first11=B. |last12=Cox |first12=S. R. |last13=Hayward |first13=C. |last14=Porteous |first14=D. J. |last15=Evans |first15=J. |date=June 2016 |title=Genome-wide association study of cognitive functions and educational attainment in UK Biobank (N=112 151) |journal=Molecular Psychiatry |language=en |volume=21 |issue=6 |pages=758–767 |doi=10.1038/mp.2016.45 |pmid=27046643 |pmc=4879186 |issn=1476-5578}}</ref> However, researchers later discovered population stratification and other environmental biases were major issues affecting basic analyses.<ref>{{Cite journal |last1=Haworth |first1=Simon |last2=Mitchell |first2=Ruth |last3=Corbin |first3=Laura |last4=Wade |first4=Kaitlin H. |last5=Dudding |first5=Tom |last6=Budu-Aggrey |first6=Ashley |last7=Carslake |first7=David |last8=Hemani |first8=Gibran |last9=Paternoster |first9=Lavinia |last10=Smith |first10=George Davey |last11=Davies |first11=Neil |last12=Lawson |first12=Daniel J. |last13=J. Timpson |first13=Nicholas |date=2019-01-18 |title=Apparent latent structure within the UK Biobank sample has implications for epidemiological analysis |journal=Nature Communications |language=en |volume=10 |issue=1 |page=333 |doi=10.1038/s41467-018-08219-1 |pmid=30659178 |pmc=6338768 |bibcode=2019NatCo..10..333H |issn=2041-1723}}</ref><ref>{{Cite journal |last1=Dandine-Roulland |first1=Claire |last2=Bellenguez |first2=Céline |last3=Debette |first3=Stéphanie |last4=Amouyel |first4=Philippe |last5=Génin |first5=Emmanuelle |last6=Perdry |first6=Hervé |date=2016-05-25 |title=Accuracy of heritability estimations in presence of hidden population stratification |journal=Scientific Reports |language=en |volume=6 |issue=1 |article-number=26471 |doi=10.1038/srep26471 |pmid=27220488 |pmc=4879529 |bibcode=2016NatSR...626471D |issn=2045-2322}}</ref> As a result, recent research has sought to estimate the SNP heritability of traits, including IQ, using within-family genome-wide association studies,<ref name=":23" /><ref name=":24" /><ref>{{Cite journal |last1=Morris |first1=Tim T. |last2=Davies |first2=Neil M. |last3=Hemani |first3=Gibran |last4=Smith |first4=George Davey |date=2020-04-15 |title=Population phenomena inflate genetic associations of complex social traits |journal=Science Advances |volume=6 |issue=16 |article-number=eaay0328 |bibcode=2020SciA....6..328M |doi=10.1126/sciadv.aay0328 |pmc=7159920 |pmid=32426451}}</ref> though researchers have identified that these are not immune to interpretive issues.<ref>{{Cite journal |last1=Howe |first1=Laurence J. |last2=Evans |first2=David M. |last3=Hemani |first3=Gibran |last4=Smith |first4=George Davey |last5=Davies |first5=Neil M. |date=2022-07-07 |title=Evaluating indirect genetic effects of siblings using singletons |journal=PLOS Genetics |language=en |volume=18 |issue=7 |article-number=e1010247 |doi=10.1371/journal.pgen.1010247 |doi-access=free |issn=1553-7404 |pmc=9262210 |pmid=35797272}}</ref><ref>{{Cite journal |last1=Veller |first1=Carl |last2=Coop |first2=Graham M. |date=2024-04-11 |title=Interpreting population- and family-based genome-wide association studies in the presence of confounding |journal=PLOS Biology |language=en |volume=22 |issue=4 |article-number=e3002511 |doi=10.1371/journal.pbio.3002511 |doi-access=free |issn=1545-7885 |pmc=11008796 |pmid=38603516}}</ref> Some earlier estimates of the SNP heritability of IQ, derived from [[Genome-wide association study|genome-wide association studies]] (GWAS), put the figure much lower, at around 10%.<ref name=":13" /> For example, one recent published analysis estimated the within-family SNP heritability of cognitive function to be 14%<ref name=":23" />. A more recent unpublished preprint estimated the SNP heritability at about 19%.<ref name=":24"/>{{Rp|page=5}}

Another set of methods attempts to generalize correlations between familial resemblance and genetic resemblance to estimate heritability.<ref name=":3">{{Cite journal |last1=Visscher |first1=Peter M. |last2=Medland |first2=Sarah E. |last3=Ferreira |first3=Manuel A. R. |last4=Morley |first4=Katherine I. |last5=Zhu |first5=Gu |last6=Cornes |first6=Belinda K. |last7=Montgomery |first7=Grant W. |last8=Martin |first8=Nicholas G. |date=2006-03-24 |title=Assumption-Free Estimation of Heritability from Genome-Wide Identity-by-Descent Sharing between Full Siblings |journal=PLOS Genetics |language=en |volume=2 |issue=3 |article-number=e41 |doi=10.1371/journal.pgen.0020041 |doi-access=free |issn=1553-7404 |pmc=1413498 |pmid=16565746}}</ref><ref name=":25">{{Cite journal |last1=Kemper |first1=Kathryn E. |last2=Yengo |first2=Loic |last3=Zheng |first3=Zhili |last4=Abdellaoui |first4=Abdel |last5=Keller |first5=Matthew C. |last6=Goddard |first6=Michael E. |last7=Wray |first7=Naomi R. |last8=Yang |first8=Jian |last9=Visscher |first9=Peter M. |date=2021-02-16 |title=Phenotypic covariance across the entire spectrum of relatedness for 86 billion pairs of individuals |journal=Nature Communications |language=en |volume=12 |issue=1 |page=1050 |doi=10.1038/s41467-021-21283-4 |pmid=33594080 |pmc=7886899 |bibcode=2021NatCo..12.1050K |issn=2041-1723}}</ref> These methods are free from most environmental biases and have typically found that twin estimates of heritability are inflated.<ref name=":25" /><ref name=":34">{{Cite journal |last1=Young |first1=Alexander I. |last2=Frigge |first2=Michael L. |last3=Gudbjartsson |first3=Daniel F. |last4=Thorleifsson |first4=Gudmar |last5=Bjornsdottir |first5=Gyda |last6=Sulem |first6=Patrick |last7=Masson |first7=Gisli |last8=Thorsteinsdottir |first8=Unnur |last9=Stefansson |first9=Kari |last10=Kong |first10=Augustine |date=September 2018 |title=Relatedness disequilibrium regression estimates heritability without environmental bias |journal=Nature Genetics |language=en |volume=50 |issue=9 |pages=1304–1310 |doi=10.1038/s41588-018-0178-9 |pmid=30104764 |pmc=6130754 |issn=1546-1718}}</ref> When tested on similar achievement phenotypes like [[educational attainment]], these methods find that twin methods significantly inflate the value of heritability.<ref>{{Cite journal |last1=Young |first1=Alexander I. |last2=Frigge |first2=Michael L. |last3=Gudbjartsson |first3=Daniel F. |last4=Thorleifsson |first4=Gudmar |last5=Bjornsdottir |first5=Gyda |last6=Sulem |first6=Patrick |last7=Masson |first7=Gisli |last8=Thorsteinsdottir |first8=Unnur |last9=Stefansson |first9=Kari |last10=Kong |first10=Augustine |date=September 2018 |title=Relatedness disequilibrium regression estimates heritability without environmental bias |journal=Nature Genetics |language=en |volume=50 |issue=9 |pages=1304–1310 |doi=10.1038/s41588-018-0178-9 |pmid=30104764 |pmc=6130754 |issn=1546-1718}}</ref> 

The phrase 'missing heritability' is commonly used to describe both the gap between classically derived heritability estimates and the variance explained by variants<ref name=":21" /> and "SNP" heritability.<ref name=":13" /><ref>{{Cite journal |last1=Oxley |first1=Florence A. R. |last2=Wilding |first2=Kirsty |last3=von Stumm |first3=Sophie |date=2024-11-01 |title=DNA and IQ: Big deal or much ado about nothing? – A meta-analysis |url=https://www.sciencedirect.com/science/article/pii/S0160289624000655 |journal=Intelligence |volume=107 |article-number=101871 |doi=10.1016/j.intell.2024.101871 |issn=0160-2896}}</ref> As the heritability estimates obtained by molecular genetic studies do not match earlier research from twin and family studies, researchers have proposed many theories as to explain these discrepancies for various traits<ref name=":27">{{Cite journal |last=Génin |first=Emmanuelle |date=2020-01-01 |title=Missing heritability of complex diseases: case solved? |journal=Human Genetics |language=en |volume=139 |issue=1 |pages=103–113 |doi=10.1007/s00439-019-02034-4 |pmid=31165258 |issn=1432-1203}}</ref><ref name=":28" />{{Reference page|pages=27, 111-112}}, including but not limited{{Efn|Some other proposed explanations that have largely been discounted include gene-gene interactions like epistasis<ref>{{Cite journal |last1=Zuk |first1=Or |last2=Hechter |first2=Eliana |last3=Sunyaev |first3=Shamil R. |last4=Lander |first4=Eric S. |date=2012-01-24 |title=The mystery of missing heritability: Genetic interactions create phantom heritability |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=4 |pages=1193–1198 |doi=10.1073/pnas.1119675109 |doi-access=free |pmc=3268279 |pmid=22223662 |bibcode=2012PNAS..109.1193Z }}</ref><ref>{{Cite book |last1=Conley |first1=Dalton |title=The genome factor: what the social genomics revolution reveals about ourselves, our history, and the future |last2=Fletcher |first2=Jason |date=2017 |publisher=Princeton university press |isbn=978-0-691-16474-8 |location=Princeton}}</ref>{{Reference page|page=52-53}}  and epigenetics,<ref>{{Cite journal |last1=Bourrat |first1=Pierrick |last2=Lu |first2=Qiaoying |last3=Jablonka |first3=Eva |date=2017 |title=Why the missing heritability might not be in the DNA |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/bies.201700067 |journal=BioEssays |volume=39 |issue=7 |article-number=1700067 |doi=10.1002/bies.201700067 |pmid=28582595 |issn=1521-1878}}</ref><ref>{{Cite journal |last1=Trerotola |first1=Marco |last2=Relli |first2=Valeria |last3=Simeone |first3=Pasquale |last4=Alberti |first4=Saverio |date=2015-07-28 |title=Epigenetic inheritance and the missing heritability |journal=Human Genomics |language=en |volume=9 |issue=1 |page=17 |doi=10.1186/s40246-015-0041-3 |doi-access=free |issn=1479-7364 |pmc=4517414 |pmid=26216216}}</ref>}} to rare variants,<ref>{{Cite journal |last1=Zuk |first1=Or |last2=Schaffner |first2=Stephen F. |last3=Samocha |first3=Kaitlin |last4=Do |first4=Ron |last5=Hechter |first5=Eliana |last6=Kathiresan |first6=Sekar |last7=Daly |first7=Mark J. |last8=Neale |first8=Benjamin M. |last9=Sunyaev |first9=Shamil R. |last10=Lander |first10=Eric S. |date=2014-01-28 |title=Searching for missing heritability: Designing rare variant association studies |journal=Proceedings of the National Academy of Sciences |volume=111 |issue=4 |pages=E455–E464 |doi=10.1073/pnas.1322563111 |doi-access=free |pmc=3910587 |pmid=24443550 |bibcode=2014PNAS..111E.455Z }}</ref> untagged variants,<ref>{{Cite journal |last1=Gymrek |first1=Melissa |last2=Goren |first2=Alon |date=2021-09-24 |title=Missing heritability may be hiding in repeats |url=https://www.science.org/doi/abs/10.1126/science.abl7794 |journal=Science |volume=373 |issue=6562 |pages=1440–1441 |doi=10.1126/science.abl7794 |pmid=34554784 |bibcode=2021Sci...373.1440G }}</ref> gene-environment interactions<ref>{{Cite journal |last=Kaprio |first=J. |date=2012 |title=Twins and the mystery of missing heritability: the contribution of gene–environment interactions |journal=Journal of Internal Medicine |language=en |volume=272 |issue=5 |pages=440–448 |doi=10.1111/j.1365-2796.2012.02587.x |issn=1365-2796 |pmc=4422871 |pmid=22934540}}</ref> or environmental confounding.<ref name=":14" /><ref>{{Cite journal |last1=Shen |first1=Hao |last2=Feldman |first2=Marcus W. |date=2020-10-13 |title=Genetic nurturing, missing heritability, and causal analysis in genetic statistics |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=41 |pages=25646–25654 |doi=10.1073/pnas.2015869117 |doi-access=free |pmc=7568332 |pmid=32989124 |bibcode=2020PNAS..11725646S }}</ref><ref>{{Cite journal |last1=Chaufan |first1=Claudia |last2=Joseph |first2=Jay |date=2013-04-01 |title=The 'Missing Heritability' of Common Disorders: Should Health Researchers Care? |journal=International Journal of Health Services |language=EN |volume=43 |issue=2 |pages=281–303 |doi=10.2190/HS.43.2.f |pmid=23821906 |issn=0020-7314}}</ref><ref>{{Cite book |last=Richardson |first=Ken |title=Genes, brains, and human potential: the science and ideology of intelligence |date=2017 |publisher=Columbia University Press |isbn=978-0-231-54376-7 |series=Columbia scholarship online |location=New York}}</ref>{{Reference page|pages=66-67}} Some researchers believe that the gap for IQ heritability will be narrowed with larger, better-powered genome-wide studies and with whole-genome sequencing,<ref name=":12" />{{Reference page|page=4}}<ref name=":26" /><ref name=":22" /> while others indicate that the gap may at least in part reflect overestimation of heritability by twin studies.<ref name=":14" /><ref>{{Cite journal |last1=Richardson |first1=Ken |last2=Jones |first2=Michael C. |date=2019-12-01 |title=Why genome-wide associations with cognitive ability measures are probably spurious |url=https://www.sciencedirect.com/science/article/pii/S0732118X1830196X |journal=New Ideas in Psychology |volume=55 |pages=35–41 |doi=10.1016/j.newideapsych.2019.04.005 |issn=0732-118X}}</ref>

== Environmental effects ==
=== Shared environment ===
{{See also|Causes of intelligence#Sociocultural}}
In biometrical studies that partition variance, "shared environment" refers to hypothetical factors that increase the similarity of relatives in a particular study design.<ref name=":36">{{Cite journal |last1=Daw |first1=Jonathan |last2=Guo |first2=Guang |last3=Harris |first3=Kathie Mullan |date=July 2015 |title=Nurture net of nature: Re-evaluating the role of shared environments in academic achievement and verbal intelligence |journal=Social Science Research |volume=52 |pages=422–439 |doi=10.1016/j.ssresearch.2015.02.011 |issn=1096-0317 |pmc=4888873 |pmid=26004471}}</ref><ref>{{Cite book |last=Harold |first=Goldsmith |title=Nature, Nurture, and Psychology |date=1993 |publisher=American Psychological Association |isbn=978-1-55798-202-5 |editor-last=Plomin |editor-first=Robert |editor-link=Robert Plomin |chapter=Nature–nurture issues in the behavioral genetics context: Overcoming barriers to communication. |editor-last2=McClearn |editor-first2=Gerald E. |editor-link2=Gerald E. McClearn |chapter-url=https://www.researchgate.net/publication/232479062}}</ref>{{Reference page|page=331-333}} Researchers draw a distinction between "objectively shared environments" (features of the environment that relatives such as siblings share) and the "shared environment" in behavioral genetics studies, which measures only the factors that increase homogeneity.<ref name=":36" /><ref>{{Citation |title=Shared and Nonshared Environment |date=2018 |encyclopedia=The SAGE Encyclopedia of Lifespan Human Development |url=https://sk.sagepub.com/reference/the-sage-encyclopedia-of-lifespan-human-development/i22712.xml |access-date=2025-10-28 |place=2455 Teller Road, Thousand Oaks, California 91320 |publisher=SAGE Publications, Inc. |doi=10.4135/9781506307633.n741 |isbn=978-1-5063-0765-7}}</ref><ref name=":41">{{Cite journal |last1=Turkheimer |first1=E. |last2=Waldron |first2=M. |date=January 2000 |title=Nonshared environment: a theoretical, methodological, and quantitative review |journal=Psychological Bulletin |volume=126 |issue=1 |pages=78–108 |doi=10.1037/0033-2909.126.1.78 |issn=0033-2909 |pmid=10668351}}</ref>

Researchers typically use the same designs to estimate shared environment effects as to estimate heritability: twin studies, adoption studies and family studies.<ref name=":37">{{Cite journal |last=Kaplan |first=Jack S. |date=2012 |title=The effects of shared environment on adult intelligence: A critical review of adoption, twin, and MZA studies. |url=https://doi.apa.org/doi/10.1037/a0028133 |journal=Developmental Psychology |language=en |volume=48 |issue=5 |pages=1292–1298 |doi=10.1037/a0028133 |pmid=22746221 |issn=1939-0599}}</ref> These methods often come to disparate conclusions about the effects of shared environment.<ref name=":37" /><ref>{{Cite journal |last1=Plomin |first1=R. |author-link1=Robert Plomin |last2=Loehlin |first2=J. C. |author-link2=John C. Loehlin |date=May 1985 |title=Direct and indirect IQ heritability estimates: a puzzle |journal=Behavior Genetics |volume=19 |issue=3 |pages=331–342 |doi=10.1007/BF01066162 |issn=0001-8244 |pmid=2667514}}</ref>

Some adoption studies show that by adulthood adoptive siblings aren't more similar in IQ than strangers,<ref name="PMIDCitationhas">{{cite journal |pmid=9549239 |year=1998 |last1=Bouchard Jr |first1=TJ |title=Genetic and environmental influences on adult intelligence and special mental abilities |volume=70 |issue=2 |pages=257–79 |journal=Human Biology}}</ref> while adult full siblings show an IQ correlation of 0.24. These findings have lead researchers such as Sandra Scarr and Robert Plomin to conclude that shared environment has minimal effect on intelligence in adulthood.<ref name=":37" />{{Reference page|page=1292|quote=The evidence most often cited in support of the claim that shared environment has little or no effect on adult intelligence comes from adoption studies}}<ref>{{Cite book |last=Scarr |first=Sandra |title=Intelligence, heredity, and environment |date=1999 |publisher=Cambridge Univ. Press |isbn=978-0-521-46904-3 |editor-last=Sternberg |editor-first=Robert J. |edition=Repr |location=Cambridge |chapter=Behavior-Genetic and Socialization theories of intelligence}}</ref>{{Reference page|page=27|quote=In contrast to the MZA studies, four studies of genetically unrelated
siblings adopted and reared from infancy to adulthood in the same homes
show sibling IQ correlations that range from .05 to -.03, with a median of
about 0.0. 
... 
Although estimates of shared nongenetic effects on individual differences in IQ from twin and adoption studies vary from 0 to 10%, one can say that they appear
to be rather small in adult populations in Western societies}} Psychologists [[Ken Richardson (psychologist)|Ken Richardson]] and Sarah Norgate question the assumptions of adoption studies for IQ, highlighting issues with assumptions of additivity and randomization.<ref>{{Cite journal |last1=Richardson |first1=Ken |last2=Norgate |first2=Sarah H. |date=2006 |title=A Critical Analysis of IQ Studies of Adopted Children |journal=Human Development |volume=49 |issue=6 |pages=319–335 |doi=10.1159/000096531 |jstor=26763905 |issn=0018-716X}}</ref> However, adoption studies have also shown that the process of adoption has a positive effect on IQ scores,<ref name=":38" /><ref>{{Cite journal |last1=Duyme |first1=Michel |last2=Dumaret |first2=Annick-Camille |last3=Tomkiewicz |first3=Stanislaw |date=1999-07-20 |title=How can we boost IQs of "dull children"?: A late adoption study |journal=Proceedings of the National Academy of Sciences |volume=96 |issue=15 |pages=8790–8794 |doi=10.1073/pnas.96.15.8790 |doi-access=free |pmc=17595 |pmid=10411954 |bibcode=1999PNAS...96.8790D }}</ref> and this is associated with factors such as the level of parental education and [[socioeconomic status]] of adoptive parents.<ref>{{Cite journal |last1=Kendler |first1=Kenneth S. |last2=Turkheimer |first2=Eric |last3=Ohlsson |first3=Henrik |last4=Sundquist |first4=Jan |last5=Sundquist |first5=Kristina |date=2015-04-14 |title=Family environment and the malleability of cognitive ability: A Swedish national home-reared and adopted-away cosibling control study |journal=Proceedings of the National Academy of Sciences |volume=112 |issue=15 |pages=4612–4617 |doi=10.1073/pnas.1417106112 |doi-access=free |pmc=4403216 |pmid=25831538 |bibcode=2015PNAS..112.4612K }}</ref><ref>{{Cite journal |last1=Capron |first1=Christiane |last2=Duyme |first2=Michel |date=1996-05-01 |title=Effect of socioeconomic status of biological and adoptive parents on WISC-R subtest scores of their French adopted children |url=https://www.sciencedirect.com/science/article/pii/S0160289696900227 |journal=Intelligence |volume=22 |issue=3 |pages=259–275 |doi=10.1016/S0160-2896(96)90022-7 |issn=0160-2896}}</ref> In a 1991 paper, psychologist Eric Turkheimer tries to reconcile results from adoption studies that demonstrate large increases in IQ from adoption and those that show low correlations between non-biologically related relatives. He argued that early adoption studies first established that genetic effects influence behavioral phenotypes, while later studies from Schiff and colleagues demonstrate that environment has an influence.<ref>{{Cite journal |last=Turkheimer |first=Eric |date=November 1991 |title=Individual and group differences in adoption studies of IQ. |url=https://doi.apa.org/doi/10.1037/0033-2909.110.3.392 |journal=Psychological Bulletin |language=en |volume=110 |issue=3 |pages=392–405 |doi=10.1037/0033-2909.110.3.392 |issn=1939-1455}}</ref> Researchers Alexandra Burt and Wendy Johnson argue that the joint study of means and variance is necessary to understand the impact of environment.<ref>{{Cite journal |last1=Burt |first1=S. Alexandra |last2=Johnson |first2=Wendy |date=2023-03-01 |title=Joint Consideration of Means and Variances Might Change the Understanding of Etiology |journal=Perspectives on Psychological Science |language=EN |volume=18 |issue=2 |pages=416–427 |doi=10.1177/17456916221096122 |pmid=36027892 |issn=1745-6916}}</ref> In a 2024 study, Burt et al. propose that measurement error or noise might inhibit the identification of specific environmental factors that influence academic achievement.<ref>{{Cite journal |last1=Burt |first1=S. Alexandra |last2=O'Keefe |first2=Patrick |last3=Johnson |first3=Wendy |last4=Thaler |first4=Daniel |last5=Leve |first5=Leslie D. |last6=Natsuaki |first6=Misaki N. |last7=Reiss |first7=David |last8=Shaw |first8=Daniel S. |last9=Ganiban |first9=Jody M. |last10=Neiderhiser |first10=Jenae M. |date=2024-05-01 |title=The Detection of Environmental Influences on Academic Achievement Appears to Depend on the Analytic Approach |journal=Behavior Genetics |language=en |volume=54 |issue=3 |pages=252–267 |doi=10.1007/s10519-024-10179-w |pmid=38587720 |issn=1573-3297}}</ref>
[[File:Sundet et al data.png|thumb|Relationship between correlation between brothers' IQ scores and age differences in Sundet et al. 2008<ref name=":42" />]]
However, some studies of twins reared apart (e.g. Bouchard, 1990) find a significant shared environmental influence, of at least 10% going into late adulthood.<ref name="BibcodeSciBCitationhas">{{cite journal |last1=Bouchard |first1=Thomas J. |last2=Lykken |first2=David T. |last3=McGue |first3=Matthew |last4=Segal |first4=Nancy L. |last5=Tellegen |first5=Auke |year=1990 |title=Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart |journal=Science |volume=250 |issue=4978 |pages=223–8 |bibcode=1990Sci...250..223B |citeseerx=10.1.1.225.1769 |doi=10.1126/science.2218526 |pmid=2218526}}</ref> Jack Kaplan suggests that the evidence from twin studies and adoption studies are inconsistent, and that twin studies suggest a role for the effect of shared environment on intelligence in adulthood.<ref name=":37" /> Researchers like Daniel Briley and Eliot Tucker-Drob have suggested that shared environment may have minimal or no effect on the variation in cognition in adulthood.<ref>{{Cite journal |last1=Briley |first1=Daniel A. |last2=Tucker-Drob |first2=Elliot M. |date=February 2017 |title=Comparing the Developmental Genetics of Cognition and Personality over the Life Span |journal=Journal of Personality |volume=85 |issue=1 |pages=51–64 |doi=10.1111/jopy.12186 |issn=1467-6494 |pmc=4670606 |pmid=26045299}}</ref> One unpublished meta-analysis of twin studies finds estimates of shared environment around 20% for 15-20 year olds, but results for adults no greater than 15%.<ref>{{Citation |last1=Dickens |first1=William T. |title=The Heritability of Cognitive Ability Across the Lifespan: A Meta-Analysis of Twins Studies |date=2015-10-23 |type=SSRN Scholarly Paper |url=https://papers.ssrn.com/abstract=2679002 |access-date=2025-10-28 |place=Rochester, NY |language=en |id=2679002 |last2=Protzko |first2=John |ssrn=2679002 }}</ref>{{Better source needed|reason=The current source is insufficiently reliable ([[WP:NOTRS]]).|date=October 2025}} Researchers have proposed many different factors that could influence intelligence through the shared environment component. One 2018 paper by Laura Engelhardt and colleagues found that measures of socioeconomic status, school demographics and neighborhood socioeconomic status statistically account for most of shared environmental influences on several metrics of intelligence.<ref>{{Cite journal |last1=Engelhardt |first1=Laura E. |last2=Church |first2=Jessica A. |last3=Paige Harden |first3=K. |last4=Tucker-Drob |first4=Elliot M. |date=Aug 2018 |title=Accounting for the shared environment in cognitive abilities and academic achievement with measured socioecological contexts |journal=Developmental Science |volume=22 |issue=1 |article-number=e12699 |doi=10.1111/desc.12699 |issn=1467-7687 |pmc=6294670 |pmid=30113118}}</ref>

Another method researchers have used to estimate the shared environmental component for intelligence is studies of siblings. In a large study, Kendler and colleagues used twins, step-siblings, and half-siblings to estimate the shared environment component of many behavioral phenotypes. They found shared environment components for IQ ranging from about 10% to 20%, depending on the relationship class.<ref>{{Cite journal |last1=Kendler |first1=Kenneth S. |last2=Ohlsson |first2=Henrik |last3=Lichtenstein |first3=Paul |last4=Sundquist |first4=Jan |last5=Sundquist |first5=Kristina |date=2019-01-01 |title=The Nature of the Shared Environment |journal=Behavior Genetics |language=en |volume=49 |issue=1 |pages=1–10 |doi=10.1007/s10519-018-9940-0 |pmid=30536082 |issn=1573-3297}}</ref>{{Reference page|location=Fig 1.}} Another study found an association between similarity in age between siblings and similarity in IQ scores, suggesting familial environment influences on intelligence scores.<ref name=":42">{{Cite journal |last1=Sundet |first1=Jon Martin |last2=Eriksen |first2=Willy |last3=Tambs |first3=Kristian |date=September 2008 |title=Intelligence correlations between brothers decrease with increasing age difference: evidence for shared environmental effects in young adults |journal=Psychological Science |volume=19 |issue=9 |pages=843–847 |doi=10.1111/j.1467-9280.2008.02166.x |issn=1467-9280 |pmid=18947347}}</ref>

A 2015 study by Johnathan Daw and colleagues argues that the additivity assumption of typical family studies is not correct and that measures of objectively measured shared environment could moderate the estimated nonshared environment component. They found that shared environments such as birth order, household size, sibling age dispersion, and school quality exhibit "large moderating effects on both verbal intelligence and academic achievement". They suggested that these results show how objectively shared environments can influence intelligence contra conclusions that objectively shared environments do not influence children's lives.<ref name=":36" />  

=== Non-shared environment ===
As nonshared environmental variance is simply the leftover variance not accounted for by genetic variance and shared environmental variance, it encompasses any factors that cause differences between siblings and estimates vary based on the estimates of heritability and shared environment.<ref>{{Cite book |last1=Plomin |first1=Robert |author-link1=Robert Plomin |title=Separate Social Worlds of Siblings |last2=Chipuer |first2=Heather |last3=Neiderhiser |first3=Jenae M. |date=1994 |publisher=Routledge |isbn=978-1-138-98166-9 |editor-last=Hetherington |editor-first=E. Mavis |edition=1st |chapter=Behavioral genetic evidence for the importance of nonshared environment |editor-last2=Reiss |editor-first2=David |editor-last3=Plomin |editor-first3=Robert |editor-link3=Robert Plomin |chapter-url=https://d1wqtxts1xzle7.cloudfront.net/112662728/1994-hetherington-separatesocialworldsofsiblings-libre.pdf?1711580030=&response-content-disposition=inline%3B+filename%3DSeparate_Social_Worlds_of_Siblings.pdf&Expires=1761810205&Signature=U70p9rrCHgEh9BL9oD0gP1A57wLXJO9vf3QW2oAoHC6S9QmK4Z6546K5xT0dGXxVeVgCIBaxU98USSwL~FQjz0FfgvnJBnuB4hu2AgzvzMTqf2tUdQmp5HeM1TchmRMO4cS37WCi3ukmGL5RPg1n9kiwDanh8Nhu370s2Y-Xm7LyhUvsa5cd-a1U~kOxhxneT1j5j-s55UoCtnj2oy6G9X6iBZ8AqBlL8DJokZujDXvndMSdf7UkDVyJ4Tv3u2hMEmMkCHs2Ww4oNnDXe-WEJL5TwJYFZRgDaGbP8DYzrftLq11ALaRpjABjlhVdJ9EIbHjDmbScEB~x1BWj7i1eDA__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA#page=9}}</ref>{{Reference page|page=22}} Robert MCall argues nonshared environment contributes 15-25% of the variance of IQ scores.<ref>{{Cite journal |last=McCall |first=Robert B. |date=1983 |title=Environmental Effects on Intelligence: The Forgotten Realm of Discontinuous Nonshared Within-Family Factors |journal=Child Development |volume=54 |issue=2 |pages=408–415 |doi=10.2307/1129701 |jstor=1129701 |pmid=6872632 |issn=0009-3920}}</ref> Although parents treat their children differently, known measures of differential treatment explains only a small amount of non-shared environmental influence.<ref name=":41" /><ref name="Plomin0103">{{cite journal |last1=Plomin |first1=R |last2=Asbury |first2=K |last3=Dunn |first3=J |year=2001 |title=Why are children in the same family so different? Nonshared environment a decade later |journal=Canadian Journal of Psychiatry |volume=46 |issue=3 |pages=225–33 |doi=10.1177/070674370104600302 |pmid=11320676 |doi-access=free}}</ref> One suggestion is that children react differently to the same environmental factors: that objectively shared measures of the environment can have different effects on siblings.<ref name=":41" />{{Reference page|page=79|quote=Thus, if an objectively shared environmental variable results in nonshared effects, the effective contribution of the objectively shared event is included with the nonshared rather then the shared component of variance. Consider, for example, parental divorce. Like most other traditional between-family environmental variables, parental divorce is usually an objectively shared event. It is possible, however, that objectively shared events may have different effects on siblings.}} One 1994 study of the National Longitudinal Survey of Youth found that a measure of the home environment accounted for nonshared environmental variance for several measures of intelligence.<ref>{{Cite journal |last1=Rodgers |first1=Joseph Lee |last2=Rowe |first2=David C. |last3=May |first3=Kim |date=1994-09-01 |title=DF analysis of NLSY IQ/achievement data: Nonshared environmental influences |journal=Intelligence |volume=19 |issue=2 |pages=157–177 |doi=10.1016/0160-2896(94)90011-6 |issn=0160-2896}}</ref> Daw et al. suggest that nonshared environment is moderated by objectively shared environment.<ref name=":36" /> More likely influences may be the impact of peers and other experiences outside the family.<ref name="unknown" /> For example, siblings grown up in the same household may have different friends and teachers and even contract different illnesses. This factor may be one of the reasons why IQ score correlations between siblings decreases as they get older.<ref>{{cite book |last1=Schacter |first1=Daniel |first2=Daniel |last2=Gilbert |first3=Daniel |last3=Wegner |title=Psychology |url=https://archive.org/details/psychology0000scha |url-access=registration |edition=2nd |location=New York |publisher=Worth Publishers |year=2010 |page=[https://archive.org/details/psychology0000scha/page/408 408] |isbn=978-1-4292-3719-2}}</ref>

Some authors highlight measurement error as one source of variance attributed to the nonshared environment, as measurement error is captured in the nonshared environment term.<ref>{{Cite journal |last=Plomin |first=Robert |date=September 2024 |title=Nonshared environment: Real but random |journal=JCPP Advances |volume=4 |issue=3 |article-number=e12229 |doi=10.1002/jcv2.12229 |issn=2692-9384 |pmc=11472802 |pmid=39411468}}</ref><ref>{{Cite journal |last1=Chen |first1=Gang |last2=Moraczewski |first2=Dustin |last3=Taylor |first3=Paul A. |date=2025-04-02 |title=Improving accuracy and precision of heritability estimation in twin studies through hierarchical modeling: reassessing the measurement error assumption |journal=Frontiers in Genetics |language=English |volume=16 |article-number=1522729 |doi=10.3389/fgene.2025.1522729 |pmid=40242473 |pmc=11999965 |doi-access=free |issn=1664-8021}}</ref> Another factor that produces differences between individuals is stochastic variation, often attributed to nonlinear epigenetic processes.<ref>{{Cite journal |last1=Molenaar |first1=Peter C. M. |last2=Boomsma |first2=Dorret I. |last3=Dolan |first3=Conor V. |date=1993-11-01 |title=A third source of developmental differences |journal=Behavior Genetics |language=en |volume=23 |issue=6 |pages=519–524 |doi=10.1007/BF01068142 |pmid=8129693 |issn=1573-3297}}</ref><ref>{{Citation |last=Vogt |first=Günter |title=9 - Disentangling the environmentally induced and stochastic developmental components of phenotypic variation |date=2020-01-01 |work=Phenotypic Switching |pages=207–251 |editor-last=Levine |editor-first=Herbert |url=https://www.sciencedirect.com/science/article/pii/B9780128179963000104 |access-date=2025-10-30 |publisher=Academic Press |isbn=978-0-12-817996-3 |editor2-last=Jolly |editor2-first=Mohit Kumar |editor3-last=Kulkarni |editor3-first=Prakash |editor4-last=Nanjundiah |editor4-first=Vidyanand}}</ref>
=== Dickens and Flynn model ===

Dickens and Flynn (2001) argued that the "heritability" figure includes both a direct effect of the [[genotype]] on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but [[feedback]] loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues.<ref name="DickensFlynn2001">{{cite journal |last1=Dickens |first1=William T. |last2=Flynn |first2=James R. |year=2001 |title=Heritability estimates versus large environmental effects: The IQ paradox resolved |journal=Psychological Review |volume=108 |issue=2 |pages=346–69 |citeseerx=10.1.1.139.2436 |doi=10.1037/0033-295X.108.2.346 |pmid=11381833}}</ref> This model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects.

The [[Flynn effect]] is the increase in average intelligence test scores by about 0.3% annually, resulting in the average person today scoring 15 points higher in IQ compared to the generation 50 years ago.<ref name="Schacter409">{{cite book |last1=Schacter |first1=Daniel |url=https://archive.org/details/psychology0000scha |title=Psychology |last2=Gilbert |first2=Daniel |last3=Wegner |first3=Daniel |publisher=Worth Publishers |year=2010 |isbn=978-1-4292-3719-2 |edition=2nd |location=New York |pages=[https://archive.org/details/psychology0000scha/page/409 409–10] |url-access=registration}}</ref> This effect can be explained by a generally more stimulating environment for all people.
Some scientists have suggested that such enhancements are due to better nutrition, better parenting and schooling, as well as exclusion of the least intelligent people from reproduction. However, Flynn and a group of other scientists share the viewpoint that modern life implies solving many abstract problems which leads to a rise in their IQ scores.<ref name="Schacter409" />

=== Indirect genetic effects ===

Various molecular genetic studies have identified an effect of alleles not carried by an individual, but some relative, on their phenotype.<ref>{{Cite journal |last=Bijma |first=P. |date=January 2014 |title=The quantitative genetics of indirect genetic effects: a selective review of modelling issues |journal=Heredity |language=en |volume=112 |issue=1 |pages=61–69 |doi=10.1038/hdy.2013.15 |pmid=23512010 |pmc=3860160 |bibcode=2014Hered.112...61B |issn=1365-2540}}</ref> These have been varyingly referred to as "dynastic effects",<ref>{{Cite journal |last1=Brumpton |first1=Ben |last2=Sanderson |first2=Eleanor |last3=Heilbron |first3=Karl |last4=Hartwig |first4=Fernando Pires |last5=Harrison |first5=Sean |last6=Vie |first6=Gunnhild Åberge |last7=Cho |first7=Yoonsu |last8=Howe |first8=Laura D. |last9=Hughes |first9=Amanda |last10=Boomsma |first10=Dorret I. |last11=Havdahl |first11=Alexandra |last12=Hopper |first12=John |last13=Neale |first13=Michael |last14=Nivard |first14=Michel G. |last15=Pedersen |first15=Nancy L. |date=2020-07-14 |title=Avoiding dynastic, assortative mating, and population stratification biases in Mendelian randomization through within-family analyses |url=https://www.nature.com/articles/s41467-020-17117-4 |journal=Nature Communications |language=en |volume=11 |issue=1 |page=3519 |doi=10.1038/s41467-020-17117-4 |pmid=32665587 |bibcode=2020NatCo..11.3519B |issn=2041-1723}}</ref> "non-transmitted coefficients",<ref>{{Cite journal |last1=Young |first1=Alexander I. |last2=Nehzati |first2=Seyed Moeen |last3=Benonisdottir |first3=Stefania |last4=Okbay |first4=Aysu |last5=Jayashankar |first5=Hariharan |last6=Lee |first6=Chanwook |last7=Cesarini |first7=David |last8=Benjamin |first8=Daniel J. |last9=Turley |first9=Patrick |last10=Kong |first10=Augustine |date=June 2022 |title=Mendelian imputation of parental genotypes improves estimates of direct genetic effects |journal=Nature Genetics |language=en |volume=54 |issue=6 |pages=897–905 |doi=10.1038/s41588-022-01085-0 |pmid=35681053 |pmc=9197765 |issn=1546-1718}}</ref> "interpersonal genetic effects",<ref>{{Cite report |url=https://www.nber.org/papers/w32404 |title=Social-Science Genomics: Progress, Challenges, and Future Directions |last1=Benjamin |first1=Daniel J. |last2=Cesarini |first2=David |last3=Turley |first3=Patrick |last4=Young |first4=Alexander Strudwick |date=2024-05-06 |publisher=National Bureau of Economic Research |issue=w32404 |language=en}}</ref> "genetic nurture",<ref>{{Cite journal |last1=Wang |first1=Biyao |last2=Baldwin |first2=Jessie R. |last3=Schoeler |first3=Tabea |last4=Cheesman |first4=Rosa |last5=Barkhuizen |first5=Wikus |last6=Dudbridge |first6=Frank |last7=Bann |first7=David |last8=Morris |first8=Tim T. |last9=Pingault |first9=Jean-Baptiste |date=2021-09-02 |title=Robust genetic nurture effects on education: A systematic review and meta-analysis based on 38,654 families across 8 cohorts |journal=American Journal of Human Genetics |volume=108 |issue=9 |pages=1780–1791 |doi=10.1016/j.ajhg.2021.07.010 |issn=1537-6605 |pmc=8456157 |pmid=34416156}}</ref> or more broadly as "indirect genetic effects".<ref>{{Cite journal |last1=Nivard |first1=Michel G. |last2=Belsky |first2=Daniel W. |last3=Harden |first3=K. Paige |last4=Baier |first4=Tina |last5=Andreassen |first5=Ole A. |last6=Ystrøm |first6=Eivind |last7=van Bergen |first7=Elsje |last8=Lyngstad |first8=Torkild H. |date=April 2024 |title=More than nature and nurture, indirect genetic effects on children's academic achievement are consequences of dynastic social processes |journal=Nature Human Behaviour |language=en |volume=8 |issue=4 |pages=771–778 |doi=10.1038/s41562-023-01796-2 |pmid=38225408 |pmc=11569812 |issn=2397-3374}}</ref>

Kong<ref>{{cite journal |last1=Kong |first1=Augustine |last2=Thorleifsson |first2=Gudmar |last3=Frigge |first3=Michael L. |last4=Vilhjalmsson |first4=Bjarni J. |last5=Young |first5=Alexander I. |last6=Thorgeirsson |first6=Thorgeir E. |last7=Benonisdottir |first7=Stefania |last8=Oddsson |first8=Asmundur |last9=Halldorsson |first9=Bjarni V. |last10=Masson |first10=Gisli |last11=Gudbjartsson |first11=Daniel F. |last12=Helgason |first12=Agnar |last13=Bjornsdottir |first13=Gyda |last14=Thorsteinsdottir |first14=Unnur |last15=Stefansson |first15=Kari |date=25 January 2018 |title=The nature of nurture: Effects of parental genotypes |journal=Science |volume=359 |issue=6374 |pages=424–428 |bibcode=2018Sci...359..424K |doi=10.1126/science.aan6877 |pmid=29371463 |doi-access=free}}</ref> reports that, "Nurture has a genetic component, i.e. alleles in the parents affect the parents' phenotypes and through that influence the outcomes of the child." These results were obtained through a meta-analysis of [[educational attainment]] and [[Polygene|polygenic]] scores of non-transmitted alleles. Although the study deals with educational attainment and not IQ, these two are strongly linked.<ref>{{cite journal |last1=Deary |first1=Ian J. |last2=Strand |first2=Steve |last3=Smith |first3=Pauline |last4=Fernandes |first4=Cres |date=January 2007 |title=Intelligence and educational achievement |journal=Intelligence |volume=35 |issue=1 |pages=13–21 |doi=10.1016/j.intell.2006.02.001}}</ref>

== Influences ==

=== Heritability and socioeconomic status ===
{{See also|Scarr–Rowe effect|Causes of intelligence#Family}}
Hypotheses about the relationship between the value of heritability and socioeconomic status originated with [[Sandra Scarr]]'s 1971 article Race, Social Class, and IQ.<ref name=":35">{{Cite journal |last=Scarr-Salapatek |first=Sandra |date=1971-12-24 |title=Race, Social Class, and IQ |url=https://www.science.org/doi/10.1126/science.174.4016.1285 |journal=Science |volume=174 |issue=4016 |pages=1285–1295 |doi=10.1126/science.174.4016.1285 |pmid=5167501 |bibcode=1971Sci...174.1285S }}</ref> Scarr notes that existing theories of environmental disadvantage explaining the relationship between social class and IQ predict that more advantaged groups will show greater genetic variance as the advantaged groups don't experience as many detrimental environmental effects. her study, she identified statistical interactions between socioeconomic environment and genetic variance, finding that genetic influences on IQ were stronger in higher socioeconomic groups where environmental conditions allowed genetic potential to be more fully expressed, whereas in lower socioeconomic groups, adverse environments constrained this expression and reduced the impact of genetic differences.<ref name=":1" /><ref name=":35" /> Other than some minor criticisms and a partial replication, the topic was not broached again until the 1996 APA report "Intelligence: Knowns and Unknowns",<ref name=":1" /> which stated that: <blockquote>
"We should note, however, that [[Poverty|low-income]] and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence."<ref name="unknown"/>
</blockquote>
[[File:Relationship between SES and heritability of IQ.png|thumb|Relationship between the genetic variance component and socioeconomic status in Turkheimer et al. 2003<ref name="Turkheimer2003" />]]
A 1999 study by [[David C. Rowe|David Rowe]] and colleagues replicated the effect in the [[National Longitudinal Study of Adolescent Health]], finding that among children from poorly-educated parents 'shared environment' was associated with most of the variance, while among children from well-educated parents, genes were associated with most of the variance.<ref name=":1" /><ref>{{Cite journal |last1=Rowe |first1=D. C. |last2=Jacobson |first2=K. C. |last3=Van den Oord |first3=E. J. |date=1999 |title=Genetic and environmental influences on vocabulary IQ: parental education level as moderator |journal=Child Development |volume=70 |issue=5 |pages=1151–1162 |doi=10.1111/1467-8624.00084 |issn=0009-3920 |pmid=10546338}}</ref> Turkheimer and colleagues (2003) argued that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in a study on seven-year-old twins, in impoverished families, 60% of the variance in early childhood IQ was accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.<ref name="Turkheimer2003">{{cite journal |last1=Turkheimer |first1=Eric |author-link=Eric Turkheimer |last2=Haley |first2=Andreana |last3=Waldron |first3=Mary |last4=d'Onofrio |first4=Brian |last5=Gottesman |first5=Irving I. |year=2003 |title=Socioeconomic status modifies heritability of iq in young children |journal=Psychological Science |volume=14 |issue=6 |pages=623–8 |doi=10.1046/j.0956-7976.2003.psci_1475.x |pmid=14629696 |s2cid=11265284}}</ref> The name "Scarr-Rowe effect" was proposed for this proposed interaction between heritability and socioeconomic status by [[Eric Turkheimer]] in 2009 paper presentation.<ref>{{Cite journal |last1=Giangrande |first1=Evan J. |last2=Turkheimer |first2=Eric |date=May 2022 |title=Race, Ethnicity, and the Scarr-Rowe Hypothesis: A Cautionary Example of Fringe Science Entering the Mainstream |url=https://osf.io/preprints/psyarxiv/md3qw_v1 |journal=Perspectives on Psychological Science: A Journal of the Association for Psychological Science |volume=17 |issue=3 |pages=696–710 |doi=10.1177/17456916211017498 |issn=1745-6924 |pmid=34793248}}</ref> Later studies found more equivocal results, some failing to replicate the finding in different locations. A 2016 meta-analysis found that the effects varied by country, with moderate interactions in studies from the United States, while the effects were zero or reversed in studies from Australia and Western Europe.<ref>{{Cite journal |last1=Tucker-Drob |first1=Elliot M. |last2=Bates |first2=Timothy C. |date=Feb 2016 |title=Large Cross-National Differences in Gene × Socioeconomic Status Interaction on Intelligence |journal=Psychological Science |volume=27 |issue=2 |pages=138–149 |doi=10.1177/0956797615612727 |issn=1467-9280 |pmc=4749462 |pmid=26671911}}</ref> A large study published the following year, however, found negligible impacts using data from twins in Florida.<ref>{{Cite journal |last1=Figlio |first1=David N. |last2=Freese |first2=Jeremy |last3=Karbownik |first3=Krzysztof |last4=Roth |first4=Jeffrey |date=2017-12-19 |title=Socioeconomic status and genetic influences on cognitive development |journal=Proceedings of the National Academy of Sciences |volume=114 |issue=51 |pages=13441–13446 |doi=10.1073/pnas.1708491114 |doi-access=free |pmc=5754768 |pmid=29133413 |bibcode=2017PNAS..11413441F }}</ref><ref>{{Cite journal |last1=Mills |first1=Melinda C. |last2=Tropf |first2=Felix C. |date=2020-07-30 |title=Sociology, Genetics, and the Coming of Age of Sociogenomics |url=https://www.annualreviews.org/content/journals/10.1146/annurev-soc-121919-054756 |journal=Annual Review of Sociology |language=en |volume=46 |issue=46 |pages=553–581 |doi=10.1146/annurev-soc-121919-054756 |issn=0360-0572}}</ref>

Recent research has focused on the relationship between socioeconomic status and heritability using genomic data. One 2020 study on educational attainment, a closely related but not identical [[phenotype]],<ref>{{Cite journal |last1=Krapohl |first1=Eva |last2=Rimfeld |first2=Kaili |last3=Shakeshaft |first3=Nicholas G. |last4=Trzaskowski |first4=Maciej |last5=McMillan |first5=Andrew |last6=Pingault |first6=Jean-Baptiste |last7=Asbury |first7=Kathryn |last8=Harlaar |first8=Nicole |last9=Kovas |first9=Yulia |last10=Dale |first10=Philip S. |last11=Plomin |first11=Robert |date=2014-10-21 |title=The high heritability of educational achievement reflects many genetically influenced traits, not just intelligence |journal=Proceedings of the National Academy of Sciences |volume=111 |issue=42 |pages=15273–15278 |doi=10.1073/pnas.1408777111 |doi-access=free |pmc=4210287 |pmid=25288728 |bibcode=2014PNAS..11115273K }}</ref> found that polygenic scores had greater predictive value in lower socioeconomic status quartiles.<ref>{{Cite journal |last1=Mostafavi |first1=Hakhamanesh |last2=Harpak |first2=Arbel |last3=Agarwal |first3=Ipsita |last4=Conley |first4=Dalton |last5=Pritchard |first5=Jonathan K |last6=Przeworski |first6=Molly |date=2020-01-30 |title=Variable prediction accuracy of polygenic scores within an ancestry group |journal=eLife |language=en |volume=9 |article-number=e48376 |doi=10.7554/eLife.48376 |pmid=31999256 |pmc=7067566 |doi-access=free |issn=2050-084X}}</ref> A 2021 study similarly found an increase in SNP heritability with increasing socioeconomic deprivation; lower socioeconomic status was associated with higher heritability.<ref>{{Cite journal |last1=Rask-Andersen |first1=Mathias |last2=Karlsson |first2=Torgny |last3=Ek |first3=Weronica E. |last4=Johansson |first4=Åsa |date=July 2021 |title=Modification of Heritability for Educational Attainment and Fluid Intelligence by Socioeconomic Deprivation in the UK Biobank |url=https://psychiatryonline.org/doi/10.1176/appi.ajp.2020.20040462 |journal=American Journal of Psychiatry |volume=178 |issue=7 |pages=625–634 |doi=10.1176/appi.ajp.2020.20040462 |pmid=33900812 |issn=0002-953X}}</ref> One 2024 study used polygenic indices and found that the Scarr-Rowe hypothesis "lacks empirical support" and suggested an alternative theory, the compensatory advantage hypothesis, that heritability was ''lower'' among populations with higher heritability.<ref>{{Cite journal |last1=Ghirardi |first1=Gaia |last2=Gil-Hernández |first2=Carlos J. |last3=Bernardi |first3=Fabrizio |last4=van Bergen |first4=Elsje |last5=Demange |first5=Perline |date=2024-08-01 |title=Interaction of family SES with children's genetic propensity for cognitive and noncognitive skills: No evidence of the Scarr-Rowe hypothesis for educational outcomes |journal=Research in Social Stratification and Mobility |volume=92 |article-number=100960 |doi=10.1016/j.rssm.2024.100960 |pmid=39220821 |pmc=11364161 |issn=0276-5624}}</ref> 

=== Heritability and age ===
[[File:Twin correlations over development.png|thumb|Twin correlations over development (data from Wilson 1983<ref>{{Cite journal |last=Wilson |first=Ronald S. |date=1983 |title=The Louisville Twin Study: Developmental Synchronies in Behavior |url=https://www.jstor.org/stable/1129693 |journal=Child Development |volume=54 |issue=2 |pages=298–316 |doi=10.2307/1129693 |issn=0009-3920}}</ref>)]]
The interaction between heritability and age over the course of development has been the interest of scientists for many years. Perhaps first described by Ronald Wilson,<ref name=":29">{{Cite journal |last=Bouchard |first=Thomas J. |date=October 2013 |title=The Wilson Effect: the increase in heritability of IQ with age |journal=Twin Research and Human Genetics: The Official Journal of the International Society for Twin Studies |volume=16 |issue=5 |pages=923–930 |doi=10.1017/thg.2013.54 |issn=1832-4274 |pmid=23919982}}</ref> researchers have noted changes in kinship correlations over the life course. Particularly in the case of twins, the correlation between monozygotic twins remains relatively constant, while the correlation between dizygotic twins decreases.<ref name=":29" />{{Reference page|pages=923}} Some early attempts at modeling this change have equivocal results, with Rao et al. finding that heritability increased in adulthood in one model, but not in another,<ref name=":30" /> while a later study by Devlin et al. did not find support for age moderation of heritability.<ref name="devlin" /> Most researchers in behavioral genetics now believe that estimates of heritability from classical family studies (adoption, twin studies) increase as individuals age,<ref name=":4" />{{Reference page|pages=179-183}}<ref>{{Cite journal |last1=Haworth |first1=C. M. A. |last2=Wright |first2=M. J. |last3=Luciano |first3=M. |last4=Martin |first4=N. G. |last5=de Geus |first5=E. J. C. |last6=van Beijsterveldt |first6=C. E. M. |last7=Bartels |first7=M. |last8=Posthuma |first8=D. |last9=Boomsma |first9=D. I. |last10=Davis |first10=O. S. P. |last11=Kovas |first11=Y. |last12=Corley |first12=R. P. |last13=Defries |first13=J. C. |last14=Hewitt |first14=J. K. |last15=Olson |first15=R. K. |date=2 June 2009 |title=The heritability of general cognitive ability increases linearly from childhood to young adulthood |journal=Molecular Psychiatry |volume=15 |issue=11 |pages=1112–1120 |doi=10.1038/mp.2009.55 |issn=1476-5578 |pmc=2889158 |pmid=19488046}}</ref>. One review described a "linearly increasing heritability of intelligence from infancy (20%) through adulthood (60%)".<ref name=":31">{{Cite journal |last1=Plomin |first1=Robert |last2=DeFries |first2=John C. |last3=Knopik |first3=Valerie S. |last4=Neiderhiser |first4=Jenae M. |date=2016-01-01 |title=Top 10 Replicated Findings From Behavioral Genetics |journal=Perspectives on Psychological Science |language=EN |volume=11 |issue=1 |pages=3–23 |doi=10.1177/1745691615617439 |issn=1745-6916 |pmc=4739500 |pmid=26817721}}</ref><ref name=":39">{{Cite journal |last1=Reis |first1=André |last2=Spinath |first2=Frank M. |date=2025-01-24 |title=The Genetics of Intelligence |journal=Deutsches Arzteblatt International |volume=122 |issue=2 |pages=38–42 |doi=10.3238/arztebl.m2024.0236 |issn=1866-0452 |pmc=12416016 |pmid=39635948}}</ref> There are two possible explanations for why measured heritability might increase over the course of the lifespan: that new genetic factors are activated (genetic innovation) or that previous genetic factors explain more variance (genetic amplification).<ref name=":31" /><ref name=":32" /> Behavioral genetics researchers believe that genetic amplification is most supported by the evidence,<ref>{{Cite journal |last1=Plomin |first1=R. |last2=Deary |first2=I. J. |date=February 2015 |title=Genetics and intelligence differences: five special findings |journal=Molecular Psychiatry |language=en |volume=20 |issue=1 |pages=98–108 |doi=10.1038/mp.2014.105 |pmid=25224258 |pmc=4270739 |issn=1476-5578}}</ref> where the same genetic factors that influence IQ at one age are involved later, but produce larger and larger phenotypic effects over time.<ref name=":4" />{{Reference page|page=182}}<ref name=":32">{{Cite journal |last1=Briley |first1=Daniel A. |last2=Tucker-Drob |first2=Elliot M. |date=1 July 2013 |title=Explaining the increasing heritability of cognitive ability across development: a meta-analysis of longitudinal twin and adoption studies |journal=Psychological Science |volume=24 |issue=9 |pages=1704–1713 |doi=10.1177/0956797613478618 |issn=1467-9280 |pmc=3954471 |pmid=23818655}}</ref> One theory proposed to explain the apparent increase in heritability values is a reciprocal effects model, where environments and genes become correlated over the course of development.<ref name="DickensFlynn2001" /><ref name=":39" /><ref>{{Cite book |title=The dynamic genome and mental health: The role of genes and environments in youth development |publisher=Oxford University Press |year=2011 |isbn=978-0-19-973796-3 |editor-last=Kendler |editor-first=Kenneth S. |location=USA |pages=103–127 |language=en-US |chapter=The Social Dynamics of the Expression of Genes for Cognitive Ability |editor-last2=Jaffree |editor-first2=Sara |editor-last3=Romer |editor-first3=Daniel |chapter-url=https://www.researchgate.net/publication/292759039}}</ref> These models incorporate phenotype to environment associations that violate the independence assumption in traditional behavioral genetic models that may overestimate heritability.<ref>{{Cite journal |last1=Beam |first1=Christopher R. |last2=Pezzoli |first2=Patrizia |last3=Mendle |first3=Jane |last4=Burt |first4=S. Alexandra |last5=Neale |first5=Michael C. |last6=Boker |first6=Steven M. |last7=Keel |first7=Pamela K. |last8=Klump |first8=Kelly L. |date=Feb 2022 |title=How nonshared environmental factors come to correlate with heredity |journal=Development and Psychopathology |volume=34 |issue=1 |pages=321–333 |doi=10.1017/S0954579420001017 |issn=1469-2198 |pmc=8081739 |pmid=33118912}}</ref><ref>{{Cite journal |last1=Beam |first1=Christopher R. |last2=Turkheimer |first2=Eric |date=Feb 2013 |title=Phenotype-environment correlations in longitudinal twin models |journal=Development and Psychopathology |volume=25 |issue=1 |pages=7–16 |doi=10.1017/S0954579412000867 |issn=1469-2198 |pmc=5746192 |pmid=23398749}}</ref> Another proposed explanation for why heritability can increase over the course of development is gene-environment interactions.<ref>{{Cite journal |last=Fischbein |first=Siv |date=1978-12-01 |title=Heredity-Environment Interaction in the Development of Twins |journal=International Journal of Behavioral Development |language=EN |volume=1 |issue=4 |pages=313–322 |doi=10.1177/016502547800100402 |issn=0165-0254}}</ref>

Recent molecular genetics research has also investigated the interaction between genetics or heritability and age over the course of development. One 2018 genome-wide association study found "comparable heritability across age".<ref name=":33" /> Another study found that the within-family estimate heritability was 0.12 in childhood, 0.13 in adolescence and 0.15 in adulthood.<ref>{{Cite journal |last1=Lin |first1=Yujing |last2=Procopio |first2=Francesca |last3=Keser |first3=Engin |last4=Kawakami |first4=Kaito |last5=Rimfeld |first5=Kaili |last6=Malanchini |first6=Margherita |last7=Plomin |first7=Robert |date=2025-06-23 |title=Polygenic Score Prediction Within and Between Sibling Pairs for Intelligence, Cognitive Abilities, and Educational Traits From Childhood to Early Adulthood |url=https://icajournal.scholasticahq.com/article/140654-polygenic-score-prediction-within-and-between-sibling-pairs-for-intelligence-cognitive-abilities-and-educational-traits-from-childhood-to-early-adul |journal=Intelligence & Cognitive Abilities |language=en |volume=1 |issue=1 |pages=5–19}}</ref> 

== Implications ==

=== Development ===
[[File:When_causes_interact.png|thumb|339x339px]]
Some researchers, especially those that work in fields like [[developmental systems theory]], have criticized the concept of heritability as misleading or meaningless. [[Douglas Wahlsten]] and [[Gilbert Gottlieb]] argue that the prevailing models of behavioral genetics are too simplistic by not accounting for gene-environment interactions.<ref>{{Cite book |last1=Wahlsten |first1=Douglas |title=Intelligence, heredity, and environment |last2=Gottlieb |first2=Gilbert |date=1999 |publisher=Cambridge Univ. Press |isbn=978-0-521-46904-3 |editor-last=Sternberg |editor-first=Robert J. |edition=Repr |location=Cambridge |chapter=The invalid separation of effects of nature and nurture: Lessons from animal experimentation}}</ref> Stephen Ceci also highlights the issues with this assumption, noting that they were first raised by Jane Loevinger in 1943.<ref name=":19" />{{Reference page|pages=139-141}}<ref>{{Cite journal |last=Loevinger |first=J. |date=December 1943 |title=On the proportional contributions of differences in nature and in nurture to differences in intelligence. |url=https://doi.apa.org/doi/10.1037/h0053767 |journal=Psychological Bulletin |language=en |volume=40 |issue=10 |pages=725–756 |doi=10.1037/h0053767 |issn=1939-1455}}</ref> They assert that the idea of partitioning variance makes no sense when environments and genes interact and argue that such interaction is ubiquitous in human development.<ref name=":18">{{Cite journal |last1=Moore |first1=David S. |last2=Shenk |first2=David |date=January 2017 |title=The heritability fallacy |url=https://pzacad.pitzer.edu/~dmoore/publications/2016_moore--shenk_the-herit.pdf |journal=WIREs Cognitive Science |language=en |volume=8 |issue=1–2 |article-number=e1400 |doi=10.1002/wcs.1400 |pmid=27906501 |issn=1939-5078}}</ref><ref>{{Cite book |last=Tabery |first=James |url=https://mitpress.mit.edu/9780262549608/beyond-versus/ |title=Beyond Versus: The Struggle to Understand the Interaction of Nature and Nurture |date=31 October 2023 |publisher=The MIT Press |isbn=978-0-262-54960-8 |language=en-US}}</ref> They highlight their belief that heritability analysis requires a hidden assumption they call the "separation of causes", which isn't borne out by biological reality or experimental research.<ref>{{Cite journal |last=Wahlsten |first=Douglas |date=July 1994 |title=The intelligence of heritability. |url=https://doi.apa.org/doi/10.1037/0708-5591.35.3.244 |journal=Canadian Psychology / Psychologie Canadienne |language=en |volume=35 |issue=3 |pages=244–260 |doi=10.1037/0708-5591.35.3.244 |issn=1878-7304}}</ref> Such researchers argue that the notion of heritability gives the false impression that "genes have some direct and isolated influence on traits", rather than another developmental resource that a complex system uses over the course of ontogeny.<ref name=":18" />

=== Between-group heritability ===
{{Main|Race and intelligence}}

Although IQ differences between individuals have been shown to have a hereditary component, it does not follow that differences in average IQ test performance between [[Race (human categorization)|racial]] and [[Ethnicity|ethnic]] groups have a genetic basis.<ref name=":9" /><ref name=":1"/><ref name=":02"/> The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.<ref name=":2">{{Cite journal |last1=Ceci |first1=Stephen |last2=Williams |first2=Wendy M. |date=1 February 2009 |title=Should scientists study race and IQ? YES: The scientific truth must be pursued |journal=Nature |volume=457 |issue=7231 |pages=788–789 |bibcode=2009Natur.457..788C |doi=10.1038/457788a |pmid=19212385 |s2cid=205044224 |quote=There is an emerging consensus about racial and gender equality in genetic determinants of intelligence; most researchers, including ourselves, agree that genes do not explain between-group differences. |doi-access=free}}</ref><ref name=":6">{{Cite book |last=Hunt |first=Earl |url=https://archive.org/details/humanintelligenc0000hunt_s6m1/page/447/mode/1up?view=theater |title=Human Intelligence |publisher=Cambridge University Press |year=2010 |isbn=978-0-521-70781-7 |page=447 |ol=24384631M |quote=[N]o genes related to the difference in cognitive skills across the various racial and ethic groups have ever been discovered. |via=[[Open Library]] |ol-access=free}}</ref><ref name=":7">{{Cite book |last=Mackintosh, N. J. |url=https://archive.org/details/iqhumanintellige0000mack/mode/2up?view=theater |title=IQ and human intelligence |date=2011 |publisher=Oxford University Press |isbn=978-0-19-958559-5 |edition=2nd |location=Oxford |pages=334–338, 344 |oclc=669754008 |ol=25211083M |via=[[Open Library]] |ol-access=free}}</ref><ref name=":02"/><ref name=":5">{{Cite journal |last=Kaplan |first=Jonathan Michael |date=January 2015 |title=Race, IQ, and the search for statistical signals associated with so-called "X"-factors: environments, racism, and the "hereditarian hypothesis" |journal=Biology & Philosophy |language=en |volume=30 |issue=1 |pages=1–17 |doi=10.1007/s10539-014-9428-0 |issn=0169-3867 |s2cid=85351431}}</ref> Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.<ref name=":02"/><ref name=":5" />{{sfn|Nevid|2014|page=271}}

Arguments in support of a genetic explanation of racial differences in average IQ are sometimes fallacious. For instance, some hereditarians have cited as evidence the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races.<ref name=":9">{{Cite journal|last=Mackenzie|first=Brian|date=1984|title=Explaining race differences in IQ: The logic, the methodology, and the evidence|url=https://content.apa.org/record/1985-25462-001|journal=American Psychologist|volume=39|issue=11|pages=1214–1233|doi=10.1037/0003-066X.39.11.1214|via=|access-date=2021-01-29|archive-date=2021-02-02|archive-url=https://web.archive.org/web/20210202100805/https://content.apa.org/record/1985-25462-001|url-status=live}}</ref> In 1972, John DeFries attempted to connect "between group heritability" or ''h{{Sup|2}}''{{Sub|''B''}}  to "within group heritability" or ''h{{Sup|2}}''{{Sub|''W''}} in an equation <math>h^2_B\approx h^2_W \frac{(1-t)r}{(1-r)t}</math>, where ''r'' is the [[Intraclass correlation|intraclass]] genetic correlation among members of the same group, ''t'' is corresponding phenotypic intraclass correlation.<ref name=":8">{{Cite journal |last1=Schraiber |first1=Joshua G. |last2=Edge |first2=Michael D. |date=2024-03-19 |title=Heritability within groups is uninformative about differences among groups: Cases from behavioral, evolutionary, and statistical genetics |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=121 |issue=12 |article-number=e2319496121 |doi=10.1073/pnas.2319496121 |issn=1091-6490 |pmc=10962975 |pmid=38470926 |bibcode=2024PNAS..12119496S }}</ref> However, this equation is tautological as the term ''r'' is defined in reference to variance among and between groups, meaning r, if anything, depends on ''h{{Sup|2}}''{{Sub|''B''}}.<ref name=":8" /><ref name=":9" /><ref>{{Cite journal |last1=Feldman |first1=M. W. |last2=Lewontin |first2=R. C. |date=1975-12-19 |title=The heritability hang-up |journal=Science (New York, N.Y.) |volume=190 |issue=4220 |pages=1163–1168 |doi=10.1126/science.1198102 |issn=0036-8075 |pmid=1198102 |bibcode=1975Sci...190.1163F }}</ref> Geneticists Joshua Schraiber and Michael Edge showed that even if this quantity r is known, the quantity ''h{{Sup|2}}''{{Sub|''B''}} is still consistent with "infinitely many configurations of genetic differences among populations".<ref name=":8" /> They conclude:<blockquote>Perfect knowledge of within-group heritability provides no information about between-group heritability. Crucially, even if the heritability of between-group differences is estimated correctly, it leaves the direction of the genetic and environmental components of phenotypic difference unclear.</blockquote>In 2017, the editorial board of ''[[Nature (journal)|Nature]]'' to issued a statement differentiating current research on the genetics of intelligence from the [[Scientific racism|racist pseudoscience]] which it acknowledged has dogged intelligence research since its inception.<ref name="Nature-2017">{{Cite journal |date=25 May 2017 |title=Intelligence research should not be held back by its past |journal=Nature |volume=545 |issue=7655 |pages=385–386 |bibcode=2017Natur.545R.385. |doi=10.1038/nature.2017.22021 |pmid=28541341 |s2cid=4449918 |doi-access=free}}</ref> It characterized the idea of genetically determined differences in intelligence between races as definitively false.<ref name="Nature-2017" /> Analysis of polygenic scores sampled from the [[1000 Genomes Project]] has likewise found no evidence that intelligence was under diversifying selection in Africans and Europeans, suggesting that genetic differences cannot be a significant factor in the observed Black-White gap in average IQ test performance.<ref>{{Cite journal |last=Bird |first=Kevin A. |date=2 February 2021 |title=No support for the hereditarian hypothesis of the Black–White achievement gap using polygenic scores and tests for divergent selection |url=https://onlinelibrary.wiley.com/doi/10.1002/ajpa.24216 |journal=[[American Journal of Physical Anthropology]] |language=en |volume=175 |issue=2 |pages=465–476 |bibcode=2021AJPA..175..465B |doi=10.1002/ajpa.24216 |issn=0002-9483 |pmid=33529393 |url-access=subscription |access-date=1 November 2024 |via=Wiley Online Library}}</ref>

== See also ==
* {{annotated link|Human intelligence}}
* {{annotated link|Outline of human intelligence}}
* {{annotated link|Sex differences in intelligence}}
* {{annotated link|Impact of health on intelligence}}
* {{annotated link|Behavioral epigenetics}}
{{clear}}

== Footnotes ==
{{Notelist}}

== References ==
{{Reflist|30em}}

==Sources==
{{refbegin|2}}
* {{Cite journal |last1=Campbell |first1=Frances A |last2=Ramey |first2=Craig T |last3=Pungello |first3=Elizabeth |last4=Sparling |first4=Joseph |last5=Miller-Johnson |first5=Shari |year=2002 |title=Early Childhood Education: Young Adult Outcomes From the Abecedarian Project |journal=Applied Developmental Science |volume=6 |pages=42–57 |doi=10.1207/s1532480xads0601_05 |s2cid=71602425 |ref={{harvid|Campbell et al.|2002}}}}
* {{Cite journal |last=Cooper |first=R. S. |year=2005 |title=Race and IQ: Molecular Genetics as Deus ex Machina |journal=American Psychologist |volume=60 |issue=1 |pages=71–76 |citeseerx=10.1.1.624.5059 |doi=10.1037/0003-066X.60.1.71 |pmid=15641923}}
* {{Cite book |last1=Daley |first1=C. E. |last2=Onwuegbuzie |first2=A. J. |title=The Cambridge Handbook of Intelligence |publisher=Cambridge University Press |year=2011 |isbn=978-0-521-51806-2 |editor-last=Sternberg |editor-first=R. |location=Cambridge New York |pages=293–306 |chapter=Race and Intelligence |editor-last2=Kaufman |editor-first2=S. B.}}
* {{Cite book |last=Nevid |first=Jeffrey S. |url=https://books.google.com/books?id=dO8bCgAAQBAJ |title=Essentials of Psychology: Concepts and Applications |date=2014-01-17 |publisher=Cengage Learning |isbn=978-1-285-75122-1 |language=en |access-date=2018-01-30 |archive-date=2023-03-20 |archive-url=https://web.archive.org/web/20230320043711/https://books.google.com/books?id=dO8bCgAAQBAJ |url-status=live}}
* {{Cite journal |last=Reichman |first=Nancy E. |date=2005 |title=Low birth weight and school readiness |journal=The Future of Children |volume=15 |issue=1 |pages=91–116 |doi=10.1353/foc.2005.0008 |issn=1054-8289 |pmid=16130543 |s2cid=23345980}}
{{refend}}

==Further reading==
* {{cite journal |last1=Johnson |first1=Wendy |last2=Penke |first2=Lars |last3=Spinath |first3=Frank M. |title=Understanding Heritability: What it is and What it is Not |journal=European Journal of Personality |date=July 2011 |volume=25 |issue=4 |pages=287–294 |doi=10.1002/per.835 |s2cid=41842465 }}
* {{cite journal |last1=Johnson |first1=Wendy |title=Understanding the Genetics of Intelligence |journal=Current Directions in Psychological Science |date=10 June 2010 |volume=19 |issue=3 |pages=177–182 |doi=10.1177/0963721410370136 |s2cid=14615091 }}
* {{cite web | url=http://blogs.scientificamerican.com/beautiful-minds/2013/10/17/the-heritability-of-intelligence-not-what-you-think/ | title=The Heritability of Intelligence: Not What You Think | publisher=Scientific American | date=October 17, 2013 | access-date=20 October 2013 | author=Scott Barry Kaufman}}
* Newson, Ainsley; [[Robert Williamson (geneticist)|Williamson, Robert]] (1999). "Should We Undertake Genetic Research on Intelligence?" ''Bioethics'' 13(3-4), 327–342. doi:10.1111/1467-8519.00161

== External links ==
* {{cite web |url=https://class.coursera.org/behavioralgenetics-001 |title=Introduction to Human Behavioral Genetics |last=McGue |first=Matt |date=5 May 2014 |website=Coursera |access-date=10 June 2014}} Free Massively Open Online Course on human behavior genetics by Matt McGue of the University of Minnesota, including unit on genetics of human intelligence

{{Human intelligence topics}}

{{DEFAULTSORT:Heritability Of Iq}}
[[Category:19th-century introductions]]
[[Category:Race and intelligence controversy]]
[[Category:Factors related to intelligence]]
[[Category:Medical genetics]]
[[Category:Behavioural genetics]]
[[Category:Quantitative genetics]]
[[Category:Genetics]]
[[Category:Intelligence quotient]]
[[Category:Intelligence]]