Editing Nature versus nurture (section)

=== Heritability of intelligence ===
{{Main|Heritability of IQ}}
Cognitive functions have a significant genetic component. A 2015 meta-analysis of over 14 million twin pairs found that genetics explained 57% of the variability in cognitive functions.<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=http://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|s2cid=205349969|issn=1061-4036}}</ref> Evidence from [[behavioral genetics|behavioral genetic]] research suggests that family environmental factors may have an effect upon childhood [[Intelligence quotient|IQ]], accounting for up to a quarter of the variance. The [[American Psychological Association]]'s report "[[Intelligence: Knowns and Unknowns]]" (1995) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Here, environment is playing a role in what is believed to be fully genetic (intelligence) but it was found that severely deprived, neglectful, or abusive environments have highly negative effects on many aspects of children's intellect development. Beyond that minimum, however, the role of family experience is in serious dispute. On the other hand, by late adolescence this correlation disappears, such that adoptive siblings no longer have similar IQ scores.<ref name="Plomin2008">Plomin, R., [[John C. DeFries|J. C. DeFries]], G. E. McClearn, and P. McGuffn. 2008. ''Behavioral Genetics'' (5th ed.). New York: [[Worth Publishers]]. {{ISBN|978-1429205771}}</ref>

Moreover, adoption studies indicate that, by adulthood, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.74), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (≈0.0).<ref>{{cite journal|jstor=41465638|pmid=9549239|year=1998|last1=Bouchard|first1=T. J. Jr.|title=Genetic and environmental influences on adult intelligence and special mental abilities|journal=Human Biology|volume=70|issue=2|pages=257–79}}</ref> Recent adoption studies also found that supportive parents can have a positive effect on the development of their children.<ref>{{cite journal|doi=10.1037/0022-0663.89.2.381|title=Same-age unrelated siblings: A unique test of within-family environmental influences on IQ similarity|journal=Journal of Educational Psychology|volume=89|issue=2|pages=381–390|year=1997|last1=Segal|first1=Nancy L.}}</ref>