Percent of variation in IQ scores in a given population associated with genetic variation
Research on the heritability of IQ inquires into the degree of variation in IQ within a population that is due to 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.[1][2]Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene,[3][4] and in the case of intelligence at least 500 genes.[5] 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 disorders, such as phenylketonuria, can negatively affect intelligence.[6]
Early twin studies of adult individuals have found a heritability of IQ between 57% and 73%,[7] with some recent studies showing heritability for IQ as high as 80%.[8] IQ goes from being weakly correlated with genetics for children, to being strongly correlated with genetics for late teens and adults. The heritability of IQ increases with the child's age and reaches a plateau at 14–16[9] years old, continuing at that level well into adulthood. However, poor prenatal environment, malnutrition and disease are known to have lifelong deleterious effects.[10][11] Estimates in the academic research of the heritability of IQ have varied from below 0.5[1] to a high of 0.8 (where 1.0 indicates that monozygotic twins have no variance in IQ and 0 indicates that their IQs are completely uncorrelated).[12] Eric Turkheimer and colleagues (2003) found that for children of low socioeconomic status heritability of IQ falls almost to zero.[13] These results have been challenged by other researchers. IQ heritability increases during early childhood, but it is unclear whether it stabilizes thereafter.[14] A 1996 statement by the American Psychological Association gave about 0.45 for children and about .75 during and after adolescence.[15] A 2004 meta-analysis of reports in Current Directions in Psychological Science gave an overall estimate of around 0.85 for 18-year-olds and older.[16] The general figure for heritability of IQ is about 0.5 across multiple studies in varying populations.[17]
^Daniele, V. (2013). "The burden of disease and the IQ of nations". Learning and Individual Differences. 28: 109–118. doi:10.1016/j.lindif.2013.09.015.
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^Bouchard, TJ (2004). "Genetic influence on human psychological traits - A survey". Current Directions in Psychological Science. 13 (4): 148–151. doi:10.1111/j.0963-7214.2004.00295.x.
^Plomin, Robert; DeFries, John C.; Knopik, Valerie S.; Neiderhiser, Jenae M. (24 September 2012). Behavioral Genetics. Worth Publishers. pp. 195–196. ISBN978-1-4292-4215-8. Retrieved 4 September 2013. Model-fitting analyses that simultaneously analyze all the family, adoption, and twin data summarized in Figure 12.6 yield heritability estimates of about 50 percent (Chipuer, Rovine & Plomin, 1990; Loehlin, 1989).
^Hunt, Earl (2010). Human Intelligence. Cambridge University Press. p. 447. ISBN978-0-521-70781-7. OL24384631M – via Open Library. [N]o genes related to the difference in cognitive skills across the various racial and ethic groups have ever been discovered.
^Kaplan, Jonathan Michael (January 2015). "Race, IQ, and the search for statistical signals associated with so-called "X"-factors: environments, racism, and the "hereditarian hypothesis"". Biology & Philosophy. 30 (1): 1–17. doi:10.1007/s10539-014-9428-0. ISSN0169-3867. S2CID85351431.