<P> Asbury and colleagues (2005) studied the effect of environmental risk factors on verbal and non-verbal ability in a nationally representative sample of 4 - year - old British twins . There was not any statistically significant interaction for non-verbal ability, but the heritability of verbal ability was found to be higher in low - SES and high - risk environments . </P> <P> Harden and colleagues (2007) investigated adolescents, most 17 years old, and found that, among higher income families, genetic influences accounted for approximately 55% of the variance in cognitive aptitude and shared environmental influences about 35% . Among lower income families, the proportions were in the reverse direction, 39% genetic and 45% shared environment ." </P> <P> Rushton and Jensen (2010) criticized many of these studies for being done on children or adolescents . They argued that heritability increases during childhood and adolescence, and even increases greatly between 16--20 years of age and adulthood, so one should be cautious drawing conclusions regarding the role of genetics from studies where the participants are not adults . Furthermore, the studies typically did not examine if IQ gains due to adoption were on the general intelligence factor (g). When the studies by Capron and Duyme were re-examined, IQ gains from being adopted into high SES homes were on non-g factors . By contrast, the adopted children's g mainly depended on their biological parents SES, which implied that g is more difficult to environmentally change . The most cited adoption projects that sought to estimate the heritability of IQ were those of Texas, Colorado and Minnesota that were started in the 1970s . These studies showed that while the adoptive parents' IQ does correlate with adoptees' IQ in early life, when the adoptees reach adolescence the correlation has faded and disappeared . The correlation with the biological parent seemed to explain most of the variation . </P> <P> A 2011 study by Tucker - Drob and colleagues reported that at age 2, genes accounted for approximately 50% of the variation in mental ability for children being raised in high socioeconomic status families, but genes accounted for negligible variation in mental ability for children being raised in low socioeconomic status families . This gene - environment interaction was not apparent at age 10 months, suggesting that the effect emerges over the course of early development . </P>

For several generations average iq scores in the united states have