This model would suggest that heritability values in mature individuals may be different for those whose environment was deficient in the necessary environmental features. Studies regarding the effects of deficient environmental conditions on the heritability of brain structures in adults are not to our knowledge currently available, and identifying and directly measuring relevant environmental factors is highly challenging. However, one might speculate that if an environmental feature during development was an integral component of developing a mature heritable phenotype, departure from the optimal range for family members would result in an increase in variance due to their shared environment. Some indirect evidence for this may be found in the IQ literature. As previously mentioned, IQ increases in heritability over childhood and adolescence, and shared environmental effects are typically nonsignificant (Plomin, Fulker, Corley, & DeFries, 1997). Studies of heritability of cognition have found that there is a G×E interaction such that shared environmental factors become more prominent relative to genetic factors as socioeconomic conditions worsen (Harden, Turkheimer, & Loehlin, 2007; Turkheimer, Haley, Waldron, D’Onofrio, & Gottesman, 2003).
