This article reports three main findings. First, socioeconomic disparities in mental ability emerged over development: SES was unrelated to mental ability at 10 months, but was related to change in mental ability from 10 months to 2 years, such that by 2 years each standard deviation of SES was associated with approximately one third of a standard deviation of mental ability. Second, at the population level, genes began to play a role in the development of mental ability between 10 months and 2 years. Third, the extent to which genes influenced mental development differed according to SES, such that by two years of age, genetic influences on mental ability were larger for children being raised in higher SES homes.
These findings are consistent with the hypothesis that the emergence of genetic variation in complex behavioral phenotypes depends on reciprocal interactions between the child and his or her environment (
Bronfenbrenner & Ceci, 1994;
Dickens & Flynn, 2001; Scarr & McCartney, 1993). According to this perspective, poor socioeconomic contexts constrain children’s opportunities to engage with supportive environments that foster cognitive growth, resulting in suppression of genetic influences on mental ability. In particular, during infancy, socioeconomic disadvantage is likely to impair an infant’s ability to elicit responsive and developmentally appropriate stimulation from caregivers (i.e., evocative processes). However, later in childhood, the role of socioeconomic status likely shifts, such that socioeconomic disadvantage restricts genetic variation in cognitive ability by limiting opportunities for individuals to actively seek out educational and social experiences that are congruent with their own genetically-influenced interests and motivations (
Scarr & McCartney, 1983). We should emphasize that this specific mechanism may not generalize to all psychological outcomes, and that there are likely to be some outcomes, e.g. Attention Deficit Hyperactivity Disorder, that are actually more heritable in higher risk social environments (
Pennington et al., 2009). Additionally, although socioeconomic status is often conceived of as a purely environmental variable, differences in the frequencies of specific genetic polymorphisms may also exist between different socioeconomic groups.
These findings highlight the importance of early life experience on cognitive development. Even though current evidence suggests that the predictive validity of infant mental ability for later cognitive ability is only moderate (
Bornstein & Sigman, 1986;
Rose & Feldman, 1995) and that children maintain a great deal of neurological and behavioral plasticity well past infancy (
Garlick, 2002;
Brehmer, Li, Müller, von Oertzen, & Lindenberger, 2007), the current findings build on a growing body of literature that highlights the importance of early life experiences for cognitive development (
Nelson, Zeanah, Fox, Marshall, Smyke, & Guthrie, 2007).
Bornstein & Signman (1986), for example, have strongly argued against the perspective “that infancy might play little or no role in determining the eventual cognitive performance of the child and, therefore, that individuals could sustain neglect in infancy if remediation were later made available” (p. 269).
Heckman (2006) has recently taken an economic perspective on this topic. He has argued that early prophylactic interventions for disadvantaged children produce much higher rates of return on what he terms “human skill formation” than later remedial interventions for older children and adults. Based on this perspective,
Heckman (2006) has concluded that “at current levels of funding, we overinvest in most schooling and post-schooling programs and underinvest in preschool programs for disadvantaged persons” (p. 1901).
This article makes an important contribution to the literature by establishing the developmental timing of gene-by-SES effects on mental ability. However, future research will be necessary to address a number of remaining issues. First, as in previous studies, the current study only examined the moderation of genetic influences by an omnibus index of socioeconomic status. In order to translate the current findings into useful recommendations for policy and intervention, it will be important for future research to examine the specific aspects of SES that contribute to the gene-by-SES effect, ranging from contextual aspects of neighborhoods, schools, and homes, to more proximal aspects of caregiver behavior. Second, it will be important to further investigate the developmental patterns of gene-environment correlation that have been hypothesized to underlie gene-by-SES effects on mental ability, and to identify specific child and caregiver characteristics that become matched to one another over time. Third, it will be important to identify the neurobiological foundations of gene-by-SES effects on mental ability. There is evidence that genetic differences in cognitive ability are strongly related to genetic differences in brain volume and cortical thickness (
Posthuma, De Geus, Baare, Pol, Kahn, & Boomsma, 2002;
Toga & Thomspon, 2005); and that the population-level heritability of brain regions that have been linked with mental ability increases with childhood age (
Lenroot & Giedd, 2008). This suggests that the gene-by-SES interaction on mental ability in early childhood may be mediated by a gene-by-SES interaction on measures of regional brain volumes.
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= .80. Here we report the results of analyses based on the zygosity determined from observers’ similarity ratings. Analyses based on the zygosity determined from parent ratings of their twins’ similarity produced the same pattern of results, as did analyses in which twins of with physical similarity ratings of 7, 8, and 9 were excluded.
