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From Neurons to Neighborhoods: The Science of Early Childhood Development
family resource differences derive primarily from parents' genetic endowments (e.g., cognitive, mental health) as behavioral geneticists argue (see, for example, Rowe, 1994), then any putative effects of resources on children's development could in fact derive from genetic endowments rather than from any of the features of families that we have just discussed. This in turn would render policy efforts aimed at these features either moot or exceedingly difficult.
There is little doubt that genetic influences need to be added to the long list of potentially important factors that deserve attention in studies that assess the effects of family resources on children 's development. The issue is how to ask the pertinent questions and how to explore them in research. Two kinds of evidence suggest that, even net of genetic endowments, family resources have important impacts on child development. These studies have used measures of family socioeconomic status (SES) (i.e., parental education and occupational status, income, family structure, and other measures of the family environment) to capture family resources.
The first study compares the importance of socioeconomic factors on children's achievement before and after statistical adjustments for parental genetic endowments. Phillips et al. (1998) used data from a nationally representative sample of mothers (the National Longitudinal Survey of Youth) to do exactly this. Specifically, they adjusted for what they called the “mother's cognitive genotype index” using her score on the battery of Armed Forces Qualifying Tests (numerical operations, arithmetic and math knowledge, paragraph comprehension), her class rank in high school, and the interviewer's assessment of the mother's understanding of the interview when assessing the association between socioeconomic status and children 's achievement. They found that genetic factors accounted for only about one-quarter of the SES-achievement association. Although far from trivial, this finding suggests that maternal cognitive endowments do not account for most of the socioeconomic contributions to children 's achievement.
The second approach compares the association of socioeconomic status to child outcomes between children raised by biological and adoptive parents. If this association is due primarily to genetic factors, then the correlation between child outcomes and the SES of adopted (and thus genetically unrelated) parents should be much lower than the correlation between child outcomes and the SES of their biological parents. Loehlin, Horn, and Willerman (1989) found that the correlation between SES and child IQ for their sample of adopted children was only 18 percent less than the correlation for biological children. Scarr and Weinberg (1976) found similar patterns in their sample of black adopted and biological children. The magnitude of these reductions are in line with those found in the direct approach of Phillips et al. (1998), and also suggests that SES impacts on childhood IQ cannot be attributed primarily to genetic factors.