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378 REVERSIBLE DISORDERS OF BRAIN DEVELOPMENTWhat Causes the SES Gapin Neurocognitive Development?From a scientific point of view, all we have done in the first three studies is todescribe the SES gap in more physiologically meaningful terms than standardizedtest results. The question of mechanism—in other words, what causes theeffects described here—has not yet been discussed. A complete scientific understandingof the effects of SES on neurocognitive development must include anaccount of the mechanisms by which different aspects of brain function come tobe associated with SES. From a practical point of view, knowing which systemsare most affected by SES has only indirect implications for intervention or preventionprograms. Knowing how the effects come about would be far more usefulin suggesting how to close the developmental gap between low and middleSES children.To address this issue, the first question we must ask concerns the direction ofcausality: Do the associations discussed so far reflect the effects of SES on braindevelopment, or the opposite direction of causality? Perhaps families with higherinnate language, executive, and memory abilities tend to acquire and maintain ahigher SES. Such a mechanism seems likely, a priori, as it would be surprising ifgenetic influences on cognitive ability did not, in the aggregate, contribute to individualand family SES. However, it also seems likely that causality operates inthe opposite direction as well, with SES influencing cognitive ability throughchildhood environment and experience. Given that the direction of causality is anempirical issue, what data bear on the issue?The methods of behavioral genetics research can, in principle, tell us aboutthe direction of causality in the association between SES and the developmentof specific neurocognitive functions. However, these methods have yet to beapplied to that question. They have been applied to a related question, namelythe heritability of IQ and SES. Cross-fostering studies of within- and between-SES adoption suggest that roughly half the IQ disparity in children is experiential(Capron & Duyme, 1989; Schiff & Lewontin, 1986). If anything, thesestudies are likely to err in the direction of underestimating the influence of environmentbecause the effects of prenatal and early postnatal environment areincluded in the estimates of genetic influences in adoption studies. Additionalevidence comes from studies of when, in a child’s life, poverty was experienced.Within a given family that experiences a period of poverty, the effects are greateron siblings who were young during that period (Duncan et al., 1994), an effectthat cannot be explained by genetics. In sum, multiple sources of evidence indicatethat SES does indeed have an effect on cognitive development, althoughits role in the specific types of neurocognitive system development investigatedhere is not yet known.