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Socioeconomic Context 379Physical and Psychological Determinantsof Neurocognitive DevelopmentWhat aspects of childhood environment and experience might be responsible forthe effects of SES on neurocognitive development? A large set of possibilitiesexist, some affecting brain development by their direct effects on the body andsome by less direct psychological mechanisms. Three somatic factors have beenidentified as significant risk factors for low cognitive achievement by the Centerfor Children and Poverty (1997): inadequate nutrition, lead exposure, and substanceabuse (particularly prenatal exposure).Two nutritional factors, iron deficiency and mild-to-moderate protein-energymalnutrition (PEM; the shortage of both protein and calories), are more prevalentat lower levels of SES. Iron-deficiency anemia afflicts about one quarter of lowincomechildren in the United States (Center on Hunger, Poverty and NutritionPolicy, 1998) and is known to impair brain development when severe. The neurocognitiveimpact mind-to-moderate PEM is not well established (see Ricciuti, 1993,and Sigman, 1995, for differing viewpoints). The Center on Hunger, Poverty andNutrition Policy (1998) has suggested that it probably has little effect on its own.The role of nutrition in SES disparities in brain development has been difficultto resolve because nutritional status is so strongly correlated with a host of otherfamily and environmental variables likely to impact neurocognitive development,including all of the potential mechanisms of causation to be reviewed here. Althoughnutritional supplementation programs could in principle be used as an“experimental manipulation” of nutritional status alone, in practice these programsare often coupled with other, nonnutritional forms of enrichment or affect children’slives in nonnutritional ways that perpetuate the confound (e.g., children givenschool breakfast are absent and late less often). The consensus regarding the roleof nutrition in the cognitive outcomes of poor children has shifted over the pastfew decades, from primary cause to a factor that contributes indirectly and throughsynergies with other environmental disadvantages (Center on Hunger, Poverty andNutrition Policy, 1998).Lead is a neurotoxin found in older house paint that accumulates in the bodiesof low SES children who are more likely to live in old dwellings with peeling paint.A meta-analysis of low-level lead exposure on IQ indicates estimated that every10 ug/dL increase in lead is associated with a 2.6 point decrease in IQ (Schwartz,1994). As with nutrition, the effect of lead synergizes with other environmentalfactors and is more pronounced in low SES children (Bellinger et al., 1987). Forexample, low iron stores render children more susceptible to environmental lead(Center on Hunger, Poverty and Nutrition Policy, 1998).Prenatal substance exposure is a third factor that affects children of all SES levelsbut is disproportionately experienced by the poor. Maternal use of alcohol, tobacco,marijuana, and other drugs of abuse have been associated with adverse