The purpose of this life course analysis was to explore the link between early-life maternal social and economic conditions with 17-year cognitive functions of their children, paying particular attention to the mediating effects of post-infancy conditions on separate domains of cognition. The results of this analysis suggest that both learning/reversal learning and attention are primarily influenced by contemporary SEP. We noted modest evidence of a link between infant-period SEP and 17-year memory, independent of childhood and 17-year socioeconomic position. Finally, SEP at multiple points in the life course was predictive of both verbal and mathematical achievement at 17 years of age. As a whole, these results suggest that SEP at different points during the young-adult life course may affect unique cognitive domains later in life.
Few studies have examined how life course socioeconomic conditions relate to adult cognitive ability. After adjusting for the participants’ own educational attainment, Kaplan et al. [7
] noted significant links between the childhood SEP and perceptual motor speed, memory, learning ability and constructional ability. No independent associations were noted for general language ability or general cognitive function. Richards and Wadsworth [9
] took a different approach by examining cognitive and achievement outcomes at multiple points in the life course. The investigators noted that greater social adversity was linked to lower reading/verbal achievement from adolescence and extending into middle age. Early life adversity was also linked to memory deficits in middle age. However, a structural equation modeling analysis of the Whitehall II study did not lend support to these findings [10
]. This analysis revealed that adult SEP better explains adult cognition than early-life SEP. However, all of the cognitive outcomes of the Whitehall II analysis were collapsed into one construct, making it impossible to determine whether different cognitive domains were influenced by SEP at different time points.
Our findings are in partial agreement with those of the prior studies. Our analysis supports prior studies with regard to an independent link between infant-period SEP and 17-year memory function. However, unlike the three previous studies, our results suggest that some measures of learning and attentional ability are linked with contemporaneous SEP. Our results also demonstrate that for some cognitive domains, links with early-life SEP are driven by later SEP. Specifically, associations of early life SEP with attention and academic achievement are at least partially mediated by contemporaneous SEP. This suggests that brains structures, such as the parietal lobe, may withstand long-term alterations resulting from early-life stress and conditions.
Each of the cognitive domains we examined is directed by unique structures of the brain, and accordingly our results suggest that socioeconomic conditions in early life differently affect these structures. Two different mechanisms may account for the independent association between early-life social factors and young-adult memory. The first could be characterized as the biological imprinting of stress during critical periods in development [22
]. At the anatomic and physiologic level, this might be explained through overactivity of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine system that mediates the body's response to stress [25
]. Recent work with rats suggests that early-life stress can cause physiologic changes in the HPA axis that promote chronic hypersecretion of cortisol, hippocampal neurogenesis and decreased HPA hormone receptor density in the hippocampus, all of which appear to persist into adulthood [26
Alternatively, SEP could exert distal effects on cognition not through the internalization of stress, but through proxy factors, such as the availability of caretakers to nurture and stimulate infants and children. More specifically, early-life SEP is related to parenting style, investment in the child's education, intellectual stimulation, and mentorship quality, all of which, in turn, are related to cognitive development [28
Several strengths are present in this analysis. This is the first analysis on this topic to employ a well-recognized test of mediation to more closely examine how contemporary SEP might mediate the early-life SEP and adult cognition association [30
]. Although further insight can be gained by newer statistical approaches (structural equation modeling and marginal structural models) [31
], the method we employed arguably allows for more straightforward and familiar interpretation of mediation effects [30
]. Additionally, we employed well-validated, standardized tests of cognition and achievement, examining specific dimensions such as memory with multiple tests, instead of relying on general cognitive outcomes. Finally, the prospective evaluation of SEP allowed us to avoid issues relating to recall bias. Notable limitations are present, however. For instance, the SCDS only collected educational attainment and occupation socioeconomic information. Therefore, this study could not explore how other dimensions of SEP relate to young adult cognition. Additionally, without multiple assessments of the cognition and academic achievement domains over time, we are unable to provide a clearer picture of the potentially dynamic relationship between SEP and cognition throughout the 17 years of study time. Finally, the present analysis only included 63% of the original mother-infant pairs, largely due to missing data; however, we did not note a significant difference among the included and excluded participants with regard to socioeconomic factors.
In conclusion, our findings suggest that programs providing resources to new families living in less than optimal social and economic conditions may beneficially affect children's cognition. More longitudinal studies that expand the number of tested cognitive domains and tease apart mediation effects are needed to confirm our observations.