This is an invited research digest by Dr. Monica Ellwood-Lowe of Stanford University.
A number of studies have shown that the hippocampus, a brain region that is particularly sensitive to stress, is smaller in children who grow up in homes with fewer economic and educational resources. The hippocampus is nestled deep in the brain, critical for learning and memory, and a key player in the body’s stress response system. Indeed, studies have shown that children growing up in socioeconomically disadvantaged homes are more likely to be at risk for experiencing a range of stressors from which those who are more well-off are typically buffered. This suggests that experiences associated with socioeconomic inequality – whether through psychosocial stress or unequal access to resources – may have a negative impact on children’s brain development.
In fact, the non-linear growth trajectories of the hippocampus make it particularly difficult to interpret the implications of these neural differences as children grow older. Compounding this difficulty is the fact that the brain is highly plastic, capable of reorganization and cell growth throughout life. Few studies have followed children longitudinally through early adulthood and assessed changes in hippocampal volume associated with such socioeconomic factors. This raises the important question of what happens over time: does the hippocampus stay consistently smaller in those who have been socioeconomically disadvantaged, or might it recover as they age?
Researchers from Stanford assessed this question in a group of girls who were followed longitudinally from childhood to early adulthood. Confirming findings from previous research, girls’ childhood family income and parental levels of education were both associated with the size of their hippocampi. Measuring the hippocampus in these girls over time, however, revealed that those who came from lower-income homes appeared to show hippocampal recovery. Specifically, differences between the highest- and lowest-income girls were greatest during late adolescence, and converged thereafter. In fact, while children from higher-income homes showed a more typically-observed pattern of hippocampal development – volume peaking during late adolescence and then gradually declining – those in the lower-income homes showed a completely different pattern, with continued growth into early adulthood.
One explanation for these findings is that the alternate growth pattern of the hippocampus exhibited in lower-income girls is due to genetic factors. Indeed, the volume of the hippocampus has been shown to be highly correlated in mothers and daughters – significantly more so than in father-daughter, mother-son, and son-son pairs – suggesting a high degree of inherited variation. To explore this possibility, the researchers also measured the size of a subset of the girls’ mothers’ hippocampi, allowing them to examine whether income-related variation was due to these inherited differences. The investigators found that income-related differences in hippocampal volume persisted even when controlling for maternal hippocampal size, suggesting that something about the environments of these girls, as opposed to genetic variation, led to the observed alternate pattern of hippocampal growth.
While the results of this study cannot tell us why family income appears to affect hippocampal volume in this way, it does underscore the importance of conducting repeated measurements for understanding the impact of socioeconomic factors on brain development. Might girls from lower-income homes actually be protected in late adolescence from some of the stressors experienced by higher-income girls? Does having smaller hippocampal volume in early adolescence have lasting consequences even if the hippocampus reaches full volume later in adulthood? How might the brain adapt to meet the demands of growing up in very different environments? Future research is needed to probe these and other important questions.