Background: Considerable research has documented age-related growth in brain size as a marker of normal brain development. This is particularly important because brain volume has a significant role in overall cognitive performance. However, less research is done on whether age-related changes in the global brain volume differ across diverse racial and ethnic groups. We hypothesized that age-related growth in brain size would be disrupted in African American children who are historically affected by racism.
Purpose: Considering race as a proxy of racism rather than genetics, this study tested racial and ethnic differences in the effects of age on global brain volume using structural brain imaging data. Built on a sociological, rather than a biological theory, we built our study on Marginalization-related Diminished Returns (MDRs) framework, which argues that under racism, resources and assets are less effective for social groups that are historically racialized, discriminated against, marginalized, and segregated. Considering age as an asset/resource that increases the global brain volume, we expected weaker effects of age on overall brain size of African American and Hispanic children, than White and non-Hispanic children, again as a result of racism.
Methods: We borrowed the structural Magnetic Resonance Imaging (sMRI) data from the Children Brain Cognitive Development (ABCD) study, which included 9,311 9-10 year old children. The independent variable was the child's age treated as a continuous measure (in months). The primary outcome was global brain volume. Sex, parental employment, parental education, household income, and parental marital status were the covariates. Race and ethnicity, as proxies of racism, were the moderators. To analyze the data, we used linear regression models.
Results: Age was positively associated with the global brain size in children. In line with the MDRs, the positive association between age and global brain volume was weaker for African American than White children, while family structure, sex, and family socioeconomic status was controlled.
Conclusions: Under racism, age has unequal effects on global brain size of diverse racial groups. In line with the MDRs, we observe diminished age-related growth of the brain for African American children, which documents detrimental effects of racism. For White children who are not affected by racism, age makes a large difference regarding global brain volume. Age-related growth of global brain size is diminished in African American children, whose daily lives are shaped by racism. School and residential segregation may have a role in reducing the effect of age on children's brain growth in African American families. The results should not be interpreted as inferiority of one group but social processes that hinder normal development of a historically oppressed group.
Keywords: MRI; age; brain development; global brain volume; racism; social determinants; structural MRI.