Socioeconomic status (SES) influences health, behaviors, and well-being. Emerging information suggests that SES effects on health may be in part be due to SES effects on brain development. Several U.S.-based studies have also established large and consistent SES influences on child brain development. This paper reviews the emerging literature from the Adolescent Brain Cognitive Development (ABCD) study on the effects of social marginalization in reducing the effects of SES on children and youth brain development. These diminished returns of SES in minoritized youth are not due to genetics; rather, they stem from systemic and structural racism, social stratification, and marginalization that generate inequalities across the SES spectrum. As a result of these diminished returns, inequalities expand from low-SES to mid- and high SES sections of US society. As such, brain development in part explain how racism effects transition across generations.