Certain social/environmental factors put some groups at extraordinary risk for adverse health outcomes, creating health disparities. We present a downward causal model, originating at the population level and ending at disease, with psychological and behavioral responses linking the two. This approach identifies how specific social environments "get under the skin" to cause disease, illustrated with the disparity in mortality from aggressive premenopausal breast cancer suffered by black women. Broadening our lens to consider the entire chain of causal factors, spanning multiple levels and interacting across the life span, heightens our ability to craft specific interventions to address group differences in health.