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. [Health Affairs 27, no. 2 (2008): 339-349; 10.1377/hlthaff.27.2.339] H e a lt h d i s pa r i t i e s o c c u r by r ac e , ethnicity, sex, socioeconomic status, and sexual orientation, with inequities in screening, incidence, treatment, and mortality across a number of diseases and conditions, including cancer, diabetes, cardiovascular disease, infant mortality, and HIV/AIDS. A black/white gap in mortality from common cancer sites has been noted since 1975, for instance, when cancer data were first collected systematically by race and ethnicity. In 1975, black women experienced 39.2 more deaths per 100,000 population from breast cancer than white women. By 2004, that gap had increased to 44.1 excess deaths per 100,000. During the same time period, breast cancer mortality rates among white women decreased from 31.8 to 23.8 per 100,000. Mortality rates