Sex differences in mortality (SDIM) vary over time and place as a function of social, health, and medical circumstances. The magnitude of these variations, and their response to large socioeconomic changes, suggest that biological differences cannot fully account for sex differences in survival. We develop a set of empiric observations about SDIM with which any theory will have to contend. We draw on a wide swath of mortality data, including probability of survival to age 70 by county in the United States, the Human Mortality Database data for 18 high-income countries since 1900, and mortality data within and across developing countries over time periods for which reasonably reliable data are available. We show that as societies develop, M/F survival first declines and then increases, a "SDIM transition" embedded within the well-described demographic and epidemiologic transition. After the onset of this transition, cross-sectional variation in SDIM exhibits a consistent pattern of female resilience to mortality under adversity, which strengthens over time.