Historical Migration Flows and Global Health Differences

In this study we provide evidence that historical migration flows impact present-day global health differences. The underlying theory is based on three physiological facts. First, vitamin D deficiency is directly associated with increased risk of premature death. Second, the ability of humans to synthesize vitamin D from sunlight (i.e., ultraviolet radiation, UV-R) declines with the level of skin pigmentation. Third, the level of human skin pigmentation is the result of an evolutionary compromise between the costs of pigmentation (e.g., higher risk of vitamin D deficiency) and its benefits (e.g., lower risk of skin cancer); people living in high UV-R regions, as a result, became more intensely pigmented. Accordingly, when individuals indigenous to high UV-R regions migrate to low UV-R regions the risk of vitamin D deficiency rises markedly, which should in turn impact average health in the recipient region. We develop an empirical measure that allows us to explore the aggregate consequences of local populations’ differential risk of vitamin D deficiency, as caused by historical migration flows. Our proposed measure of risk of vitamin D deficiency holds strong explanatory power vis-à-vis health outcomes in a world sample as well as across US states.