The dark skin of tropical peoples is likely to be an adaptation to the strong ultraviolet (UV) radiation near the equator, perhaps protecting against sunburn or degradation of folate. By contrast, the adaptive value of light skin is questionable. In particular, the relevance of vitamin D deficiency rickets as a selective factor has been cogently criticized. Population genetic studies on the melanocortin 1 receptor (MC1R) gene (one of the genes responsible for normal human skin colour variation) also cast doubt on the role of positive natural selection in the evolution of light skin. Natural selection may favour dark skin everywhere, though to a lesser extent at higher latitudes. Darwin believed that racial differences in skin colour were caused by sexual selection. Available evidence suggests that in each society a lighter-than-average skin colour is preferred in a sexual partner. Such a preference would generate sexual selection for light skin that counteracts natural selection for dark skin. The observed latitudinal gradient in skin colour may result from the balance between natural and sexual selection.