Hemispheric difference in human skin color.

Previous studies of human skin color have shown a strong relationship between skin color and distance from the equator, which has been interpreted as a link between skin color, latitude, and the intensity of ultraviolet radiation. The underlying assumptions are that UV radiation is greatest at the equator and that it diminishes with increasing latitude to the same extent in both the Northern and Southern Hemispheres. The standard analysis of human skin color is based on these assumptions, such that skin color is assumed to be darkest at the equator, and the decrease of skin color with latitude is assumed to be the same in both hemispheres. A nonlinear piecewise regression model was developed to test these assumptions and applied to mean skin reflectance data from 102 male samples and 65 female samples from across the Old World. For both males and females, skin reflectance (%) is lowest at the equator (darkest skin). Among males, skin reflectance increases roughly 8.2% for every 10 degrees of latitude in the Northern Hemisphere but only 3.3% for every 10 degrees of latitude in the Southern Hemisphere. Among females, the corresponding numbers are 8.1% in the Northern Hemisphere and 4.7% in the Southern Hemisphere. These results indicate that human skin color is darker in the Southern Hemisphere than in the Northern Hemisphere at equivalent latitude. Recent research shows that UV radiation is higher in the Southern Hemisphere than in the Northern Hemisphere at similar latitude. This difference, relating to astronomical and climatic conditions, may have existed in the past at different times and perhaps influenced the evolution of human skin color.