Donald Templer and J. Phillippe Rushton have recently published a study of the 50 U.S. states which finds strong correlations among the following varaibles: skin color, IQ, infant mortality, life expectancy, violent crime, HIV/AIDS, and income. This pattern mirrors what Templer has observed internatioanlly. Skin color tends to be more strongly correlated to the list of variables than does income, so the authors conclude that this points to biology rather than economic environment as the more important explanation of the pattern of correlations. They rely on Rushton's life history theory and Lynn's cold climate theory to make sense of why the variables hang together, but whatever the explanation, there seems to be a worldwide pattern of desirable (or undesirable traits) running together, and this is closely associated with skin color.
They review some interesting research by Ducrest et al. (2008) which found that in 20 wild vertebrate species, darker individuals are more aggressive, sexually active, and resistant to stress. They also tend to have a larger body mass and greater energy and physical activity. For example, darker maned male lions are reported to be more aggressive and sexually more active, and darker barn owls show stronger immune response. Darker coloration is associated with enhanced fertility, female sexual receptivity, male sexual motivation and performance, and higher levels of testosterone. Ducrest et al., however, cautioned that these findings might not apply to humans.