Second to fourth digit ratio, sexual selection, and skin colour

Abstract

Skin pigment may be related to mate choice, marriage systems, resistance to microorganisms, and photoprotection. Here we use second to fourth digit ratio (2D:4D) to disentangle the relationships among these variables. There is evidence that 2D:4D is negatively associated with prenatal testosterone and positively with prenatal oestrogen. We show (i) a negative association between skin colour and 2D:4D in Caucasian women, but not in men, suggesting that skin colour in women is partly dependent on prenatal oestrogen; and (ii) Caucasian subjects with low 2D:4D reported higher susceptibility to sun-burn, athlete's foot and eczema than subjects with high 2D:4D, suggesting that prenatal testosterone increases susceptibility to sunburn and skin diseases. Frost [ Hum. Evol. 9 (1994) 141] has reported that with latitude controlled, highly polygynous peoples have relatively dark and monogamous peoples relatively light skin. We suggest that polygynous populations incur selection for high prenatal testosterone and low prenatal oestrogen because of competition among men for wives. Such groups have low 2D:4D, and high susceptibility to sunburn and skin infections which may result from the immunosuppressive effects of prenatal testosterone. Where polygynous groups are found at low latitudes, they have evolved dark skin for protection against UV and microorganisms. More monogamous peoples experience selection for low prenatal testosterone and high prenatal oestrogen as a result of mate choice for light-skinned oestrogenised women. Such groups have high 2D:4D, resistance to sunburn and skin infections, and light skin. The association between very dark skin and low latitude exists only when polygynous societies are found at low latitudes, as is common in sub-Saharan Africa, but not in the New World.