2nd to 4th digit ratio and a universal mean for prenatal testosterone in homosexual men.

Male homosexuals are said to have experienced high prenatal testosterone, and testosterone may have deleterious effects on the immune system. The question of the prenatal androgen status of male homosexuals is therefore of importance. There is evidence that the relative length of the 2nd and 4th digit (2D:4D) is negatively correlated with prenatal testosterone. We review the available literature, and provide further data on 2D:4D in male heterosexuals and homosexuals. Normative values of male mean 2D:4D ratio vary between 0.94 to 1.00 across populations. However, mean 2D:4D ratio is relatively constant among male homosexuals from different populations, with a mean of 0.96 to 0.97. Therefore homosexuals appear to be more androgenised (lower 2D:4D) than heterosexuals in some populations, while in others they appear to be less androgenised (higher 2D:4D) than heterosexuals. Speculations regarding the effect of early sex steroids on the health of male homosexuals should therefore focus on the effects of prenatal testosterone levels associated with 2D:4D ratios of between 0.96 and 0.97.