The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D).

We used self-reported direct finger measurements from 255,116 participants in a BBC Internet survey to investigate the measurement of 2D:4D ratios and their association with sex, ethnicity, and sexual orientation. We found significant sex differences such that males had lower 2D:4D than females and the effect size of the sex differences was greatest for right hand 2D:4D. Mean 2D:4D was lower for right hands than for left hands in men, but lower for left hands compared to right hands in women. The sexual dimorphism in 2D:4D was present across ethnic and country groupings, suggesting that it is universal in humans. However, there was also evidence that mean 2D:4D varied across ethnic groups with higher ratios for Whites, Non-Chinese Asians, and Mid-Easterners and lower ratios in Chinese and Black samples. There were significant differences in 2D:4D across sexual orientation groups but these were confined to men. Male homosexuals and bisexuals had higher mean 2D:4D (suggesting exposure to lower prenatal T) than heterosexuals. The effect was present in Whites, but there was no evidence for the pattern among Black and Chinese participants. In women, there were no significant effects of sexual orientation on 2D:4D. Most studies of sexual orientation effects on 2D:4D have measured finger length from photocopies of the hands. In comparison, our self-reported measures gave higher mean 2D:4D, lower effect sizes, and, in some instances, different patterns of effect size. The implications of our findings for future research into 2D:4D are discussed.