Fetal exposure to androgens, as indicated by digit ratios (2D:4D), increases men's agreeableness with women

The ratio of the length of the second finger, or digit, to the fourth finger (2D:4D) is influenced by fetal exposure to androgens; a smaller ratio indicates greater androgen exposure. We used event contingent recording to investigate the relation between the 2D:4D ratio and social behavior. Participants completed multiple records of their behavior in events in naturalistic settings; records included information about situational features such as the gender of the person with whom the person was interacting. Men were more agreeable towards women than men; this effect was significantly greater in those with smaller 2D:4D ratios. Men with smaller 2D:4D ratios were also less quarrelsome towards women than towards men. The 2D:4D ratio did not influence social behavior in women. The hormonal environment in which the male fetal brain develops may influence adult social behavior in specific contexts.