‘Born this way’? Prenatal exposure to testosterone may determine behavior in competition and conflict

Fetal exposure to sex hormones can have long lasting effects on human behavior. The second-to-fourth digit ratio (DR) is considered a putative marker for prenatal exposure to testosterone (vs. estrogens), with higher exposure resulting in lower DR. Even though testosterone is theoretically related to competition, the role of DR in human behavior is debated; and in situations such as bilateral conflict is unknown. We investigate this through a laboratory experiment using a repeated 2-person Tullock contest played in fixed same-gender pairs. Based on a previously obtained large sample of student subjects, we selectively invited participants to the laboratory if their right-hand DR was in the top (High-DR) or bottom (Low-DR) tercile for their gender. Unbeknownst to the subjects, we performed a controlled match of the DR types (Low-Low, Low-High, High-High). This novel methodology allows us to analyze the causal effect of DR on behavior for the first time in the literature. We find that Low-DR (vs. High-DR) males compete more aggressively regardless of the counterpart’s type. For females’ conflict behavior, the counterpart’s type matters more than the decision-maker’s type: Low-DRs are non-significantly more aggressive but everyone is more aggressive against High-DRs. Limitations due to sample size are discussed.