Working Paper

‘Born this Way’? Prenatal Exposure to Testosterone May Determine Behavior in Competition and Conflict


It is documented that fetal exposure to sexual hormones has long lasting effects on human behavior. The second-to-fourth digit ratio (DR) is a putative marker for prenatal exposure to testosterone (compared to estrogens) while in uterus, with higher relative exposure to testosterone resulting in a lower DR. Although the existing literature documents the correlation of DR with various decisions, and testosterone has been related to competitive behaviors, little research has studied the effect of DR on competition in conflict situations where skills do not matter. We investigate this question in the laboratory. Based on a previously obtained large sample of student subjects, we selectively invite subjects to the laboratory if their right-hand DR is in the top (High type) or bottom (Low type) tercile for their gender. Unbeknownst to the subjects, we perform a controlled match of High and Low types as opponents in a 2-person Tullock contest. We find that Low type (higher exposure to testosterone) males expend significantly higher conflict effort than High type males, that is, they are more aggressive, which reduces their opponents’ earnings. Among females, however, everyone is more aggressive against the High type (who respond less aggressively). These results can partially be explained through high joy of winning and/or spitefulness for Low type males, and high spitefulness for Low type females. This investigation sheds light on the importance of biological aspects in the ex-ante determinants of conflict, and on contest design.

Keywords: Contests, Digit Ratio, Tullock