||Imbalances in the number of men and women-the adult sex ratio-are related to patterns of marriage and reproduction cross-culturally. Specifically, a shortage of men is associated with more divorce, more children out of wedlock, and more short-term, casual relationships. There are two competing hypotheses to explain this in the literature: one suggests this results from women's competition over scarce mates, and another suggests this results from men's increased mating effort when the returns to mating effort are the greatest. This dissertation tested whether women's mate competition drives this pattern. The research design was a natural experiment, comparing women's marital and reproductive histories, as well as self-reported aggressive competition, on two outer islands of Yap, which share a similar cultural history but differ in the sex ratio. The data show that women were not more aggressive when men were scarce, but sex ratio did appear to affect the consequences of aggressive competition. Sex ratio was also related to women's reproduction in ways that suggest that some men likely do exhibit more mating effort when there is a surplus of women. This dissertation adds to the growing literature on the adult sex ratio as a predictor of mate competition, highlighting that a scarcity of mates does not appear to predict women's aggression but suggests factors that might.