In this paper, the authors argue that polygyny creates social inequality in which economically advantaged men marry multiple women and economically disadvantaged men marry late in life or potentially never. The institution of polygyny results in a higher proportion of single men without families ("excess men"), who, the authors propose, may turn to violence to achieve higher wealth or prestige. Following this theory, the authors hypothesize that societies with more polygynous neighbors will be at higher risk for intergroup conflict, for which they find robust support. They also find that young men in polygynous societies, who are more likely to be economically disadvantaged and have less prestige, are also more likely to feel as though they are treated unequally and more ready to resort to violence, supporting the authors' theorized underlying mechanism.