This paper examines why developed countries are monogamous while rich men throughout history have tended to practice polygyny (multiple wives). Wealth inequality naturally produces multiple wives for rich men in a standard model of the marriage market. This paper argues that the sources of inequality, not just the level of inequality, determine the equilibrium degree of monogamy or polygamy. In particular, when inequality is determined more by disparities in human capital versus non-labor income (such as land, capital, corruption), the outcome is more monogamous. This explains why developed countries, where human capital is the main source of income and inequality, are monogamous while less-developed economies tend to be polygynous. The results are driven by the larger inequality in the value of women in the marriage market in modern economies. When the value of human capital increases, rich men increasingly value quality women who can help them raise quality children more efficiently. As a result, high quality women are valued much more than low quality women, which makes polygyny less affordable for rich men. In this manner, we show that male inequality generates polygyny, but female inequality reduces it. Using data from Cote d'Ivoire, we provide evidence for all the main implications of the model. In particular, we control for a man's total income and show that polygyny increases with non-labor income but decreases with labor income and education. These patterns are strong even within social groups where norms regarding polygyny are likely to be constant.