We examine why developed societies are monogamous while rich men throughout history have typically practiced polygyny. Wealth inequality naturally produces multiple wives for rich men in a standard model of the marriage market. However, we demonstrate that higher female inequality in the marriage market reduces polygyny. Moreover, we show that female inequality increases in the process of development as women are valued more for the quality of their children than for the quantity. Consequently, male inequality generates inequality in the number of wives per man in traditional societies, but manifests itself as inequality in the quality of wives in developed societies.