This paper* explains why modern societies are less polygynous than less-developed societies:

Men in less-developed economies prefer quantity over quality [=skill] in wives and children. The explanation is rather intuitive. Rich men in less-developed economies are not efficient at producing quality children because they tend not to have high human capital themselves. Therefore, they have a low demand for quality children and, consequently, a low demand for quality women who can help them produce quality children. As a result, women in less-developed societies are valued only for the quantity of children they can produce, and not the quality. This makes all women very close substitutes for one another, which keeps the price of all women low enough for richer men to acquire multiple wives.

In more advanced economies, richer men tend to have high human capital and, therefore, they are more efficient at producing human capital in children. This creates a high demand for quality in children and in women, because quality women are complements in the production of high-quality children. Thus, all women are not close substitutes in the marriage market in advanced societies. Higher-quality women are a scarce resource, which drives up their price in the market marriage market and makes polygyny less affordable for wealthy men.

The main implication of the model can be summarized as follows: male income inequality generates polygyny, but female inequality in the marriage market reduces it. As the return to human capital increases, women who can create high-quality children more efficiently are increasingly valued in comparison to low-quality women.

The main empirical prediction is that the composition of inequality, not just the level, is an important determinant of the degree of polygyny in society. Secifically, societies should be more polygynous in countries where variation in overall wealth inequality is determined more by differences in nonlaber income (capital and inherited wealth) versus income variation generated by differences in the levels and returns to human capital investments.

* Eric Gould et al., The Mystery of Monogamy, American Economic Review, American Economic Association, vol. 98(1), pages 333-57, March 2008.