We discuss and test competing explanations for polygyny based on household economics, malecentered kin groups, warfare, and environmental characteristics. Data consist of codes for 142 societies from the Standard Cross‐Cultural Sample, including new codes for polygyny and environmental characteristics. An explanatory model is tested for the worldwide sample using regression analysis, and then replicated with regional samples. We obtain convergent results with two different measures of polygyny, cultural rules for men's marriages and the percentage of women married polygynously. We conclude that the best predictors of polygyny are fraternal interest groups, warfare for capture of women, absence of constraints on expansion into new lands, and environmental quality and homogeneity.