Objectives Sexual selection theory suggests patterns of covariance among polygynous mating behaviors and ecological variables at the cross‐national level. We quantified national levels of polygyny using the human polygyny index (HPI), a ratio of men's to women's variability in the numbers of sex partners over the past year. Methods HPI scores were available for 48 nations from the International Sexuality Description Project (Schmitt, 2005), and were used to test three hypotheses: (1) human polygyny should be associated with increased intrasexual competition (e.g., high male‐male aggression and resource competition), (2) human polygyny should be associated with features of natural and intersexual selection (e.g., high pathogen stress and an emphasis on physical attractiveness in mate choice), and (3) human polygyny should be associated with early and more prolific reproduction. Results All three hypotheses received at least partial support. Conclusions Discussion focuses on the limitations and implications of the current findings.