The Human Polygyny Index and its Ecological Correlates: Testing Sexual Selection and Life History Theory at the Cross-National Level * David P. Schmitt, Bradley University Percy A. Rohde, Universit¨ at Kassel 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. We quantified levels of polygynous mating behavior across 48 nations 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 as measured in the Interna- tional Sexuality Description Project (ISDP) (Schmitt, 2005). We used the HPI to test hypotheses derived from sexual selection and life history theories, including predictions that heightened human polygyny would be associated cross-nationally with increased intrasexual competition (e.g., high male-male aggression), higher pathogen stress, and more prolific levels of reproduction. Support for these predictions was expected given what is known about polyg- yny, reproductive success, and sexual selection in the natural world. ∗ Direct correspondence to David P. Schmitt, Department of Psychology, 75 Bradley Hall, Bradley University, Peoria, IL 61625 〈dps@bradley.edu〉, or to Percy A. Rohde, Universit¨ at Kassel, Fachbereich 01, Institut f¨ ur Psychologie, Holl¨ andische Str. 36–38, D-34127, Kassel, Germany 〈percy.rohde@t-online.de〉. The authors shall share all data and coding for replication purposes. The authors would like to thank Melinda Jacobs, Jessica Nadler, and Jeffrey Taylor for their comments on an earlier draft of this article. SOCIAL SCIENCE QUARTERLY C  2013 by the Southwestern Social Science Association DOI: 10.1111/ssqu.12030