Preliminary Abstract: The Human Biology subfield within Physical Anthropology seeks evolutionary explanations for biological variation among living humans. Much of the work in Human Biology related to female reproductive function has been heavily focused on evolutionary models to explain the responsiveness of ovarian steroid production to ecological conditions. Underlying functionally significant, genetic variation that also likely impacts reproductive phenotypes has seldom been investigated, as has the potential for genotype x environment interactions in reproductive physiology. The proposed project addresses this problem by using an additive-multiplicative model to investigate whether a high-frequency, functionally significant genetic variant of the human progesterone receptor (PROGINS) interacts with certain environmental or life history variables in the production of menstrual cycle characteristics. A small population of women in the Philadelphia, PA area will be screened for the PROGINS polymorphism, and gynecological history, physical activity, menstrual cycle diary, menstrual blood loss, and ovarian hormone data will be collected to test the model.