Worldwide, ~100 million women use oral contraceptives (OCs). Discontinuation of OC use due to concerns about possible side effects is common among women who do not intend to become pregnant. Many recent studies have reported that women using OCs and women not using OCs have different mate preferences, raising the possibility that OCs alter mate preferences. Although these findings have been widely reported in the popular press as evidence that OCs alter womens mate preferences, the between-subjects designs used in these studies mean that it is not known whether such differences reflect pre-existing differences in the mate preferences of OC users and non-users or occur because OCs alter mate preferences. If OCs alter mate preferences, knowledge of this phenomenon would likely figure in womens decisions about contraception. If OCs do not alter mate preferences, reports in the media that ascribe causality to the effects of OCs in between-subjects studies are very misleading. My research will combine within-subjects and between-subjects designs to establish if OCs alter womens mate preferences and if there are pre-existing differences in the mate preferences of OC users and non-users. The project is groundbreaking in 4 important ways: (1) it will resolve whether OCs alter womens mate preferences, (2) it will be by far the most comprehensive study of hormone-mediated social perception ever undertaken, (3) it will use sophisticated computer graphic methods to assess womens mate preferences using photorealistic static face images and more ecologically valid video stimuli, and (4) it will use powerful new statistical techniques (multilevel statistical methods) that have not previously been used to investigate hormone-mediated social perception. Importantly, the proposed research will provide essential information for womens contraceptive choices and illuminate the roles of endogenous and exogenous hormones in the regulation of social perception.