Blood Sample Collection and Assays: Baseline saliva samples for estradiol, progesterone and a baseline blood sample for OT will be collected from each participant 15-30 minutes prior to the OT/PBO administration. Blood will be collected in tubes containing ethylenediaminetetraacetic acid (EDTA). Tubes will be centrifuged at 1500 rpm at 4˚C. Plasma will be stored at −70˚C. Determination of OT will be performed using a commercial ELISA kit (Enzo Life Sciences). Intra-assay coefficients of variation for OT average ~3-5% and inter-assay variation is typically less than 10%.