Based on the existingknowledge of maternal PPD, weconjecture that PPD experienced bya father might be caused byhormonal changes occurring duringhis partner’s pregnancy andpostnatal period. We will proposeseveral biological factors for theonset and development of thepaternal PPD.First, paternal PPD might berelated to changes in histestosterone level, which decreasesover time during his partner’spregnancy and postpartum period.Testosterone levels started todecrease at least a few monthsbefore the childbirth and maintainlow levels for several months afterthe childbirth among most offathers.