Oxytocin is a powerful hormone that acts as a neurotransmitter in the brain. It influences social interaction and sexual reproduction, playing a role in behaviors from maternal attachment to an infant and milk release to empathy, generosity, and orgasm. When people hug or kiss a loved one, oxytocin levels increase; hence, oxytocin is often called "the love hormone." In fact, the hormone plays a huge role in all pair bonding. The hormone is greatly stimulated during sex, birth, and breastfeeding. Oxytocin is the hormone that underlies individual and social trust. It is also an antidote to depressive feelings. For all its positivity, however, oxytocin has a dark side. Or, more accurately, it plays a more complex role in human behavior than is commonly thought. As a facilitator of bonding among those who share similar characteristics, the hormone fosters distinctions between in-group and out-group members, and sets in motion favoritism toward in-group members and prejudice against those in out-groups. Ongoing research on the hormone is a potent reminder of the complexity of biological and psychological systems.