increased oxytocin did not predict where they donated their money. But there are some caveats. The more marginalized a group felt on campus, the more likely they were to circle their wagons and favor their own in-group (presumably, the band nerds weren’t as generous as frat boys to other groups). The effects of oxytocin could also change depending on what else was happening in the body: If Zak’s lab induced stress or acted to jack up testosterone, participants could, in fact, become more aggressive toward out-groups.

oxytocin is involved with attachment and social bonding, but that neural system can get tangled up in fear and anxiety—it gives us a visceral memory of those who have harmed us, as well as those who have cared for us.

oxytocin doesn’t simply make you all lovey-dovey, suggests this study. It also keeps you faithful to your partner—and wary of her rivals.

Each group watched a series of images and the individuals in the group voted for which ones they found most attractive. The results: The oxytocin-influenced participants tended to go with the flow of their group, while the placebo-dosed participants hewed to their own individualistic path. The implication: Oxytocin is great when you’re out with friends or solving a problem with coworkers. It might not be so great when you need to pick a leader or make some other big decision that requires independence, not conformity.

oxytocin doesn’t just bond us to mothers, lovers, and friends—it also seems to play a role in excluding others from that bond. (And perhaps, as one scientist has argued, wanting what other people have.) This just makes oxytocin more interesting—and it points to a fundamental, constantly recurring fact about human beings: Many of the same biological and psychological mechanisms that bond us together can also tear us apart. It all depends on the social and emotional context.

“The narrow thinking that medications are the only way to control persistent pain,” Dr. Arnstein concluded, “has resulted in a lot of suffering.” Researchers have discovered a physiological basis for the warm glow that often seems to accompany giving. “The benefits of giving back are definitely biological,” says bioethicist Stephen G. Post, co-author of Why Good Things Happen to Good People. “Contemporary neuroscience has confirmed the connection between the physiological and psychological. We know now that the stress response, hormones, and even the immune system are impacted by, and impact, the pathways in the brain. MRI studies of the participants’ brains revealed that making a donation activated the mesolimbic pathway—the brain’s reward center.”