The love hormone.
Look up “oxytocin” and you’ll find articles praising its life-facilitating and social bonding roles, along with ads for pills and nasal sprays. Practically a celebrity in the hormone world!
A now-famous 2005 study published in Nature titled “Oxytocin increases trust in humans” kickstarted the popularity of this neuropeptide. Despite the golden reputation of oxytocin, its effects aren’t all sunshine and roses. Like many factors in psychological behavior, it depends on the situation. Context, context, context.

Oxytocin is especially important during childbirth and lactation. It’s also been shown to play a role in relationships, face recognition, and maternal-infant care. It promotes the strengthening of social and pair bonds, such as during a fascinating process known as social synchrony, when mother and child coordinate their behaviors. It even helps facilitate the bond between a person and their pet dog! Yet it also increases xenophobic and ethnocentric behaviors, and the explanation behind us seeking heartwarming social moments can help reveal why.

Humans have an evolutionary need to understand social cues and be prosocial, especially to those on our side, since those people can help us survive. Oxytocin promotes these behaviors, and because not everyone can be on our side, it then makes us quite nasty to those who aren’t. Our tendency to form these in-groups and out-groups can be based on quite arbitrary boundaries, but this also makes them easy and quick to form.

In several studies published in 2011, Dutch scientist Dr. De Dreu showed that oxytocin makes us lousy and uncooperative towards people who we see as Other. For context, all the subjects were men, and the extra oxytocin was administered through a nasal spray. Oxytocin encouraged participants to share more money with teammates, demonstrating the previously known ease of forming groups and the generosity of oxytocin.

Here’s where it gets interesting. When given the choice to share money with or backstab a player from the other team (in what is known as the Prisoner’s Dilemma), oxytocin actually increased backstabbing behavior. It made biases against people of other races more pronounced when participants took an Implicit Association Test, which tests unconscious bias. Finally, in one of the most shocking parts of the studies, oxytocin promoted bias against people simply because they had names stereotypical of other cultures, even in life-or-death situations. Participants had to choose whether to kill a named person to save five unnamed people and if the person’s name was from a different culture, the participants were more likely to sacrifice them.

The extent that oxytocin affects individuals depends on, well, the individual and their environment.
Both sides of oxytocin’s effects reflect another important part of psychology and neurobiology: hormones don’t explicitly cause behaviors but instead increase those we are already prone to. This demystification of oxytocin is not meant to make the hormone–or humans in general–seem evil or nasty, but rather emphasize the importance of context in studying human behavior.