Oxytocin, the so-called love hormone, activates feelings of trust and attraction between people when it is released in the brain, and it rises in the early stages of romantic love. Yet it is not just a Cupid's arrow that spurs you to fall in love with the nearest person, according to a recent study published in the Journal of Neuroscience. Instead oxytocin's social magic depends on whether or not a person is in a monogamous relationship.

A team of researchers from the University of Bonn in Germany and elsewhere monitored a first encounter between straight male study participants and an attractive woman in a laboratory setting. When given oxytocin via an intranasal spray before the meeting, men who had indicated they were currently in a stable relationship kept a greater physical distance from the woman in the lab compared with single guys given oxytocin and with single and “taken” guys given a placebo. Although they stayed only 10 to 15 centimeters farther away, the extra distance left the woman outside of what most people consider “personal space,” a zone reserved for loved ones. Moreover, it was not because they did not find her attractive: the monogamous men who received oxytocin rated the woman just as good-looking as the other men did.

The results suggest that oxytocin has a role in maintaining relationships after they are sparked and add to growing evidence for differences in how the hormone acts to modulate social interactions—for example, promoting bonds with familiar people but provoking aggression with strangers. “It's not all positive with oxytocin,” says Dirk Scheele, a psychologist involved with the study. “And what you call prosocial or antisocial depends on your perspective.”