Oxytocin is often thought of as a "love drug," and is linked with all kinds of feel-good emotions in people such as trust, empathy and generosity. Increasingly, however, scientists are finding that the hormone has a dark side—and now researchers have discovered it also can promote ethnocentrism, potentially fueling xenophobia, prejudice and violence.

Past studies have shown that oxytocin fosters social feelings—between mates, for example, or mother and child—which explains why this "cuddle chemical" might be linked with goody-goody behavior such as altruism. Social feelings, however, are not always positive ones, reasoned social psychologist Carsten de Dreu of the University of Amsterdam in the Netherlands. For instance, ethnocentric people view their own group as better than others—they feel closer to their compatriots, but potentially at the expense of outsiders.

In five experiments 280 male Dutch volunteers first inhaled a spray containing either oxytocin or a placebo. In the first two experiments participants hit one button if they saw a positive word such as "wonderful" or the name of a fellow Dutchman on the computer, such as Maarten; they pushed another key if they saw a negative word such as "awful" or a foreign name. These groupings were then reversed—the volunteers pushed one button if they saw a positive word or a foreign name, and the other if they saw a negative word or a domestic name.

Arab names such as Ahmed were used in the first experiment and German ones such as Helmut in the second. The researchers found that in both cases, volunteers given oxytocin were faster at linking fellow Dutchmen with positive words. There was also significant evidence that Arabs were more often linked with negative words, and weaker signs that Germans were.

In a third experiment the scientists investigated the extent to which volunteers ascribed uniquely human emotions such as admiration or contempt to fellow countrymen or foreigners as opposed to more basic ones such as exhaustion and pleasure. Participants given oxytocin were significantly more likely to associate more human emotions with Netherlanders than with Arabs, although they were equally likely to associate positive and negative emotions to either group.

In the last experiments volunteers were given the option to save the lives of five nameless people by sacrificing one other person. These lone individuals either had typical Dutch male names, or foreign ones—Arab names in the fourth experiment and German ones in the fifth. Participants given oxytocin sacrificed foreigners more often than fellow Dutchmen. This isn't because they mercilessly sacrificed more outsiders than volunteers given placebo did—rather, they protectively sacrificed fewer fellow Dutchmen.

The net result is that oxytocin makes people prefer their own group over other groups "and thus sets the stage for prejudice and social discrimination," de Dreu says of findings detailed online January 10 in Proceedings of the National Academy of Sciences. "Our findings and those of others indicate that oxytocin should not be used for recreational purposes. And while oxytocin may help in therapeutic settings, much more controlled research is needed—there is a lot we do not yet know about oxytocin."

"Recent classes of theories explaining the role of oxytocin in social behavior have focused only on prosocial positive behaviors such as trust and generosity. However, oxytocin is also involved in maternal aggression and territoriality," says neuroscientist Simone Shamay-Tsoory at Haifa University in Mount Carmel, Israel, who did not take part in this study. "Administration of oxytocin may provoke a wide range of emotions and behaviors related to social behavior and parenting, such as trusting collaborators, feeling love and compassion as well as attacking potential intruders and competing with rivals."

"So are we hard-wired for prejudice?" asks social neuroscientist Susan Fiske at Princeton University, who also did not participate in this research. "You have to remember that groups are defined socially and culturally, and they can change over time based on short-term context and long-term history. So for people who hear about this and are discouraged about the state of the world and intergroup conflict, this doesn't mean prejudice is inevitable. The key is who you define as part of your group, and we know that's changeable."

One question that de Dreu and others may have is why oxytocin sometimes did and did not lead to bias against outsiders. "There may be specific situations in which oxytocin motivates out-group hate, and new research is needed to find out," de Dreu says. Research should also investigate the effects of oxytocin in female subjects, he notes.

It would be interesting to see if oxytocin could be used to overcome bias against outsiders, Fiske adds. "It could be that if an out-group is the only life preserver one has and if one is under personal threat, you could see attachment," she speculates. "After 9/11, a lot of people in the United States commented that a lot of racial tensions temporarily took a backseat."