Bad news for anyone who… well, everyone, really.

Men given a dose of oxytocin, a hormone known to promote feelings of love and trust, have revealed the chemical’s dark side: It made them more ethnocentric.

When asked to resolve a moral dilemma, such as choosing to save five lives from a runaway train by sacrificing one life, oxytocin-sniffing Dutch men more often saved fellow countrymen over Arabs and Germans than those who didn’t get a hormonal whiff.

“Earlier research of oxytocin paints a very rosy view of it. We thought it was odd a neurological system that survived evolution would make people indiscriminately loving toward others,” said social psychologist Carsten De Dreu of the University of Amsterdam, co-author of a Jan. 10 study in the Proceedings of the National Academy of Sciences. “Under oxytocin we saw an increase of in-group favoritism, which has the downside of discrimination against people who are not part of your group.”

Oxytocin is a hormone made in the brain and [euphemism alert] some reproductive organs. The body releases the biggest doses of it into the bloodstream during intimate situations (such as caressing), and from there it can dampen fight-or-flight instincts and calm down organs such as the heart.

As a neurotransmitter, it’s also intricately involved in social behaviors such as mother-child bonding, feelings of trust and love, and group recognition.

De Dreu’s “research flies in the face of how we’ve thought about oxytocin for decades. It’s not all about free love and warm fuzzies,” said neuropsychologist Sarina Rodrigues of Oregon State University, who was not involved in the study. “It complements recent data showing oxytocin can promote envy when someone you don’t like wins something, or gloating when you win over someone you don’t like. It’s key to defining where and who we are in society.”

Continues: http://www.wired.com/wiredscience/2011/01/oxytocin-social-favoritism/