Once bitten, twice shy, unless your brain is flooded with oxytocin.

Swiss researchers have shown that the hormone first discovered for its role in labor and lactation also helps people learn to trust again after betrayal. The hormone has been linked to trust before, but the new study, published in the May 22 Neuron, is the first to demonstrate that oxytocin works on a brain region involved with fear processing and areas involved in anticipating reward and resolving conflict.

“When trust has been broken, something has to allow you to move on with your life and learn to trust again,” says Mauricio Delgado, a cognitive neuroscientist at Rutgers University in Newark, N.J. That something is oxytocin.

The chemical is important in being able to balance forgiving and forgetting with learning from mistakes, he says.

Researchers led by Thomas Baumgartner at the University of Zurich in Switzerland gathered 49 male volunteers to play games of trust and risk while in an fMRI scanner. Some of the volunteers got a nasal spray of oxytocin, while the rest received a squirt of placebo. The men could not tell the difference between the two nasal sprays.

One game involved the volunteers “investing” money (provided by the researchers) with a trustee. Half the time the trustee would share money with the investor. The rest of the time, the trustee pocketed all of the cash, violating the investor’s trust. In the other game, the volunteers played the lottery. This lottery paid off half the time just like the investment with the trustee, but the men didn’t feel betrayed if the lottery didn’t pay off. Researchers used the lottery game to determine how likely the men were to take risks.

After being betrayed, men were less likely to want to invest with a new trustee and were slower to hand over money if they had received the placebo spray than if they had received the oxytocin.

Both groups of men were equally likely to risk money in the lottery game.

Brain scans of the volunteers revealed that oxytocin dampened activity in the amygdala, a part of the brain that helps regulate emotions such as fear. The amygdala has been shown to be involved with judging the trustworthiness of faces, but the new study is the first to show that oxytocin can alter activity in that part of the brain during a trust exercise, Baumgartner says.

This study addresses oxytocin’s role in people’s general trust for other human beings after being betrayed, but does not show how oxytocin shapes the future of a relationship after a friend or family member breaks trust. The study also takes place under controlled laboratory conditions and doesn’t address what happens in the real world.

“What happens with free-roaming humans? How do they learn to trust each other?” asks Paul Zak, a neuroeconomist at Claremont Graduate University in California. “They certainly don’t do it by spraying stuff up each other’s noses.”

Women are more powerfully affected by oxytocin, Zak says. He expects that a similar study on women might show larger differences in amygdala activity between the group on oxytocin and the placebo group.

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