A whiff of oxytocin lit up reward center in men's brains when they looked at their partner's face.
Scientists, and women everywhere, have long wondered exactly what keeps a man from straying with a stranger.
From a biological perspective, at least, cheating is easy to understand. The more sexual partners a man has, the more likely he'll be to pass on his genetic material.
So why do so many men settle down, get married and stick around to raise their kids?
Researchers think they may have found a clue in oxytocin, a hormone released during sex and other intimate gestures like hugging or holding hands that's been proven to strengthen social bonds in other mammals.
They found that the hormone appears to boost men's attraction to their mate -- even when presented with pictures of other women.
The findings are published in this week's issue of the Proceedings of the National Academy of Sciences.
In the study, 20 men who were in committed relationships for 28 months, on average, took whiffs of either oxytocin nasal spray or an inactive placebo spray.
For the first test, the men looked at pictures of their partner, a woman they'd never met, or a house. The photos of the women were carefully matched so one wasn't more attractive than the other.
In the second experiment, they looked at pictures of their partners or of women they knew but weren't related to, perhaps someone they saw at work every day.
Then the men rated the attraction they felt to the various faces. Men consistently rated their partners as being more attractive and arousing than the other women and, in most cases, a whiff of oxytocin boosted that effect compared to the placebo.
But what really fascinated the researchers was what happened inside the men's brains.
Under the influence of oxytocin, two areas of the brain responsible for feelings of reward and pleasure lit up when men saw their partner's faces. But the sight of other women had the opposite effect, suppressing feelings of pleasure.
"Oxytocin triggers the reward system to activate on the partner's face, the presence of the partner," said study author Dr. Rene Hurlemann, a professor of psychiatry at the University of Bonn, in Germany.
"Sexual monogamy is actually quite costly for males, so there must be some form of mechanism binding males and females together, at least for some time," Hurlemann said. "There must be some benefit, and reward is actually the strongest motivation underlying human behavior."
An expert who was not involved in the study said the results suggest that couples who keep a high level of intimacy in their relationships can maintain stronger bonds.
"When you're first becoming intimate, you're releasing lots of dopamine and oxytocin. That's creating that link between the neural systems that are processing your facial cues, your voice and the reward system" of a partner's brain, said Larry Young, a professor of psychiatry at Emory University in Atlanta. He studies the role of oxytocin in social bonding.
As time goes on, and couples become less intimate, Young noted that linkage can decay. But activities that release oxytocin, such as really looking into another person's eyes, holding hands, kissing and having sex may help restore the connection.
"To me, it suggests that it may be a way to help prevent the decay that can occur that leads couples to separate," he said.
Hurlemann agreed: "I think this is the only reason that we do hug and touch each other all the time. I think this is the mechanism that keeps oxytocin levels high in relationships."
