Secret of Sticking to partner REVEALED!

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.