The usual rules of sexual attraction go out of the window when men are stressed, say psychologists

Men are drawn to a wider range of women when they are feeling stressed out, according to research into the psychology of sexual attraction.

People are usually attracted to partners with similar facial features to their own, but after a brief but stressful experience, men's preferences changed to include a wider variety of women, the study found.

Relaxed men who took part in the study rated women on average 14% less appealing if they looked very different from themselves compared with women who looked similar. But a group of stressed men found dissimilar women 9% more attractive.

Johanna Lass-Hennemann, who led the study at the University of Trier in Germany, said the findings echo research suggesting that animals lose their normal mating preferences when they are under stress.

"Men have a tendency to approach dissimilar mates and to rate these to be more pleasant when they are acutely stressed," Lass-Hennemann said. "[But] we are not sure how this might reflect in true mating decisions."

Scientists suspect the appeal of similar-looking partners may be linked to our tendency to have more trust in a familiar face, a factor that is important for long-term relationships. Under stress, however, the importance of pairing up with someone similar-looking seems to vanish.

Lass-Hennemann speculates that stress might increase men's tendency to "outbreed", or reproduce with more genetically dissimilar women, with the potential benefit that any children born from the relationship might be better equipped to cope with a stressful environment.

"We think that chronically stressful environments should increase outbreeding, because inbreeding may lead to offspring that are not genetically diverse enough to deal with the varying circumstances that a risky and stressful environment imposes on them," she said.

In the study, 50 healthy heterosexual male students were divided into two groups. Those in the first group were asked to plunge one arm into a bucket of icy water for three minutes before taking part in the test. Those in the second group were asked to do the same, but with water heated to body temperature.

Measurements of the volunteers' heart rates and levels of the stress hormone cortisol indicated that the men in the first group were significantly more stressed before the test began than those in the second.

In the test itself, the men were shown a series of images on a computer screen. Some were of household objects, but others were of naked women. Some of the women's faces had been digitally altered to resemble either the person being tested or another man in the group.

Throughout the test, the scientists played occasional bursts of noise to startle the men and recorded their reactions. Previous research suggests people startle less when they are looking at something they find attractive. The men were also asked to rate the images by how appealing and arousing they were.

While men in the control group performed as expected and were more attracted to women who looked like them, the stressed men consistently rated the unfamiliar women as more appealing. Their startle reactions confirmed their preferences.

The research is published in Proceedings of the Royal Society B.

Lass-Hennemann said it is highly unlikely that the acute stresses of everyday life can switch someone's tastes when it comes to choosing a partner, but long-term stress might shift male preferences towards women who are more dissimilar.