BEIJING, Sept.15, 2010 (Xinhuanet) --

 Calm, collected guys seem more attractive to women in real life, and now scientists think they have found out the biology behind it, said media reports Wednesday.

The findings will be published online Proceedings of the Royal Society B. 

Researchers often focus on testosterone when investigating into what makes men desirable, but human behavioral ecologists at the University of Abertay Dundee in Scotland focus on the stress-linked hormone cortisol.

Persistently high levels of cortisol can suppress not just the immune system, but also reproductive function. As such, it would make sense if women preferred men with low cortisol levels -- that is, those who are not stressed out.

The researchers recruited 39 healthy young male students from the same university and measured their cortisol and testosterone levels, and then had 42 straight female students from a different university rate photos of these men for attractiveness, masculinity and health.

Men with low cortisol levels were often rated as more attractive than guys with high cortisol levels. Testosterone levels were not significantly linked with attractiveness, masculinity or health.

The researchers also had 43 heterosexual female university students look at the composite images at times both inside and outside the fertile phases of their menstrual cycles. This enabled the scientists to see what effects female hormones and fertility might have on perceptions of male desirability.

When women were in the fertile phase of their menstrual cycle -- that is, when they were more likely to get pregnant -- those men with low cortisol levels (so were likely more chill) were seen as more attractive than men with high cortisol levels.

"We speculate, then, that males with low cortisol possess something desirable that women seek to secure for their offspring," one of the researchers Fhionna Moore said. "This could be, for example, good health or a healthy response to stress."

(Agencies)