Science might be able to explain our fascination with Brad Pitt's chiseled jaw and George Clooney's smoldering eyes.

Women seem to judge potential mates by how masculine their features are, new research shows. Men with square jaws and well-defined brow ridges are seen as good short-term partners, while those with more feminine traits such as a rounder face and fuller lips are perceived as better long-term mates.

In the study, 854 male and female subjects viewed a series of male head shots that had been digitally altered to exaggerate or minimize masculine traits.  The participants then answered questions about how they expected the men in the photos to behave.

Overwhelmingly, participants said those with more masculine features were likely to be risky and competitive and also more apt to fight, challenge bosses, cheat on spouses and put less effort into parenting.  Those with more feminine faces were seen as good parents and husbands, hard workers and emotionally supportive mates [compare examples]. 

Despite all the negative attributes, when asked who they would choose for a short-term relationship, women still selected the more masculine looking men.  Brad and George then would be picks for a brief romance, if not the long haul.

Makes sense

The study, detailed in the December issue of the journal Personal Relationships, reached conclusions similar to research published earlier last year in Britain.

The new study's author, Daniel Kruger at the University of Michigan's School of Public Health, said that from an evolutionary perspective, it makes sense women would view more masculine-looking men as potential flings and less masculine-looking ones as long-term partners. 

The key, he said, is testosterone, the hormone responsible for development of masculine facial features and other secondary sexual characteristics.

Testosterone is necessary for development, but can also have detrimental health effects. It has been shown, for example, to interfere with the body's immune response, so men who are able to maintain high levels of the hormone are typically strong and healthy—traits women would want to pass on to their progeny. 

Increased testosterone has also been linked to male cheating and violence in relationships, so while these men might produce high quality offspring, they don't always make great parents or faithful mates, Kruger says. 

The study suggests women could be equipped to use seemingly superficial characteristics "as a cue to pick up on trends in these behavioral strategies," Kruger said.

Get a clue

There are plenty of these signals in the animal world. Male peacocks' huge, outrageous tails can make foraging for food and evading predators difficult, but the plumage, which many researchers say indicates male fitness, is so effective at luring females that the trait has been preserved in the population, Kruger points out.

While the findings are compelling, the scientific community has typically greeted the field of physiognomy, which links facial characteristics to certain behavioral traits, with skepticism.

Kruger argues, however, that the research is a valuable tool for understanding mating strategies. And, of course, for explaining how Pitt and Clooney managed to snag People Magazine's "Sexiest Man Alive" title two times each—it might have to do with their genes, but could also have something to do with ours.