WEDNESDAY, February 22, 2012 (Health.com) — Scientists have been trying to confirm what Hollywood has known for decades: Women are often attracted to men with chiseled cheekbones and lantern jaws.

These and other masculine facial features are associated with high testosterone levels, and women seem to know this fact intuitively. Studies suggest that women are especially drawn to rugged-looking guys when they’re in the mood to mate (i.e., when they’re ovulating), while at other times, when they’re more interested in companionship, they tend to prefer a softer, more delicate look—think Ryan Gosling versus Burt Lancaster.

It’s not entirely clear why testosterone-fuelled features are attractive, but some experts have proposed that they’re a sign of a healthy immune system. Testosterone dampens immune function, so a man with high testosterone who’s still standing, the theory goes, is likely to have a stronger-than-average constitution. And he’ll pass those hardy genes on to his offspring, making him an attractive mate choice.

A new study, published this week in the journal Nature Communications, provides the strongest evidence yet to support this theory. It also adds a new wrinkle: stress.

n the study, men with higher testosterone levels tended to have stronger immune systems and faces that were more attractive to women. This relationship, however, was especially pronounced in men with low levels of cortisol, a stress hormone involved in the so-called fight-or-flight response.

This finding, researchers say, suggests that a man’s stress levels may play a key role in whether his testosterone is free to work its magic on women. Cortisol may even have a direct impact on a man’s facial features, although it’s much too soon to say if that’s the case or not, says Benedict Jones, Ph.D., a psychologist at the University of Aberdeen, in Scotland, who researches the influence of hormones on attractiveness.

“The important thing here is that…we’re seeing evidence that the relationship between men’s facial attractiveness and their hormone levels is probably much more complex than we once thought it was,” says Jones, who was not involved in the new study. “The interplay between cortisol and testosterone seems to be more important than the effects of any one hormone.”

Why is testosterone so sexy and cortisol, apparently, such a turnoff? The answer may lie in something called the “handicap hypothesis.”