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Washboard abs and a chiseled jaw are a draw, but a recent study suggests it may be a man’s healthy immune system that really turns women on.

That makes sense, since a strong immune system signifies a healthy guy — one who’s likely to survive long enough to pass his rugged genes to the next generation. But the more surprising thing is that women can apparently spot good immunity by looking at a guy’s face.

The key factor may be testosterone, the male hormone that has been associated with a man’s hardy good looks. According to a study published in the journal Nature Communications, testosterone levels are not only linked with facial attractiveness, but also with a strong immune system response.

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To figure out the complex relationship between hormones and hotness, researchers from Abertay University in the U.K. studied 74 Latvian men in their early 20s. They gave the men a Hepatitis B vaccine, which triggers the immune system to produce antibodies to fight the virus, and took blood samples before and after. Using the blood samples, researchers measured the men’s antibody levels, as well as levels of testosterone and the stress hormone cortisol.

The researchers then showed photographs of each man to 94 Latvian women, who were also in their early 20s, and asked the women to rate the pictures on a 10-point scale of attractiveness.

Turns out, the most highly rated men were those with stronger immune responses and higher levels of testosterone. “The more antibodies a man produces in response to a vaccine, the more attractive his face,” Fhionna Moore, co-author of the study and a psychologist at Abertay University, told Cosmos magazine.

Researchers also found that the link between testosterone, immune strength and attractiveness was most robust in men who had the lowest levels of the stress hormone cortisol. What does that mean? The findings suggest that a man’s stress levels (high stress equals high cortisol) may interfere with his testosterone’s mate-attracting allure, reports Health.com. It all goes back to the “handicap theory” of sexual selection, according to the authors.

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Health.com’s Anne Harding explains:

Biologists who study mate choice across species have suggested that attention-getting male features — such as the peacock’s tail, or the deer’s antlers — are attractive to females because they’re largely useless and impractical. They’re often a disadvantage in areas of life besides attracting a mate, but they signal to females that a male is healthy enough to divert some energy away from surviving and put it towards looking good. “The very fact that you’re able to stay alive with this handicap means that the female choosing you is getting a high-quality mate,” says Anthony Little, Ph.D., a research fellow at the University of Stirling, in the United Kingdom, who did not participate in the new research but has studied how human faces communicate information.

Being handsome isn’t exactly a handicap in the way that having a head full of antlers is, but human mating still involves attention to good looks, appearance and other aspects of fine plumage.

“The idea is that there are always individuals with good genes who can afford to have high testosterone — to be highly attractive — without paying the cost for this,” Dario Maestripieri, a professor of comparative human development at the University of Chicago, told Cosmos. And the new research suggests these men might well be those who know how to handle their stress.