Women may drool over George Clooney and Brad Pitt, but their lust may be more for these macho guys' immune systems than their pretty faces and chiseled abs, new research suggests.

Men with high levels of the sex hormone testosterone are seen as more hunky — and these same men have stronger immune responses, researchers report Tuesday (Feb. 21) in the journal Nature Communications. The findings suggest that women may be attracted to manly facial types because the macho look signals good health.

Researchers led by Fhionna Moore of Abertay University in the United Kingdom recruited 74 Latvian men in their early 20s to give blood samples right before and one month after their first dose of the Hepatitis B vaccine. The vaccine triggers the immune system to create antibodies against the virus. The researchers measured these levels of antibodies as well as testosterone levels and levels of the stress hormone cortisol.

Next, 94 Latvian women, also in their early 20s, rated photographs of each man on a 10-point scale of attractiveness. The researchers then looked for links between the immune response as measured by Hepatitis B antibodies, hormone levels and attractiveness.

They found that high testosterone correlated with both sexy faces and a strong immune response. Men with the strongest immune responses were rated as better looking than those with weak immune responses. The link between testosterone and hotness was strongest in men with low levels of the stress hormone cortisol, suggesting that stress might take a toll on the immune system, and thus women's ratings of attractiveness.

While much research has hinted at a relationship between testosterone and the immune system, this study is the first to directly link women's opinions of a man's looks with the strength of his immune system. Researchers next hope to tease out whether the results hold across cultures and across different age ranges.

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