Washboard abs and a chiseled jaw are a draw, but a recent study suggests it may be a man's healthy immune system that turns women on.
That makes sense since a robust 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 spot good immunity by looking at a guy's face.
The critical factor may be testosterone, the male hormone that has been associated with a man's healthy, 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 robust immune system response.
To figure out the complicated 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.defense-1403067_640
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.
It turns out that 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.