Forget the chiseled abs, strong cheekbones, and iron biceps. It turns out that the biggest turn-on for women is a man's powerful... immune system. A new study in Nature Communications found that women can somehow tell just by looking at a man that he's got high levels of the manly hormone testosterone and a rock-solid disease-fighting system, and that they find the combination extremely sexy. The hottest men, the study found, had great immune systems, lots of testosterone, and low levels of the stress hormone cortisol. Here's what you should know:

How did the study work?
Psychologist Fhionna Moore and her team at the Abertay University in Scotland selected 74 Latvian men in their early 20s, and tested their testosterone, cortisol, and antibody levels before and after an immune-system-challenging hepatitis B shot. Then they showed headshots of the men to 94 Latvian women in the same age group, having them rate the men's attractiveness on a 10-point scale. The hotness ratings were then matched to each man's hormone levels and immune response. "Bottom line," says Shauna Wright at The Checkup: "Macho men look healthier."

Why is a good immune system so attractive?
Ruggedly handsome facial features have long been tied to high testosterone levels, which are in turn tied to healthy immune systems. If a man is both healthy and sexy, he's a good bet for creating strong offspring. It makes sense that powerful immune systems would be attractive in men, says Cassie Murdoch in Jezebel. "They produce sturdier children, they don't cough all the time, [and] you don't have to take care of them when they're always sick."

What role does stress play?
Cortisol tends to weaken the immune system, and men with high cortisol levels also had low testosterone levels. Researchers suggest that cortisol might keep testosterone from working its sexy magic on women, or even make a man's face less attractive. "Hollywood seems to have beaten science to the punch," says Anne Harding at "It's no secret that there's something attractive about a man who seems relaxed and cool under pressure."

What can a guy do to up his sexy quotient?
More research needs to be done to determine how to mix up a "healthy hormone cocktail." But the study suggests you might have better luck with the ladies if you're in good shape and not stressed out. There also has to be an upper limit on testosterone levels, since "body builders who dope themselves on steroids until they look like they're going to pop" look neither healthy nor sexy, says Jezebel's Murdoch. But assuming these findings pan out for people who aren't Latvians in their early 20s, "it's probably only a matter of time before we're eagerly awaiting the release of People's Sexiest Immune System Alive issue."

Sources: The Checkup, CNN/, Cosmos, Jezebel, LiveScience, Medical News Today