When a guy catches your eye, what draws your attention?

Some people may say nice eyes get them going, while others say a manly, chiseled jaw or big strong arms. Evolutionarily, though, a man's body fat, not masculine features, is a better indicator of how attractive of a mate he would be, a new study suggests.

With any luck, these traits of an "optimal" mate would be genetically encoded and would be passed on to the man's offspring — so a female wants to choose a man with good genes and a healthy immune system. We just need to know how to figure out if a potential mate has these traits.

Masculine looking males were believed to be a good indicator of good mate choice because their looks were thought to indicate better genes and a stronger immune system, one that could take on the negative effects of high testosterone levels (which make men more prone to infection).

A new study, published today Nov 27 in the journal Proceedings of the Royal Society B: Biological Sciences, switches that on its head. The researchers found that a man's waist size is more important than how masculine his face is, when it comes to attractiveness and good immune systems.

Twenty-nine women in the fertile stage of their menstrual cycle (meaning they were more receptive to sexual cues, and more responsive to masculinity) rated pictures of 69 young white males of all body sizes by their attractiveness, masculinity, and body fat.

They then took these rating and compared them to how well the men's immune system responded to a Hepatitis B vaccine. They also tested circulating testosterone levels in the participants.

The results showed that a man's masculinity rating didn't mean he was more attractive, and also didn't mean that his immune system worked better. How masculine a man was also wasn't an indicator of how much testosterone he had either.

On the other hand, body fat was linked to attractiveness, high testosterone levels, and a stronger immune system. Too much fat was bad too, though: Body fat attractiveness had a bell-shaped curve. The most attractive men had 12 percent body fat.

When he was above normal body fat — obese or overweight — the immune system was negatively impacted.

From their data, the researchers made a composite image of the bottom eight men with the cruddiest immune systems on the left, and the eight men with the best on the right. Which do you think is more attractive?