Penis size is an important factor in male attractiveness, a new study suggests. It's as important as height and more important than shoulder-to-hip ratios, the study says.

Of course, this is a kind of weird study, because usually we meet mates while wearing clothing — and they shouldn't be able to determine penis size.

It would be relevant, though, before humans invented clothing. And it could have influenced how the penis evolved, because without clothes, humanity's upright posture a man's "protruding, non-retractable genitalia" would have been clearly visible to potential mates.

Females may have preferred to mate with males with larger penises, hoping to pass on the attractive trait to their sons. This could be what led to the human penis being longer and thicker than it is in any other primate.

Science Magazine suggests:

Women may have selected for larger penises because they're linked to higher rates of female orgasm and sexual satisfaction, which may explain why the human penis is proportionally larger than those of our evolutionary cousins.

Luckily, this has diminishing returns as the penises get bigger and bigger. As Nature News puts it, "men with substantially larger-than average features were not found much more attractive than those with only slightly above-average features."

The study was published in the journal Proceedings of the National Academy of Sciences. And yes, PNAS does sound a lot like penis. The researchers used 3D models of naked men with varying body shapes, height, and flaccid penis length — up to 5.1 inches. Here's an example of the three extremes in the mock up, which they projected onto the wall life-size:

They showed images like these to 105 Australian women, to see how they would judge each man for attractiveness.

Discover Magazine's D-brief blog explains:

Female participants in the new study who viewed random subsets of the life-size figures were also quick to rate those they found unattractive, but spent significantly longer gazing at the computer models they found appealing, particularly those with longer penises. This pattern is consistent with a human tendency to view attractive things for a longer time than those perceived as unattractive.

That being said, having a longer penis had a bigger effect on tall people. Short men were less attractive, even if they had a longer penis, the researchers write in the paper: "This result could be because perceived penis size was smaller when assessed relative to the height of a taller man; or because of general discrimination against short men irrespective of the value of other traits, so that even a larger penis did little to increase their net attractiveness."