Evolutionarily speaking, it's kinda weird how people are super into butts, since every derrière is a source of gross bacteria. And yet, men are attracted to behinds for procreative reasons. As biopsychologist Dr. Gordon G. Gallup, Jr. of the University of Albany told MTV last year:

“The reason narrow waists and broad hips are so prized -- the reason males rate these as being attractive, even though they don’t have any insight into why they do -- is two-fold. It means, if a woman has a narrow waist, she’s not pregnant. And if she has broad hips it means that the underlying skeletal morphology is probably such that she’d be able to have a relatively unencumbered childbirth."

That settled that. But(t) we couldn't help wondering: Since men don't usually pop out babies, what's the possible scientific reason for women thinking a guy's butt is cute, as some of us have been informed by our significant others when we try on a new pair of boxer-briefs?

We googled around the interwebs, and found ... nothing. Well, nothing aside from NSFW pics and nonscientific comment forums. Take it away, Yahoo Answers:

There's also a Reddit thread devoted to the topic, and while it's pretty entertaining, there's not much in the way of traditional clinical research:

OK, point taken...

...SERIOUSLY POINT TAKEN...

Apparently, despite the extensive research on why bros are all about that bass, there isn't a single study on why ladies are. Double standard! Underrepresentation of women in the sciences!

However, there is one woman in the sciences who has conducted some related studies: Dr. Kerri Johnson of UCLA, a leading specialist in the psychology of human attraction, who extrapolated her findings to explain why you hate to see him go but love to watch him leave.

Women aren't likely attracted to the butt itself -- as its own separate entity -- but rather attracted to how it enhances male body movement.

“Our paper on attractiveness of body shape in motion focused on waist and hips, and how curvy the body is, which incorporates the behind region,” Dr. Johnson says. What she found is that heterosexual women make two snap judgments within microseconds: Is this a male, and if so, is he masculine?

“Bodies that were judged to be men ... were judged to be more attractive when they were also perceived to be masculine," Dr. Johnson says. "That combination produced the most attractive ratings for men.” And how a guy moves enormously informs a woman's opinion of that.

It's not just his butt that matters -- it's his shoulders in relation to his butt.

“There are many differences in the body movements of men and women," Dr. Johnson says. "One of the biggest is lateral shoulder movements, side-to-side movements. Men do that much more; women in contrast move their hips ... whereas men’s walk goes side-to-side and has high levels of shoulder motion, low levels of hip motion."

According to her research partner, Dr. Louis G. Tassinary of Texas A&M University, "A masculine walk is one in which the shoulders 'twist' more relative to the natural 'twist' of the hips. In old school terms, think John Wayne vs. Marilyn Monroe." He specifically says "a .85 [waist-to-hip ratio] combined with wide shoulders" is, scientifically speaking, the best look for a guy.

The butt is made of muscle, and there's a Golden Rule about muscles.

A flat butt is no good, according to science (sorry, Hank Hill), but a steroid butt is no good either if it puts the max in gluteus maximus (sorry, Powdered Toast Man).

“I like the Goldilocks [comparison]," Dr. Johnson says. "Goldilocks only evaluates her man to be more attractive when he’s just right -- the hyper-masculinity is perceived as kind of threatening. ... Having large muscles is not the same thing as having toned muscles; big, bulky muscles are perceived as less attractive than well-toned, moderately sized muscles."

There's a delicate balance in the middle, where "too little is unattractive and too much is unattractive."

And Dr. Johnson says that logic could be applied to booties: "There’s a sweet spot -- just enough muscles, but not enough that you look scary or formidable. You might take that to the appreciation of the man’s backside; I suspect there is an optimal level of muscularity. ... You probably don’t want, on a man, a muscular tush connected to a curvy body."

If women find themselves obsessed with Olympic swimming every four years, there may be a quantifiable reason besides love of aquatic sport.

"The prototypical body shape is that swimmer’s v-body, where there’s broad shoulders that taper down to the waist, straight down to the hip," Dr. Johnson says. "We have some unpublished research showing that a tapered [male] body with just the right level of muscularity is going to be found the most attractive.”

By the way, gay dudes are probably looking for those same attributes. “Men who exhibit more feminine body motions are more likely judged to be gay," Dr. Johnson says, but “research suggests they find cues attractive in much the same way as straight observers. Gay men on average prefer masculine men and lesbian women on average prefer feminine women."

For sustained attraction, however, even the most well-formed butt can't make up for a mismatched personality.

“We know body movement does impact attractiveness," Dr. Johnson says, but it's not the end-all, be-all. "Aspects of personality -- kindness, sense of humor -- can influence attractiveness [just like] the body’s shape and motion can influence that."

Ultimately, she says, we're most attracted to someone who reminds us of ourselves: “Intelligence, political party affiliation, physical attractiveness levels, interests -- from all of the research on personal attraction, the overwhelming conclusion is that ‘birds of a feather flock together’ has more basis than ‘opposites attract.’"

In other words: No matter how perfect you think his ass may be, a relationship with him is going nowhere if you also think he's talking out of it.