What explains the rise of the rump? Is it an of-the-moment fixation brought on by celebrities who've got buns? Or is it more deep-seated, our desire for deep seats? Has humankind—or mankind, specifically—always preferred girls with cheeks? And if so, why?

Some men have trouble putting words to this predilection, as I learned when I conducted an unscientific survey of three straight males I know.

One could not lie: He liked big butts.

“Why though?” I pressed.

[[Hemming, hawing]]

“It’s like ... butt, but more butt,” he answered, suggesting that studies based on the behaviors of cavemen are applicable today, after all.

Another brother similarly could not deny: “A healthy, toned butt is great.”

The last interviewee was made visibly uncomfortable by the question.

“I don’t objectify women!” he prefaced. But then he quietly admitted that, if all the world’s behinds were lined up end to end, so to speak, he would likely gravitate toward the more voluptuous half.

On reflection, he added, “I think I’m more drawn to curve than size.”

He might be on to something. Researchers at the University of Texas at Austin recently found in a study that men are, in fact, all about that bass. Or at least, a specific kind of spinal curvature that creates the illusion of bass. That is to say, if we were talking about actual bass rather than butts, men would be equally appreciative of the earthy growl produced by a synthesizer as they would one that's emitted by a Rickenbacker 4003.

For the study, which was recently published in Evolution and Human Behavior, researchers asked about 100 men aged 17 to 34 to evaluate pictures of female silhouettes that had been manipulated to look like their lower spines curved at different angles. (I hate to get crass in what I hope is overall a highbrow psychological deep-dive, but essentially these lumbar differences made the drawings look like they were sticking their butts out to varying extremes.) As you can see:

Evolution and Human Behavior

The researchers found that the participants preferred the middle drawing, which is closest to the “hypothesized optimum”—a 45-degree lumbar curvature. In a second experiment, the researchers tested whether men would prefer women with larger behinds even when they did not possess the “ideal” lower-back swoop. The men consistently preferred silhouettes with the 45-degree angle curve, regardless of their buttock size.

From left to right, figures that show a physically fit butt, fat distribution suggestive of fertility, and a curved lumbar area. (Evolution and Human Behavior)

The explanation–can you handle this?–is that the 45-degree curve allowed “ancestral women to better support, provide for, and carry out multiple pregnancies,” according to a release put out by the university. (And yes, despite today's date, this is a real study, not a joke.)

In their literature review, the authors build a case that because pregnancy shifts the center of gravity forward, women whose backs were not positioned at 45 degrees “would have been subjected to a nearly 800 percent increase in hip torque during pregnancy.” The hip torque, in turn, would have impeded their ability to forage for food, which would have put the women and their children at risk of starving. Women with the proper lumbar curvature, by contrast, ain’t missin’ no meals.

Men want healthy offspring, so, the thinking goes, they wanted women who could get their butts out there, literally, and forage like champs. This preference was passed down from the era of mammoths to the epoch of Match.com, and to this day, it might explain why some women do everything from wear 5-inch heels to balance stemware on their rears.

“This adds to a growing body of evidence that beauty is not entirely arbitrary, or ‘in the eyes of the beholder’ as many in mainstream social science believed, but rather has a coherent adaptive logic,” said David Buss, a University of Texas psychology professor and co-author of the study, in a statement.

These findings are not without their haters. I reached out to Agustín Fuentes, an anthropologist at the University of Notre Dame, who said, “Bottom line (no pun intended)—this is a fishing expedition to find some form of sexual selection, ignoring what we know about human morphology and positional behavior.”

(I think we both know that pun was intended.)

Fuentes argues that findings based on “early hominins aren't valid for us anymore. Our bodies are substantially different. We have different birthing patterns than homo erectus.” The evidence is scant, he says, that a 45-degree curvature leads to better pregnancies. If men prefer backsides that are wedged at 45 degrees, and this desire has been around for millennia, then why are there still women whose bottoms protrude at every which angle? Wouldn’t those genes have been bred out by now?

Finally, “they don't even nod to the possibility that there could be cultural norms at play,” he said. “All of these young men might be watching porn. If you look at models, you'll see that stance is a common one in heels.” Might not seeing girls dressing up in heels as early as middle school, or watching the math-themed adult film A Beautiful Behind, have influenced these men’s derriere-based desires?

David Lewis, who now teaches at Turkey’s Bilkent University but who authored this study while at UT, countered that modern-day humans do resemble their predecessors in the sense that “women uniquely face the adaptive problem of a bipedal fetal load. That is an argument that is true for both early hominins and is—and always has been—valid for humans, from the beginning of our species’ history up until and including the present day.”

Lewis agrees that wearing heels makes women look like their spines curve the “right” way, but he argues that men prefer this look thanks to evolutionary underpinnings, not just because of cultural norms. “Has [Fuentes] considered that wearing high heels is a behavior designed to enhance women’s physical attractiveness, and that one of the bodily features that is accentuated by heels (in addition to the legs) is the curvature of the spine?”

After all, every morning when I’m trying to decide what shoes to wear, I think to myself, Let’s see, how many berries am I planning to gather after that 11 o’clock call? If the answer is “quite a few,” I go with stilettos. If they happen to simultaneously make my tush look more tempting, so be it.

I kid, of course. Having started this butt fight, I now feel I must bow out (pun inten ... I don’t even know anymore). This is probably one of those studies that, for the lay man or woman, is worth filing away for a particularly racy, yet nerdy, cocktail shindig, but not to live one’s life by.

It’s great that we’re living in the golden age of dumps like a truck. But at various points in history, prominent tuchuses have been about as sexually appealing as Snuggies. The flappers of the 20s strived for an androgynous, pancake-flat physiques. Even just a few years ago, butt implants were practically unheard-of in plastic surgeons’ offices. As the lucky individuals who stocked up on mom jeans in the mid-90s might now attest, fashion changes faster than you can say, “Do these pants make my butt look big?”

There’s also the issue that, in suggesting that lust for certain attributes is innate, we risk disparaging people who didn’t happen to be born with them. As one of my male interlocutors said during the butt survey, “I think some men are far too influenced by pop culture and shouldn't fixate on one part of a woman. Even ostensibly positive things like ‘real women have curves’ can hurt a lot of feelings because a lot of other women don't have curves.”

It’s true. Pour one out for us Scandinavian box-women.

Studying male sexual preferences can help us better understand human sexual behavior. But it's important that the findings not take precedence over what women want for themselves. As Stony Brook University anthropologist Gabrielle Russo said when I sent her this study: “How about what women prefer?”