Man evolved to seek out women who could procreate, Buss says. If a woman has a larger rear, that's a signal to your primitive brain that she's probably carrying enough fat to become pregnant, he says. And if her bum is perky and round, that means she's probably young – which, in caveman days, would make her a good candidate to carry your child.
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And then there's the naughty factor.
"It's a taboo," says Eric M. Garrison, a sex expert and author of Mastering Multiple Position Sex. "We're not allowed to go there. And when it comes to sex, there's something animalistic and dirty about mounting from the rear."
Geography and culture can sway just how into bums you are. For example: 59 per cent of Argentinian men dig butts more than boobs, a University of Buenos Aires study found. But British men don't pay all that much attention to the booty, according to University College London research.
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Regardless of where you live, though, bottoms are rising to the top.
"Pop culture is celebrating the butt more and more these days," says Ava Cadell, a doctor of human sexuality and author of NeuroLoveology. "With the mainstream appeal of celebrities with big butts like Jennifer Lopez and Kim Kardashian, women want to emulate the way they look and flaunt their own booty to get attention."
Thanks to the trend, doggy-style sex and spanking are more popular than ever, Cadell says, and even anal sex is going mainstream. (Here are 8 Things You Need To Know About Anal Sex)
Yes, the ass is definitely having a moment.
