S2E12: Butts

September 23, 2021

Why do we have such big butts? Why does society admire women's butts more than men's? And what does race have to do with all this?

Featuring Jeanne Bovet, senior lecturer at Northumbria University in Newcastle.

Citations and further reading:

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Taboo Science is written and produced by Ashley Hamer. Theme music by Danny Lopatka of DLC Music.


TRANSCRIPT

Ashley Hamer: I don’t have to tell you how popular the butt is as a body part. I mean, there are songs about them, leggings specially adorned for them, exercises you can do to make them bigger and rounder and out there.

The easiest evidence to find is on the unfortunate Instagram hashtag “belfie.” That’s short for butt selfie, if you haven’t been acquainted. This is a trend where women — it’s almost entirely women — take a picture of their backside, whether that’s in leggings, in a bikini, or a tight pair of jeans. There are nearly 400,000 of these images on Instagram. It’s a popular style.

 And while you’ll hear people admire men’s butts from time to time, it’s not with the fervor society has for women’s butts. A woman’s butt is the lower half of the classic hourglass figure, after all, and there seems to be something primal about the attraction to that silhouette. 

So today, we’re going to find out why humans love butts, why our butts are so big, and what exactly makes women’s butts the apple of our eyes — or, peach of our eyes, more appropriately. 

 I’m Ashley Hamer and this is Taboo Science, a show that answers the questions you’re not allowed to ask. 

The human butt is unique in the natural world. Forget things like language and tool use — our booty is what sets us apart from the animals. Humans have the biggest butts in the animal kingdom. Our primate cousins don’t even come close.

Scientists think that’s all thanks to the fact that we walk upright. See, in both humans and non-human primates, the butt is made up of three muscles: the gluteus maximus, the gluteus medius, and the gluteus minimus. Kind of a Goldilocks and the three bears situation. In humans, the gluteus maximus is the poppa bear. It’s big and thick and mostly handles thigh extension: stuff like rising from a sitting position, climbing, or running. In primates, that muscle is way smaller. It doesn’t even connect to the same bones: our gluteus maximus runs from the top of the pelvis to the upper thigh bone. The primate gluteus maximus connects to the femur too, but it only starts at the ischium or sit bone, the part of your butt that hurts on a long bike ride. It’s smaller and less powerful. As a result, primates can stand upright — long. This mini gluteus maximus also, obviously, gives primates less of a big round booty than the one humans have. 

The ability to stand upright is a fundamental difference between us and our primate cousins. Scientists aren’t sure what, exactly, led our ancestors to stop walking on all fours. Maybe they started by trying to reach higher in trees when they foraged, maybe they wanted to shield their backs from the sun or see further over tall vegetation or keep their hands free to hold food or babies while they traveled. But whatever it was, we got up on two legs, and our butts changed as a result. We got junk in our proverbial trunks, and the world was never the same again.

Of course, muscle isn’t the only thing that makes our booties so juicy. Fat is a big player, too. Humans are also the fattiest primates, and, again, while we aren’t sure why, some think we need to store a lot of fat to fuel our big brains. And when it comes to butt fat, women are the reigning champions. They store more fat in their butts, hips, and thighs than men do. In fact, that’s another trait that’s unique to humans: the way that fat distribution is different between men and women. No other animal does that.

Women’s butts are also the ones that get the lion’s share of the attention: in advertisements, in music videos, in workout routines, in clothing designs with lettering across the backside and zero pockets to be found. Sure, you’ll hear people talk about men’s butts, but it’s not nearly to the same degree as women’s. Why is that?

Jeanne Bovet: Women's bodies are really linked to their reproductive success in a way that is not the case for men's bodies. 

Ashley Hamer: That’s Jeanne Bovet. She’s a senior lecturer at Northumbria University in Newcastle. 

Jeanne Bovet: So just because we have to carry the baby and there's a pregnancy on this breastfeeding and all of that is really demanding for the body. And so, in a way the body before your pregnancy is going to affect the way the pregnancy is going to go, it's going to affect the baby and the children later with breastfeeding and everything. And of course having a baby and a pregnancy is going to affect your body in return. So you have a really close relationship between bodies and reproduction in women, a bit less in men.

And so that's why scientists think that men are really focusing on a woman's body a bit more than women focus on men's bodies.

Ashley Hamer: In her work, Dr. Bovet has been trying to untangle what, exactly, we’re attracted to when it comes to the objects of our desire. Those kinds of questions are really important to evolutionary biologists, for a pretty huge reason.

Jeanne Bovet: Well, so I was trained as a biologist and mate choice is always something really fascinating in animals and something really important for evolution because that's a way selection is happening: when you choose a specific mate.

Ashley Hamer: Evolution happens through reproduction — reproduction of babies, sure, but especially reproduction of genes. Like, the best genetic trait in the universe isn’t worth much if you never find a mate and pass that trait onto your offspring. And the mate you choose will put half of their genes into your offspring, which makes mate choice a really big deal in the evolutionary scheme of things. More attractive mates are more likely to be chosen to have offspring, which means the genes that code for traits that make an individual more attractive as a mate are likely to spread throughout the population. So finding out what those traits are is super important for scientists studying evolution — human evolution included.

Jeanne Bovet: So I'm working on mate choice, but mostly on physical attractiveness. And I think it's just also the fact that physical attractiveness is so important in our lives — too important, I would say. It's a source of anxiety when we are teenagers or even before. You can affect a lot of things in our lives. And I was like why?

Why is it so important and why do people care so much? And I think I was never really, like, completely satisfied with the usual answer which is well, you know, it's media and society which wants us to care about physical appearance. But for me it was asking other questions like: so why do the media care about physical appearance in the first place? And why do they represent only certain types of bodies? So that's why I think some studies in psychology and with an evolutionary perspective I'm hoping maybe to answer these questions.

Ashley Hamer: You might not be surprised to learn that there aren’t a whole lot of studies about our attraction to butts, specifically. It’s more about what’s around the butt — how the butt fits in with the overall picture. The way scientists like Dr. Bovet measure it is with the waist-to-hip ratio. A low waist-to-hip ratio is that classic itty-bitty waist and a round thing in your face, to quote Sir Mix-a-Lot. The higher the ratio, the less difference there is between the circumference of the waist and the circumference of the hips and butt. I...constantly get this confused, so my rule of thumb is that a low waist-to-hip ratio means a low waist measurement, and vice versa.

But what is it about a low waist-to-hip ratio that makes a woman attractive?

Jeanne Bovet: In the literature you often find like, really general statements or vague statements. So a low waist-to-hip ratio is a sign of good health or it's a sign of fertility.

And it's not clear enough. Let's put it that way. So the waist-to-hip ratio is linked with really important characteristics, but they are not really mysterious. 

Ashley Hamer: Number one: it’s a sign that she’s a woman.

Jeanne Bovet: Women and men have really different waist-to-hip ratios. So this is important information when you choose a mate.

Ashley Hamer: Number two: it’s a sign that she’s of childbearing age.

Jeanne Bovet: So the waist-to-hip ratio changes with time. It's high when you're a kid and then it's decreasing and then it's going to increase again with age. 

Ashley Hamer: Number three: it’s a sign of pregnancy status.

Jeanne Bovet: So if you're currently pregnant you will have a high waist-to-hip ratio. 

Ashley Hamer: And finally, it’s a sign of how many kids she’s already had.

Jeanne Bovet: So the more pregnancies you had in the past, the higher will be your waist-to-hip ratio. And so these characteristics are going to influence your reproductive potential.

So if you had 10 children in the past, you will have a different fertility than if you had none or one kid. And of course age is going to influence a lot if you're going to be able to get pregnant really quickly and the risks linked to pregnancy and things like that. 

Ashley Hamer: A lot of this is directly tied to the extra fat a woman carries on her butt and hips. Big butts make better babies. 

Jeanne Bovet: And so there is a hypothesis that the more fat you will have around the hips and so the better the baby's going to develop.

There is definitely something specific about the fat in the hip region and it's used during pregnancy and during breastfeeding, more than the fat in other parts of the body. And this is why after several pregnancies, you have a different waist-to-hip ratio. It's one of the reasons you have other reasons. So the effect on the muscles and the effect of the hormones, et cetera , but in part is because the baby is using this fat when it's growing and when you breastfeed the baby. There is preliminary evidence that it's really used for the brain. Theoretically, it sounds great and it makes sense because it’s used in pregnancy, etc. We don't have a lot of data to measure it, but it's a promising hypothesis.

Ashley Hamer: Now, of course, it’s not true that all people attracted to women like the same types of bodies — much less the same types of butts. But that’s why waist-to-hip ratio is such a handy measurement tool: its ideal doesn’t seem to change that much from culture to culture or time period to time period, even if other metrics like body-mass index or bodyfat percentage do. It does vary a little bit: some studies suggest that men in harsher environments prefer women with a higher waist-to-hip ratio, possibly because in those circumstances, a mate that can acquire more resources is more important than one that can have more babies. But when it comes to the question of universal preferences as a whole, the evidence is kind of all over the place.

Jeanne Bovet: So people used to look at the preferences across countries. And the results are mixed. You have some preferences that are relatively universal like for example preference for relatively young or young age body, healthy body. And those preferences are very regular between countries. And waist-to-hip ratio, there are studies showing that men have similar preferences across countries and also studies saying no look like this country prefers lower waist-to-hip ratios than other countries. 

So we decided to look at the universality of men's preferences according to time. So across time and not across countries. But of course you cannot ask dead men from 300 years ago to tell us about their preferences.

Ashley Hamer: So instead, they did something pretty ingenious: they looked at art pieces through time. 

Jeanne Bovet: So paintings and sculptures of women who are supposed to be beautiful. 

Ashley Hamer: To make sure they knew the subject was supposed to be beautiful, they stuck to classic symbols of beauty: characters from mythology whose whole thing is being beautiful.

Jeanne Bovet: So basically like Venus the goddess of beauty. We'd guess that the artist wanted to represent a beautiful woman when they were depicting Venus. 

Ashley Hamer: They used 160 paintings and 56 sculptures representing women from 500 BC to present-day. That’s nearly 2500 years. Well...1500 years.

Jeanne Bovet: Then you have basically 1000 years without any woman depicted naked in paintings or sculptures because it was forbidden by the church. 

Ashley Hamer: But still, through all that time, the waist-to-hip ratio of the most beautiful women depicted in art? It didn’t really change. 

Jeanne Bovet: And what we found is that during antiquity, the waist-to-hip ratio depicted was relatively stable. It was not changing that much. And when we start again we can see that the waist-to-hip ratio depicted is decreasing a bit, so, toward the more feminine waist-to-hip ratio until today. 

So you have some variation according to time. Also, it's always a relatively feminine waist-to-hip ratio. It's never really really high.

Ashley Hamer: The waist-to-hip ratio of the subjects in these pieces never went above the current medical recommendation — so even when the ratio was on the high end, it was still what modern medicine considers healthy. 

But like I said, waist-to-hip ratio is a lot different than something like body mass index, which has its own effects on the derriere. That does seem to change among time and place, and Dr. Bovet hopes to study that too.

Jeanne Bovet: But what we know when we look at different countries is that the preferences for body mass index, for example, is quite variable. So I won't be surprised to see that actually women depicted across time are, like, a wider range of body size than, compared to the waist-to-hip ratio. I'm expecting to see more variation and variation according to time like that, yes.

Ashley Hamer: But there’s a dark side to our love of the booty. I quoted Sir Mix-a-Lot a second ago because he’s got one of the most famous songs about butts there is: Baby Got Back. And when you get right down to it, that song is all about how 1980s fashion held up white women with flat butts as icons, leaving women of color — who tend to be curvier than their white counterparts — out in the cold. This was not new. 

There’s a long history of Europeans both fearing and fetishizing the figures of women of color. As white European colonizers traveled to new places in the world, they encountered new places and people — and quickly came up with their own classifications for them. Usually, they classified the people they encountered by their visual characteristics, including the butt. They considered the large butts of African people as signs of an underlying animal or sexual nature, which became one of many things they used as justification for enslaving them. Europeans even came up with a medical diagnosis for curvy African women: steatopygia , defined as an excessive accumulation of fat around the buttocks. It was considered a terrible, debilitating condition, despite the fact that big butts were fashionable for European women at the time.

The posterchild for Europe’s kinda creepy fascination with the bodies of African women was Saartjie "Sarah" Baartman, also known as the Hottentot Venus. She was kidnapped from her home in Southwestern Africa by Dutch colonists and paraded as a sort of sideshow freak or circus animal, and attracted crowds curious to see her large backside. When she died in 1815 at the age of 26, her body was dissected and written up in a book about mammals of the world — she was the book’s only human subject. The way she was treated, both in life and death, was truly shocking. It shows that even if certain human preferences are universal, dignity and respect are only afforded to some.

Alright, I’ve spent a lot of time talking about women’s butts. But all that doesn’t mean that men’s butts don’t matter. I mean, you can find plenty of man butts in movies and TV shows. Magic Mike is basically an entire movie about men’s butts and the women who love them. But I would argue that people appreciate a man’s butt in context. That is, if he’s fit and muscular everywhere else, his butt will be too. There’s a difference between a flexing bicep and a rippling glute, but I’d argue that difference more comes down to social dynamics. When you look at someone’s butt, they usually don’t know you’re looking. That makes it different.

I asked Dr. Bovet about my theory: that is, maybe the male butt is a signal of his physical fitness?

Jeanne Bovet: Again, I'm really not a specialist but I see it is all I'm really speculating right now. But I guess if it's different muscles it's going to be linked to different abilities. And so maybe in some cases it's more useful to have muscles in the lower body parts or in the upper body part depending on what you need to do on a daily basis. So you would have to look at what kind of tasks you need in the environment.

Ashley Hamer: Another theory out there is that the shape of a man’s body is directly linked to his testosterone levels. When a cis woman goes through puberty or a trans woman takes estrogen, those so-called female hormones send extra fat to her butt, hips, and thighs. But testosterone has a different effect on men: they actually lose fat on their butts and redistribute it to their stomach and shoulders. So for men, a small, lean butt is a signal that he’s got high testosterone levels — and studies show that heterosexual women are more attracted to men with a higher waist-to-hip ratio, of roughly 1 to 1. But still, testosterone levels only tell you so much.

Jeanne Bovet: But again it’s going to be — it would probably be more efficient for other people to just look at the outcome of what the men are able to do than to look at the body. Which you cannot really do with the female body because it's an outcome that never really happened. It's going to be the future outcome of the pregnancy. So yeah, there are more direct ways to look at what matters for male mates, I would say, than the body.

Ashley Hamer: In other words, it’s not about what his body looks like, it’s about what he can do with it. Makes sense.

So it turns out that not all butts are created equal. Women’s butts send an evolutionary signal about their ability to have babies. Men’s butts...not so much. 

Thank you so much for listening! 

Taboo Science is written and produced by me, Ashley Hamer

The theme was by Danny Lopatka of DLC music.

Big thanks to Jeanne Bovet. You can find links to her other research, along with a transcript of this episode, in the show notes. 

This is the final episode in Season 2! We did it! As a result, I’m going to take a nice long break. So this is the perfect time to let me know what you think in a review on Apple Podcasts, or to drop me a line at ashley at tabooscience dot show. And make sure to follow the show on your favorite podcast app so you don’t miss season 3. 

Catch you next time!

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