I made a video about hypotheses for the evolution of breasts in women. Sort of a video. It’s not very visual, and is just me talking, because I didn’t want anyone distracted by sexy pictures — that wasn’t the point. I even left my sexy face out of it.
Since it’s just me droning on, I include my script below, so you can skip the video altogether and just read what I said.
Why do women have breasts?
A warning: I’m going to be talking about breasts, but I am consciously refraining from showing any photos of breasts — the closest I’ll come to showing a boob is a short clip of Jordan Peterson. So if you’re hear for prurient reasons, you might as well stop right now. I’m going to be dry and pedantic and academic. So sorry, not sorry.
It is, however, a valid question, and one that arises legitimately from a comparative approach: other female primates have small, non-protuberant breasts, so it’s a noticeable difference that human females have this prominent secondary sexual characteristic. It’s distinctive to our species, and it’s certainly a product of our evolution. So why?
I’m going to go through several hypotheses, but first I have to bring up an important caveat to the question. Breast sizes are highly variable. Not all women have large breasts, and furthermore, small breasts are demonstrably not a problem for lactation — the enlargement is of fatty tissue, not glandular tissue — and further, breast size is variable in its relevance as a sexual cue. You may think the Western obsession with large breasts is entirely justifiable as a biological response, but it’s not — other cultures place little emphasis on breasts as a sexual stimulus. The sexual role of breasts is…don’t cry, sexists…a culturally determined response. It is a variable, socially-constructed response to a variable, biologically determined stimulus, which means we’re delving into complex territory.
This variability is a significant factor upon which many of the hypotheses founder. If you’re going to argue that breast size has been selected for, you either have to show that small-breasted women have a reduced fitness, or that there are trade-offs with other factors that can favor a variable distribution. You also have to do the hard work of actually gathering observational evidence and assessing correlations — you don’t get to do experimental work on humans — and another of the failings of many of these hypotheses is that the evidence just isn’t there.
Another problem many of these hypotheses have is that they fail to address another dimension: why does your hypothetical advantage only apply to women? You have to explain why your explanation applies to only half the human population.
So let’s get into it. I’m going to discuss 5 different hypotheses. This is not an exhaustive list, since many of these really are nothing but just-so stories, which are limited only by your imagination, but these are the primary proposals I hear.
The buttock substitute hypothesis
The nipple mobility hypothesis
The honest signaling hypothesis
The sexual maturity hypothesis
The byproduct hypothesis
The buttock substitute hypothesis
I can’t believe I have to discuss this one, because it is so ludicrous. If you’re a biologist, it’s only going to provoke laughter, but unfortunately, it is amazingly popular with the general population, and keeps getting brought up by the media and evolutionary psychologists and other cranks. The idea here is that other primates mainly copulate with the male entering the female from the rear, and in some species, the female exhibits prominent, even colorful swellings when she is receptive. Humans, when they became bipedal, often copulate face to face, so they required a ventral signal for sexual receptivity. Thus, evolution conveniently provided a butt substitute on the front of the chest.
There are more than a few problems with this idea.
a) It completely ignores the variability issue. Do flat-chested women have difficulty arousing men in the bedroom? Are men unable to perform without proxy buttocks?
b) Breasts are not sexual signals in many cultures. It’s bizarre to assume that a peculiar Western obsession is a human universal.
c) I don’t know about you, but really, I am not fooled. Breasts & butts actually look very different. It is one thing to propose that humans evolved novel sexual triggers, and another to claim that diverse primate signals are displaced in humans.
d) Another difference between humans and other primates is that we don’t have a cycle of estrus — human females are potentially always sexually responsive, so why did we need to evolve a receptivity signal? The biological “open for business” signal is always open, it’s the social and personal signals that are variable.
But the biggest problem: no evidence. No evidence at all. The logic is that I, a man, find women’s bodies sexually attractive, therefore evolution must have shaped women to fit my desires. And that, ultimately, is why this wild-ass guess is so popular — I should have called it the “universe serves male gratification” hypothesis.
But there are so many men who accept this kind of nonsense as fact — they assume women are like these non-autonomous objects whose entire history is one of being sculpted biologically to serve men’s needs. This idea came straight from Desmond Morris and his book, The Naked Ape.
“The answer stands out as clearly as the female bosom itself. The protuberant, hemispherical breasts of the female must surely be copies of the fleshy buttocks, and the sharply defined red lips around the mouth must be copies of the red labia.”
It was a bad idea when it was first published, but it was controversial and attention-getting — so many reviews mention this “observation”. It was published when I was 10 years old, and when I read it as a teenager, I noticed and marveled at it — and I assumed, since it was stated with such absolute certainty, it must be true. You can say total balderdash, but if you say it with a confidence that brooks no questions, you’ll always find someone gullible enough to accept it.
But nope, sorry. There is no evidence for Morris’s claim. They aren’t even any logically sound arguments. This is pure just-so story-telling. It’s also an appeal to adaptationist thinking, as so many of these hypotheses are, drawing on the satisfaction of a causal explanation, especially one that draws on the appeal of male gratification.
This next hypothesis is also an adaptationist just-so story, but at least it doesn’t regard women as evolving to fit male sexual desires.
The nipple mobility hypothesis
I actually like this one a little bit — it breaks away from the usual male-centered idea. The hypothesis is that humans, when they became bipedal, also acquired a new flexibility in carrying things with their arms now liberated from the task of locomotion. Among the things they could carry in new ways were babies, babies that may need to nurse while you’re walking along, or when you’re sitting around a hearth. In other primates, the nipple is located in a fixed position on a flat chest; in humans, it can be moved around more, allowing more degrees of freedom in holding a baby.
Hmm. Interesting. The hypothesis has a few positives:
a) it explains the sexual dimorphism, since men don’t lactate.
b) it assumes that female morphology evolved to benefit women, not men.
But it also has some negatives.
As I will continue to harp upon, it doesn’t explain the variability within women. Variation is data! Don’t ignore it!
Studies of hunter-gatherer societies tend to show that all members of the culture tend to have very low body fat, and that in fact maintaining low body fat while lactating is a useful physiological trait for regulating fertility. The women striding across the savannah with a baby on their hip would have been lean and relatively flat-chested.
NO EVIDENCE. It’s a purely speculative hypothesis. The author of this idea was Elaine Morgan, of aquatic ape fame, and while I like that she was good at thinking outside the box, there was no gathering of evidence or experimental testing, and ultimately it’s appeal lies in the fact that it treats women as autonomous agents in their own history, but not in any science.
The honest signaling hypothesis
This another idea with a reasonably sound foundation. Large fat deposits are expensive, and require a lot of energy to accumulate. If a woman has large breasts, she must be strong, well fed, and relatively free of parasites, and therefore flaunting heavy fatty organs is an unambiguous signal of health and nutrition, and therefore is a good choice for a mate.
Up to a point, this is certainly a reasonable idea. To some degree, we do judge prospective mates on their health and appearance; young, healthy, and fertile are selectable characteristics, and they represent a suite of traits with sex appeal to modern humans, so why not in our ancestors as well?
A few flaws, though:
As always, explain the variability please. Why aren’t slender women rejected by men more — they’re clearly carrying a draining load of parasites, or have been unable to feed themselves, or are wasting away from disease.
This doesn’t explain sexual dimorphism. It assumes that males are the selective force for traits in females, but it ought to go both ways: wouldn’t it be an honest signal of strength and skill in food gathering if males sported a pair of large breasts, too?
Why breasts? Gentlemen, if you find fatty growths attractive as indicator of health, would you feel likewise about gigantic bellies or huge thighs? The transition would have been awkward; would it have been seen as “healthy” when an ancient ancestor sprouted unusual bulges in places no previous ape would have considered normal?
Men and women have different distributions of body fat. Do women find pot-bellies as sexually attractive as men find large breasts?
The sexual maturity hypothesis
This is the idea that breasts are basically a visible meter to allow one to judge the sexual maturity of the female — you can easily assess whether a female is prepubertal, and therefore infertile, and the breast shape changes also allow one to see at a glance when a female is old and post reproductive.
Notice that once again we’re in a male-centered domain: men are judging, women are being passively judged. No one ever seems to consider that it is to the woman’s selective advantage to exercise control over her reproduction, so why are we assuming it is evolutionarily favored for her to advertise? We can hypothesize about women evolving cryptic ovulation to hide their fertility, but at the same time argue that males were selecting females for visible fertility signals.
I’ll say it again: we’re ignoring the variability. Are women with small breasts bad at advertising their sexual maturity? Then there’s the flip side: there’s variability in the male response. Creepy as it is, there are men who are attracted to small, childlike bodies, where small breasts are an indicator of youth. What is being signaled? How is it being received? We are in a world of mixed messages here, which could, in part, be one of the reasons there is so much variability…but it also means we lack a consistent context to explain the origin of the phenomenon.
Unlike other hypotheses, this one does have some observations to back it up: the age of first menstruation is inversely correlated with the amount of body fat. That is, women reach reproductive maturity earlier when living on a rich diet and having high body fat indices, and conversely, low body fat, as is seen in athletes, for instance, is correlated with menstrual irregularities. Those hunter gatherers I mentioned earlier? They have low fat diets, are walking for many miles every day, and are simultaneously lactating, and may not ovulate at all for years at a time. But note please: this does not address the origin of breasts, specifically. It’s about the endocrine effects of subcutaneous body fat, or the ratio of fat to lean body mass.
I’d give this hypothesis a bit of a shrug. It’s true that there are physiological changes to the human body with development and age, so in that sense it is a valid, but trivial explanation, lacking in specificity.
The byproduct hypothesis
I saved my preferred explanation for last: breasts are a product of an initial chance distribution of subcutaneous fat, followed by sexual selection for male vs. female markers. This is basically the null hypothesis.
What does that mean? We know there are sex differences in the distribution of fat, just as there are sex differences in the distribution of hair. These are arbitrary — that is, there is no specific functional reason why females tend to accumulate subcutaneous fat in the thighs, while males accumulate it in the belly — it probably involves localization of, for instance, steroid receptors during development. It really had no significant effect on survival or behavior, and variations in genes affecting that distribution can accumulate in the population, leading to the variation we see now.
So the first cause for breasts: pure chance. An intelligent primate species could just as well have evolved that had fatty deposits on male chests, while the females grew beards. Evolution doesn’t care.
However, humans care a great deal about sex. Once these differences emerged, human brains would become finely tuned to perceived differences between sexes — we know that people are exquisitely sensitive to even the most delicate differences between males and females. We’re amazingly good at picking up on tiny variations in faces, for instance: female eyebrows are, on average, infinitesimally higher and more arched than those of males, creating an illusion of larger eyes. Male noses are slightly larger, on average, than those of females. Female jaws are less robust than those of males, and they have smaller chins. We can pick up on those tiny differences, and larger differences, like width of the hips and differences in gait, are even easier to see — and yes, shape of the chest is another obvious marker. Again, all these differences are on average, and it’s the totality of the whole that we unconsciously assess to recognize male vs female bodies. Or, more accurately, we’re really good at recognizing the outcomes of the human body’s responses to different hormone regimes.
These are all human bodies, after all: no one sex is the standard for the species, and evolution happens to all sexes simultaneously, so your perspective on the evolution of sex-specific attributes shouldn’t be seen only through the lens of males vs. females. There has to be a lot of co-evolution going on: if there’s evolution of larger breasts in response to estrogen, there also has to be simultaneous evolution of favored responses to larger breasts by brains conditioned by testosterone, or more likely, by cultural influences.
But it really is, at it’s root, an arbitrary expansion of a chance distinction in our physiology. For another example of how it could have gone, look at the proboscis monkey.
Male and female proboscis monkeys have large noses, but they’re immense in the male. The most likely explanation is sexual selection — that females favored males with large honkers, perhaps because of the greater resonance of their mating calls, and from that initial chance variation, sexual selection has led to this result.
Imagine if, in the early human root stock 150,000 years ago, estrogen tended to produce slightly larger noses, rather than slightly larger breasts, and that males had cued in to that difference, and found large noses to be a signifier of female beauty, rather than large breasts. We could have ended up with a species in which this photo was pure pornography, very sexy, and we’d be kind of grossed out by species with pendulous fat bags on their chests.
But we’d also be finding a way to try and argue that large noses are naturally and biologically favored in some functional way. It’s a common phenomenon, in which we try to justify some biological attribute as the best solution, an indicator that we live in the best of all possible worlds, and we assume that all properties of our behavior and structure are righteously optimized.
We also tend to personalize evolution in inappropriate ways. Think back to that Desmond Morris quote, which is all about his biases: “Well, I find breasts sexually stimulating, therefore evolution must have generated them hundreds of thousands of years ago to make me happy today”, just as proboscis monkey females would assume gigantic noses are nature’s way of sexually stimulating her. The outcome is not intrinsically valuable, and we commit the fallacy of retrospective coronation when we assume its effects now tell us about its origin. That only leads to ludicrous conclusions.
Here’s another example of a fallacious interpretation. It’s from that infamous interview of Jordan Peterson with Vice.
There is so much wrong with so much of this interview, and I’m only picking on this one tiny part. But here are a few difficulties: red is a popular lipstick color, but how does he explain these other colors, which can also be quite attractive? What about the implicit racism in that claim — it’s a white trait to have a greater contrast in color between the lip and the face? What about the men — why do we have a vulva-mimic between our nose and our chin?
Where is Peterson’s evidence? This is another example of the Desmond Morris effect: he simply asserts that his interpretation is the obvious one, therefore it must be true.
Allow me to offer a better, alternative explanation.
Women are not putting on make-up in order to invite random men to have sex with them — that’s the explanation of a sociopath, someone who can’t empathize with another human being. Women are putting on makeup in order to conform to culturally defined standards of attractiveness. Often this involves accentuating attributes associated with femininity — under the influence of estrogen, human bodies develop fuller lips, so additional color is used to exaggerate female features. They are simply announcing, “I am a woman, and I look good”.
That is not synonymous with announcing “I am a sex object.”
It’s about taking pride in one’s appearance, making an effort to conform with society’s expectations, and about identifying with a group. Peterson is doing exactly the same thing by refusing to wear makeup and having a conventionally male hairstyle — he’s taking a position on how men ought to look, and conforming to social pressures.
Maybe we should break that down by having a social convention that everyone in the workplace ought to take enough concern in their appearance that they put on at least a little lipgloss and some eyeshadow to make themselves presentable.
What’s surprising about Peterson’s assertion is that this is the same fellow who famously offers the basic advice that men ought to “stand up straight and throw their shoulders back”, that they ought to take responsibility for their appearance and their lives and clean up their room. Yet he fails to notice that while society allows considerable male slovenliness, women are under greater pressure to take more concern with their appearance, that they are expected to show up at work or in public with nice clothing and appropriate makeup … and he mistakes that for posing for sex.
The bottom line is that there are biological sex differences, and we do not deny them … but we also do not consider sex to be the only role for women in society, and we reject Peterson’s pseudo-scientific rationalizations to compel rigid sexual identities on everyone. Walking around with breasts or lips is not advertising one’s availability for sex, and it’s demeaning when someone uses expression of self-respect and appreciation of one’s identity as, instead, a statement about sexual submission. That’s an interpretation that is entirely in Peterson’s head, and he should be working to change it, rather than trying to indoctrinate more people to accept his faulty perceptions.