Have you ever noticed how often the origin of breasts is explained as “for men”?

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.


  1. reedcartwright says

    For several years, I’ve been trying to identify the genes that evolved in our lineage to change human mammary phenotype. I have one candidate gene that we’re working to characterize. Here are a few things that I have learned.

    (1) It’s not a pregnancy phenotype. Pubertal breast development is triggered by estrogen. Pregnancy and lactation shape mammary development via progresterone and prolactin. Nulliparous breasts are different than parous breasts.

    (2) It’s not about fat. The distinction between human and NHP mammary tissue is the amount of firm tissue present. Early breast development begins with growth of firm tissue. Fatty tissue come late in development and/or later in life.

    (3) Breast size varies, but breast presence doesn’t. All women if they produce estrogen and have mammary tissue, will experience breast growth in puberty. There is no evidence in medical literature of a woman having an ancestral/NHP mammary phenotype. This suggests that there may be strong purifying selection or canalization for breasts.

    (4) Female fitness in humans (and probably our ancestors) is driven primarily by age of first live birth. So if breasts are due to selection on female fecundity, then one has to figure out how women with breasts reproduced at an earlier age then women without them.

  2. says

    Hold on a minute… I was traveling this road with you, but this just made me put the “skids on”… you say “
a) it explains the sexual dimorphism, since men don’t lactate.”, they do & can.
    AND their nipples move around just like human women’s do.
    Come on, while attempting to be a-sexist you have fallen into a common & ridiculous fallacy.
    Certainly YOU can play with your own nipples, can’t you?
    They move & you CAN lactate.
    There is a group called, “Milkmen”, that support men who choose to breastfeed their children.
    LaLecheLeague has refused to assist them in the past, but I am not currently up to date on LLL morays.

  3. kestrel says

    This is great. And LOL, I must be a dope! Here all this time I thought breasts were for transferring milk to infants! I didn’t realize they were there to make men happy –

    I can observe just how divergent breast material can be by observing goats (I raise dairy goats). Your final hypothesis makes much more sense than the ideas you had covered first. Some of these ideas I had never heard about so this was very educational.

  4. Michael says

    Just a comment regarding your problem (c) in the buttock substitute hypothesis. There is a Benny Hill sketch which is parodying the old Playtex bra commercials of the 70’s/80’s, where he is pointing out the features of the bra on a mannequin. The joke is that toward the end of the commercial, the camera pulls back to show that he has placed the bra on the mannequin’s buttocks! When caught on this not insignificant error, he turns the mannequin around to show that one of the breasts is damaged, which is why he placed it elsewhere. Similarly, I also saw another tv program (can’t remember if it was about nature, or sexuality), which made the same comment that zooming in to the right point made it difficult to tell whether you were looking at cleavage in breasts or buttocks, and claimed that this was the replace the estrus signal in our evolutionary ancestors.

    However I take your point (b), and wonder if there is a correlation to breastfeeding. Could it be for the infant, rather than the mother (nipple mobility), that the change took place? ie. infants prefer to nurse on larger breasts, or needed to distinguish between fathers and mothers. One thing that might explain the Western fascination might relate to this, eg. were men (or women) who fixate on breasts breastfed less (so were hunting for the signal in infancy, and never forgot the hunt); while those who fixate on buttocks or other body parts were breastfed more?

  5. says

    I would also like to comment on a little known facet of American life re: breastfeeding… it would appear that American women are somehow physically challenged in this area vs the rest of the human world.
    Far less American women breastfeed while other cultures breastfeed their babies for a minimum of 1 year up to and including 4 years.
    While my hypothesis is intended to be a criticism of American women and their lack of desire to use those breasts for what they are intended for, many women agreed with it.
    American obsession with breasts does not work well for the infants of this country.

  6. says

    Men *can* lactate, but they usually don’t, and they don’t produce the volume that women can. Is there any culture in history in which fathers have shared nursing duty with mothers?

    By nipple mobility, I (or rather, Elaine Morgan) means extensive movement — a moderately well endowed woman can move her nipple from around her armpit to her sternum. Even as a relatively flabby man, I can’t come close.

    But I will emphasize: I favor hypothesis #5.

  7. rietpluim says

    which means we’re delving into complex territory

    This is true for virtually every part of reality, something that racists, sexists, creationists, miscellaneous quacks, and climate change deniers just do not wish to accept.

  8. JustaTech says

    dragonessfredriksson @5: Have you considered that the low rate of breastfeeding among US women might have something to do with the appalling maternity leave policies here? It’s very hard to breastfeed if you have to go straight back to work. Pumps are expensive and regardless of the minimal legal protections not all women are allowed breaks to pump.

    And then there’s the issue of feeding or pumping in public, where so many people are scandalized at the sight of a breast because they are so sexualized in this culture. If you’re not brave enough to ignore the people who are upset at feeding a baby in public then you can end up basically housebound (this happened to a close friend).

    So I would not assume that women in the US don’t *want* to breastfeed so much as they *can’t*.

  9. says

    I will have to disagree with you re: volume.
    We know for certain, the size of the breast does not influence volume.
    I often have given the example of adoptive mothers wanting to breastfeed and they are fortunate to know ahead of time that they will be adopting a newborn.
    They, too, can start up their ability to lactate even if they have never been pregnant before.
    Size never plays into it.
    And, it can be stimulated manually, just as in men.
    Perhaps size can influence nipple movement, however, a “flat chested” woman has no better nipple movement than a man does.
    (Former LLL leader… all sorts of experience in different sized breasts).

  10. monad says

    The idea of a byproduct makes sense, and I imagine could be developed into more than just a null hypothesis, since potentially someone could figure out a byproduct of exactly what changes. But it seems strange to compare that to the noses of male proboscis monkeys, which presumably started as a byproduct, but are generally understood to have gotten much larger through sexual selection since.

    And I assume you’re not suggesting that has happened to the breasts of female Homo, right? Because that stops being a null hypothesis, but again makes it about their role in attraction. And so your other criticisms should apply: not everyone has or cares about large breasts, and it doesn’t seem to give them trouble finding mates or having children. Whereas proboscis monkey harem sizes are related to nose sizes.

    Wouldn’t it make as much sense to assume the whole range was no more than incidental variation, without needing any subsequent reinforcement at all? Since humans went through a genetic bottleneck not too long ago, I would have expected we would have an unusual amount of that, but maybe the new mitochondria article means I should reconsider that.

  11. says


    While my hypothesis is intended to be a criticism of American women and their lack of desire to use those breasts for what they are intended for, many women agreed with it.
    American obsession with breasts does not work well for the infants of this country.

    As a woman who breastfed both her kids to around a year of age, can I just tell you to stuff that bullshit to where the sun doesn’t shine?
    Breast are not “intended” for breastfeeding. They just are and they can produce milk which is handy, but no part of my body is “intended” for the use of somebody else. My vagina is not for men to fuck. My uterus is not for fetuses to grow in. My breasts are not for infants to eat from.
    All those things are my choices to do with my body.
    And formula isn’t fucking child abuse. It’s adequate nutrition for babies and offers quite some advantages, like somebody else being able to care for the baby.
    A woman can choose not to breastfeed for any fucking reason, including “I don’t want to” and it’s absolutely none of your business. Nobody needs your “criticism” of those uppity American women (and I’m not even American)
    And last but not least, do you know how much harm your bullshit is causing to people who have difficulties with breastfeeding? Do you know how much pain you’re causing the very people you claim to support?
    Because believe it or not, despite the mantra “everybody can breastfeed”, some people can’t or experience great difficulties. So fuck your attitude and the boob you rode in on.

    Re: large breasts
    Actually, the difficulties I experienced with breastfeeding were exclusively due to being very well endowed. At another time and place, without access to formula and technical aid, the kids might as well have starved as newborns.

  12. says

    to: Giliell, professional cynic -Ilk-
    I make no apologies for what I said.
    Somehow, the human population made it without the difficulties American women seem to exclusively have.

  13. Dunc says

    I can’t believe you missed the most obvious problem with the buttock substitute hypothesis: “conventional” primate copulation works just fine for humans. Face-to-face copulation is absolutely not required, or even particularly beneficial.

  14. reedcartwright says

    No, we don’t have a preprint or publication ready. It’s a side project for my collaborator and I. If the wetlab work prodivids good data, we will probably be motivated enough to get it out. The difficulty in writing it up is going to be in how much we refer to adaptationist stories about breasts versus just ignoring the literature in its entirety and focus on the developmental biology.

  15. says

    Somehow, the human population made it without the difficulties American women seem to exclusively have.

    1. Lots of us used to die. Both the ones giving birth and the ones being born. That’s literally the most stupid thing I read all day and that says something given that I spend a lot of time on Twitter.
    2. I’m not American, as you would know if your reading comprehension was equal to your hatred of female autonomy.
    3. Go on, hate women for choosing what to do with their bodies. Hurt new mothers who experience difficulties. But don’t claim to be on our side.

  16. says

    Face-to-face copulation is absolutely not required, or even particularly beneficial.

    Indeed. Trying to make an argument that relies on humans only using one sexual position is just ludicrous. Are we really supposed to believe that two horny cave people couldn’t figure out how to turn around? And that this happened often enough to breed an entirely new sexual characteristic into the entire species?

  17. Nerd of Redhead, Dances OM Trolls says

    Are we really supposed to believe that two horny cave people couldn’t figure out how to turn around?

    Dang, recall a movie about cave-people from years ago (at least 25 since I was still watching movies) where that scene happened. IIRC, it was the woman who changed position.

  18. Pierce R. Butler says

    dragonessfredriksson – Pls remember your male-lactation hypothesis needs to account for ~0 reported male lactation in all mammals, even among those domesticated species bred specifically for maximum milk production over several thousand years. Bulls, for example, have visible nipples, but this farm boy never saw a calf nursing at one.

    From O.P: … we don’t have a cycle of estrus …

    – which raises, at least to me, a plethora of evolutionary questions. My current w.a.g. involves selective advantage(s) for females who had less seasonal/cyclical hormone swings and thus, maybe, more autonomy in mate selection. Assuming (uh-oh!) that this occurred simultaneously with the period of ever-enlarging brains within human-ancestral populations, some sort of runaway feedback cycle may have occurred – and, since growth in brain size evidently outstripped growth in pelvic width and consequent increased female/infant mortality, may have involved some fierce selective factors.

    Alas, lacking any serious know-how in the multiple fields required to explore this hypothesis, I’ve already exceeded my capacity to support any such proposal. Worse yet, Morris and several other pop-“sci” writers poisoned the wells for such speculations since the middle-third of the last century, richly earning the shoot-downs (some listed above by our esteemed host) they received but driving off all but the ignorant and imprudent (like myself) from attempting to analyze the roots and consequences of a uniquely human anomaly.

  19. says

    “Another difference between humans and other primates is that we don’t have a cycle of estrus — ”

    Actually, you do. It’s just a monthly cycle rather than yearly (or longer).

  20. chrislawson says

    Another problem with hypothesis #4 is that there are plenty of other markers of sexual maturity. Why would this one anatomical novelty gain so much selection pressure if existing signals already fill that function?

  21. cartomancer says

    It’s not exactly difficult to study how make-up is used differently in different cultures across time and space. Surely if there were anything to the red lipstick nonsense then we would expect to see many cultures, past and present, settling on some kind of red lip colour for women in their panoply of body decorations? And yet, in all the dizzying array of body decoration that humans have used, it doesn’t seem anywhere near ubiquitous.

    I do wish that people proposing evolutionary hypotheses for features of our cultures would do some historical and anthropological research first, to see if what they’re interested in is anywhere near a human universal rather than a parochial cultural foible.

  22. cartomancer says

    Someone has probably looked into this already, of course, but it would be interesting to know exactly when in our history this fashion for mammaries as sexual portions of the anatomy came about. It’s not really apparent in Greek and Roman society, for instance, where breasts tended to be seen as symbols of motherhood and nursing – baring a breast tended to be a symbolic act reminding one’s children of their filial obligations. Medieval European society, as far as I recall, wasn’t terribly hung up on the things either.

  23. Nerd of Redhead, Dances OM Trolls says

    Anat #22, I think you are right. The title of the movie was the “AHA” moment. I’ve always had trouble putting a face to a name, but eventually getting there given time. Much to the amazement of the Redhead.

  24. Pierce R. Butler says

    cartomancer@ # 27 – From what I’ve read of the cultures you mention, large breasts there mostly connoted wetnurses (breastfeeders for hire), which implied peasant-class status; hence, of course, unsexiness.

    The story of Lola Montez and “Mad King Ludwig” of Bavaria (in which she impulsively cut open her bodice strings when Ludwig asked if her boobs were real) indicates that large breasts had reached “sexy!” status by the 1840s.

  25. ridana says

    dragonessfredriksson #15

    Somehow, the human population made it without the difficulties American women seem to exclusively have.

    I’m not sure why you’re singling out American women for your disdain. This is a worldwide issue. A UNICEF report released last month showed that while 95% of babies are breastfed at some point in their lives, both Ireland (55%) and France (63%) had lower rates than the US (74%), which was only slightly lower than Spain (77%), the UK (81%) or Germany (82%). There is some ambiguity about these figures in that “ever breastfed” doesn’t seem to be well-defined, and “exclusively” breastfed is very different from “ever,” but the fact remains that this is not a uniquely American issue.

    In a study from last August by UNICEF and WHO none of the 194 countries surveyed fully met recommended standards for breastfeeding.

    “If a significant proportion of women are not breastfeeding their children, it is, by and large, because mothers…and families aren’t getting the support they need to do so.” So maybe think about offering support rather than condemnation?

  26. blf says

    An idea which one rarely hears, but which has always struck me as plausible, is that of Gillian Bentley, a biological anthropologist now at Durham University but then (2001) at University College London. From The flatter the face, the bigger the breast (April-2001):

    The […] explanation came to Bentley while she was feeding her daughter. Bentley looked down and realised that if her breast didn’t protrude, her daughter’s nose would be buried in flesh while she was trying to suckle.

    She would be in danger of being smothered. Could the breast have evolved and enlarged precisely to give infants room to breathe?

    Most primate infants aren’t at risk of suffocation, she realised, because they have a protruding jaw and lips. So she suggests that the breast co-evolved with human facial features. As the face became flatter, the breast became larger to compensate. “If infants were dying, that would have provided a very strong selection,” she says.

    The above-cited article’s link to the original New Scientist article is useless, a more useful link is Comfort feeding (paywalled).

    However, as David Bainbridge in Curvology: The Origins and Power of Female Body Shape points out (copied by hand (cannot copy-and-paste), any transcription errors are my own):

    […] One suggestion is the modifications to the shape of the human head — the dramatic reduction in jaw size and flattening of the face — meant that infants had trouble breathing during suckling. And so, instead of an infant muzzle protruding towards a flat breast, a protuberant breast grew out to engage with the flat infant face. This is a reasonable idea, but there are other flat-faced primates in existence, such as gibbons and marmosets, and their females do not have permanently swollen mammary glands. And women with very small breasts due not seem to have undue trouble with the mechanics of breastfeeding.

    I’m not entirely sure about those objections as stated: The idea does not claim to explain the permanent protrusion, only why they protrude; and it’s not the women but the babies who have a problem if the breasts did not protrude. (Bainbridge’s book did not get very good reviews, and has been cited as an example of mansplaining.)

    Bentley’s idea is, as Bainbridge says (not included in above excerpt), based on function. Which, at least to me, suggests why only women have protruding breasts. And as long as the breast protrudes enough, it’s functional; the actual “size” is determined by other factors. That is, the variability in size is to be expected. And the permanence of the protrusion could also be due to other factors.

    I have no idea if anyone has ever followed up on Bentley’s idea; indeed, Bainbridge’s comments are the only mention I’ve found.

    (Obviously, I am not a biologist !)

  27. rq says

    baring a breast tended to be a symbolic act reminding one’s children of their filial obligations

    Something to try next time they refuse to help out with chores. Though I doubt they’ll get the message…

  28. cartomancer says

    I have occasionally wondered, though, whether there are people who are attracted to women who find breasts to be a turn-off. Not just neutral about the things but find them actively off-putting. I’ve certainly never heard of such people, but given the cultural climate it’s not really something many would feel comfortable admitting to.

    I say this because there are plenty of people who are attracted to men who find beards and body hair a turn-off. I’m one of them. Can’t stand the stuff. Our cultures sanction the removal of hair for the sake of appearance though, so I suppose that’s an acceptable thing to admit to.

  29. Tethys says

    I agree that all of those are pretty much just-so stories. I’ve been considering what would else would drive changes to a body that are involved with sexual signaling, but variability would not affect. Women just don’t grow breasts, they also put down fat deposits in the thighs and buttocks, and as noted by Reed at #1, the fatty tissues in breast develops after the firm tissue.

    The nursing theory is right out. All nursing mammals have enlarged breasts, which disappear after weaning.

    Variability is also the norm, so perhaps that is actually a feature of what has selected for breasts outside of lactation? The same trait would also have to apply to the males in some manner. Would the distinctive broader shouldered shape of a male torso as compared to a female torso qualify? Since this trait develops secondarily, along with adulthood, and human males take longer to become mature, that has to also be factored into any theory of selection.

    I propose that all of these physical changes have the basic effect of changing an individuals profile, so as to make it easier to identify at a safe distance your individual mother or mate. Humans evolved on the savannah, and given that we have poor eyesight and smelling ability as compared to most other mammals, and a very strong in-group selection bias, this seems to be a plausible driver for both selection, and the observed varibility.

  30. Azkyroth, B*Cos[F(u)]==Y says

    On an obliquely related note, does anyone have sourcing suggestions for affordable 30EE bras?

  31. estraven says

    Well, I have small breasts and a small butt, but bore and nursed two children, and my spouse seems to like me the way I am, as we’ve been together nearly 49 years. Some of this crap just amazes me. He’s built kind of like a teddy bear and I’ve always loved it. Yeah, the boobs got bigger during pregnancy and nursing, but they shrank back again afterwards. Who cares? What is it with these people?

  32. Tethys says

    Bumping this in hopes of an actual biologist opinion about my enhanced profile identification theory. The human behavior of accessorizing, adornment, and personal individual style seems to be an outgrowth of this same trait. I’ve never heard of any other primates being concerned with their appearance.

    On a related note, since breasts don’t preserve, do we know the approximate time when humans evolved breasts? I’ve seen reconstructions of Lucy that show her with small, lightly furred breasts, but that seems like pure conjecture on the part of the artist.