Why Women Have Breasts


Why Women Have Breasts

Many people may suppose that the question of the title is a stupid one, given that the answer is so obvious: women have breasts for feeding babies. In fact, the question is a good one, because it is a mystery why the vast majority of women has breasts. Most women are, at this moment, not lactating, and yet they have breasts. If breasts were merely for feeding babies, then most women would not have them. They would develop them only during pregnancy, and would lose them again when they stopped breast-feeding. Humans are unique in the animal world, in that they develop breasts at puberty and retain them into old age, whether or not they ever get pregnant. This requires an explanation.

A heifer develops an udder when it is time to produce milk for feeding her calf. Once the calf has stopped feeding, the heifer (now a “cow”) dries up and the udder disappears. Growing an udder, like any body tissue, is a cost. It takes nutrients to grow and maintain the udder, and the udder itself is a hindrance to quick movement. Indeed, prominent udders are not necessarily needed at all. Seals manage to feed milk to their young without any such protuberances which would spoil their hydrodynamic form. A woman pays a cost in nutrition and agility by growing breasts when she doesn’t need them. If there is a cost, then there must be a benefit to explain why this human trait has evolved. Given that the trait is unique to humans, it is very likely indeed that the explanation will be found in some other characteristic that is unique to humans.

There is surprisingly little literature on this topic. There are a few short papers, many written long ago, and most of which have been soundly criticised by more recent writers. There is little point in repeating the demolition of several early theories here. See the bibliography for further reading (Cant 1981 especially).

Herbivorous women, carnivorous men

Our ancestors were hunters, who survived principally from eating other animals. With other hunter species, like dogs, cats, and stoats, the adult females hunt, unless perhaps they are heavily pregnant or tending young offspring. Hunting is difficult enough without udders, and so the hampering effect of udders is minimised – they only grow them when lactating. Herbivorous herd beasts like cows, sheep, and deer must be able to escape predators by running faster than their neighbours in the herd, and so one with useless udders is just asking to be eaten. Most mammals rely on speed to escape being eaten, so even solitary herbivores would be badly advised to grow udders until the proper time.

Humans, however, are importantly different, in that they do not rely mainly on speed to escape predation. Few animals will attack a group of humans, and humans will use group tactics, fire, weapons, missiles, and the like to defend themselves. Any animal powerful enough to bring down an animal as big as a grown woman, can out-run her so easily that speed of foot is not the issue. Human females gather food in groups, and in parts of the world where this is dangerous, tribes post armed teenage boys, and the occasional older man, to guard them. Much of their time they spend in camps, in the presence of fire, and very few animals will venture there. Human females, therefore, paid a much smaller price for developing udders than other animals, during their evolution. Very few women in the Pleistocene of child-bearing age would have been lost to predators. They were free to be hampered by non-lactating breasts.

In other words, division of labour made nulliparous breasts possible. Our ancestral fathers behaved like carnivores, and needed to be able to run and jump freely to hunt, and our ancestral mothers behaved more like herbivores, harvesting in groups. A nullipara is a female who has not yet borne children. It is an interesting quirk of our culture that we all know the word virgin, but few know nullipara.

The above explains how it could have been possible for non-lactating breasts to evolve, but it still falls a long way short of saying why they actually did evolve.

Beautiful plumage, beautiful breasts?

Once it has been established that it is possible for breasts to evolve, then an evolutionary scientist can argue that breasts can evolve as sexual characteristics. They could be like peacocks’ tails – arbitrary signals to the opposite sex. Peacock tails are big, fan-shaped and green, with lots of “eye-spots”. Other birds have long tapering tails or bright red plumage. It seems that it doesn’t really matter what sort of plumage evolves, as long as it belongs recognisably to a certain species, and as long as the opposite sex develops a preference for it. Once peahens start finding fan-shaped green tails with spots sexy, then it is in their interests to mate with the male with the best tail of that description. Human breasts could have evolved in the same arbitrary way. This can lead to “Fisherian run-away selection” which very rapidly causes a species to evolve such things as bright plumage, wattles and combs, and, perhaps, breasts. Fisher’s logic would say that once men start to find breasts attractive, women have selective pressure on them to grow them, and thus do such women become sexier, and thus is there a greater pressure on men to find them sexy.

The classic Fisherian example is the peacock’s tail, and I apologise to evolutionists who have read about this too many times already. In peacocks, however, the males are the pretty ones. In humans, somehow this is reversed: women are the pretty ones, and men are all astonishingly ugly. In most species, the females are the replicators, and must be sensible about camouflage and the like, and so can’t afford to be showy, but even if most of the males die because of their hampering plumage, this doesn’t matter, since those left can fertilise all the females, and will probably have the best genes anyway, since they managed to survive. This occurs in its most extreme form in species where the males play little or no part in the rearing of offspring. Humans have fairly high male parental investment, however, and so the males are more valuable, and as said before, mortality due to predation in adult females, even those hampered by udders, would be so low, that this rule could in humans be sex-reversed.

That breasts were selected for by sexual selection is convincingly argued by Geoffrey Miller in his book The Mating Mind (2000). Like other such traits, they exaggerate the different between the sexes; they are invisible on the skeleton; they vary greatly between individuals; they enlarge after puberty; they engorge with blood during sexual arousal; all cultures value them as sexual symbols and many mutilate them for sexual offences; all around the world they are emphasised to look sexy or played down to avoid sexual attention. That breasts vary so much in size is a good clue to their not being utilitarian like hands or eyes. Pre-lactating breasts consist mainly of fat. Lactating breasts produce the most milk about eight months after the start of lactation. During the first eight months of lactation, however, the breasts are at their largest. It seems that the fat is actually hampering to milk production, and that it takes eight months to change the design of the breast to maximise efficiency, and get rid of the decorative fat.

A peacock’s tail seems to have some innate qualities of beauty, in that humans find them beautiful, and they were not designed for us to gaze upon. A human breast involves a circle within a circle within a circle, all presented to the viewer. It is easy to imagine that there might be some innate beauty in this design. The nipple is not placed for efficient feeding. When tapping a barrel, one does not put the tap half way up the barrel. A much more efficient nipple would be on the bottom of the breast. This would be better for getting milk to (gravity feed) and easier for the infant to suckle from. One possible explanation for the location of the nipple is that it accentuates pertness when it points upward, and hides sagginess, in that it almost never points directly downward (see below for why this might be important). Another is that the geometric perfection of the design allows the viewer to see the tiniest of imperfections. The eye can detect very easily whether a circle is properly circular or not, and whether a circle within it is exactly at the centre. If a breast’s form is for display, then its design might be expected to be something which is particularly difficult to get just right.

Breasts, when not lactating, have an important decorative function.

So, the bigger the better, right?

If Fisherian run-away selection got a hold on the evolution of the human breast, then one might expect breasts to get ever bigger and bigger. The process has a number of limiting factors, however. For a start, breasts still do have another important function: they will be needed one day for feeding babies. Women with very large breasts report that they find it very difficult to breast-feed their babies. The infant finds it difficult to latch on to the nipple, and the baby can be smothered. Often, the heavily-encumbered mother will need both hands to feed the baby. In the environment in which our ancestors evolved (the “EEA” – Environment of Evolutionary Adaption), breast-feeding would have been vital. Without mother’s milk, the baby would die (1). Breast-feeding can take a lot of time, and Pleistocene babies may have been breast fed for longer than modern babies. It is unwise to underestimate the cost of the loss of time which needing both hands to breast-feed would incur. Frequent small feeds are far more efficient and comfortable for the mother than infrequent larger feeds. Storage of milk in the breast for more than a few hours leads to a prompt reduction in milk production, and breasts can become uncomfortably engorged. Our female ancestors would have wanted to get on with tasks such as food gathering, suckling their infants as they went, and they might do this for two years per child.

Another false “bigger is better” argument is that which says that a man will find big breasts sexy because he knows that any children he fathers by the breasts’ owner will not go hungry. In fact, the breasts on a nullipara contain mainly fat, not milk-producing tissue. They are almost no indication at all of the amount of milk a woman might produce in the future. Breasts typically increase in size for the first eight months of lactation. Besides, an ability to produce more milk than is needed is no advantage. Once a woman has produced enough to feed her baby well, any excess production is expensive waste. Almost all women can produce enough to feed a child. Milk production increases to meet demand, so a woman bearing twins will produce more (2).

Another limitation on the size of breasts would be that the larger they get, the sooner that gravity will take its toll over time. Low-drooping breasts are a sign of age, and in all cultures youth is sexy in women. A young bride will probably bear a man more children than an old one, so it is unwise to grow breasts which make you age more quickly.

Another limitation is that the larger a pair of something gets, the harder it becomes to keep the pair symmetrical. The symmetry of a body is a good clue to health and fitness, and seems to be considered important in judging mates in most species, including humans. Only very healthy women would have been able to grow a large and symmetrical pair of breasts.

It is possible, therefore, that Fisherian run-away selection took place in the evolution of the human breast as a sexual ornament, but that a few factors then later put a limit on the degree of change in the human form.

They’re sexy, you fool!

The next obvious answer to the mystery is that men find breasts sexy, and that is why women have evolved them. Again, however, this does not suffice. Yes, men do find breasts sexually attractive, but the question is why do they find them sexy? One could say that we get food because we like eating. This is not an answer to “Why do we get food?” We get food because if we don’t eat we die. We like eating because people in the past who enjoyed eating worked harder to get food, and passed on more genes as a result. Pleasure has a purpose other than itself. Men must find breasts sexy because they signal something worth knowing. That they find breasts especially sexy suggests that breasts are a very reliable indicator of some good trait in women – that they are “honest indicators”.

One objection to all this is the claim that men from cultures in which the women habitually go about topless, do not find breasts sexy. I refute this: we do not say that women’s faces are sexy, largely because we see them all the time, but were I to show a photograph of Celia Johnson‘s head to any red-blooded Englishman, then he would surely say that the image he sees is one of an attractive woman. Any body part of a woman that conveys information of her good qualities to a man is sexy.

Rival theories

Dr. Boguslaw Pawlowski of Wroclaw suggested a theory to explain permanent breasts in 1999. He writes that humans evolved a layer of subcutaneous fat for insulation against the cold of a new environment, after our species stopped living in trees in forests, and moved out onto the plains. The same hormone, oestrogen, is responsible for the growth of this fat in girls, and the development of breasts. Having breasts would be a sign of having a fair amount of fat, and men would find this sexy, because fatter women would give rise to fatter children, who would survive better. A snag with this is that men too would surely have benefited from this subcutaneous fat, and men have far less of this than women. He explains this away by writing that the oestrogen that created the fat would have “disturbed the masculinization process in males”, and that males had to be nimbler, for hunting and fighting. I’d have thought that even a nimble active man might freeze in the night if a layer of fat could make the difference. Even men have to sleep sometimes.

That fat is important to women is pretty certain. To become fertile, a woman has to have a certain amount of fat on her body (3). In the past, it would have been a lot more difficult to become fat. Modern diets make this easy. Today, most successful professional female athletes in track and field events are so lean, that they are not fertile. They do not ovulate. Men have evolved to find fertile women sexy, and infertile women unsexy. Naturally enough, most men prefer the curvaceous look of glamour models to the lean look of infertile athletes (Polivy 1986). This is reason enough for women to put on a bit of fat. That women did not have to hunt in the ancestral past also means that they were free to put on fat, and the benefits of food storage and insulation were probably useful too, and worth having since the costs of being a little plump were low.

Breasts could be a signal to a man that a woman has put on enough fat to be fertile. I do not find this explanation good either. Humans are very good indeed at gathering information from a glance at another person. Women could signal to men that they have enough fat to be fertile, simply by having enough fat to be fertile. It would show (4). Also, if men saw breasts as a cue to adequate fat for fertility, then women might evolve to grow breasts or not to grow breasts, as a dishonest signal. They might hide their fat by growing no breasts, or might suggest that they were fat and fertile by putting what meagre fat resources they had into deceitful breasts. This does not accord with observation. If breasts were simply a cue to a fat reserve, then why can men find the combination of large breasts and slender body attractive? Why would a woman who is very fat still have to grow breasts? Singh (1995) found that men preferred large breasts and symmetrical breasts, regardless of what the rest of the woman looked like, for both long and short-term relationships.

Pawlowski’s theory in my view suffers from the weakness that it relies to a great extent on the effects of hormones. It is a common error to believe that hormones have some innate effect. Imagine that a man is a forward observer for the artillery in a war. He is told to fire up a green flare if he sees the enemy approaching in the west valley, and a white flare if he sees the enemy in the north valley. He sees the enemy in the north and fires a white flare. A few minutes later, he spots a mass of explosions amongst the enemy troops. “My goodness!” he thinks to himself, “This flare gun is powerful!” In fact, the flare gun had no effect whatsoever on the enemy. It was an arbitrary signal to the hundreds of artillery crews behind him. The crewmen responded to the signal by following a pre-arranged plan, and firing at a map grid reference. If the artillery crews had not been there, or had not been told what a white flare meant, the flare would have had no effect. Another week, different codes might be arranged, and a white flare could mean something else entirely.

In animals, hormones are like those signal flares. They have no innate effect, and the response is variable. The same hormone can have one effect in one species and another in a different species. It can have a different effect on a given animal at different stages in the life of that creature. The same hormone in different individuals of the same species can lead to different responses. Creatures evolve different reactions to the presence of hormones. Human females evolved the response of growing breasts and subcutaneous fat in response to oestrogen. They also evolved many other responses to the same chemical. Testosterone would be a remarkable chemical indeed if it could cause both a deep voice and a beard. The truth is that it is a signal, and men have evolved to respond to that signal by growing beards and growling. They might have evolved to develop patches of green skin on their ears in response to testosterone, but they haven’t.

That the same hormone causes women to grow breasts and to grow subcutaneous fat does not explain the fact that women grow breasts. Women with Cushing’s Syndrome become very fat due to a hormonal problem, but they do not grow large breasts. Here, cortisol, not oestrogen, is the hormone at work (5). The woman’s body reacts to the signal by growing fat on the body, but not by growing huge breasts. If the fat were for insulation, then putting fat into breasts would be inefficient. It seems that the body can develop mechanisms to grow insulating fat without growing breasts, and if it can, then it would. There must be another reason for permanent breasts.

The big snag

I feel that where most other theories fall down is that they don’t start far enough back in time. There is every reason to suppose that there was a time when our ancestral mothers did not have breasts when they were not lactating. We share common ancestry with chimpanzees, and these do not have permanent breasts. At some point in the past, proto-humans only grew breasts when they needed to lactate. If permanent breasts evolved as a sexual ornament, then we must first think what the lusts of men at that time might have been.

If breasts only developed when milk was needed, then when a man saw a woman with breasts, he knew that she was either about to give birth, giving birth, or breast-feeding a young child. A pregnant woman cannot be made pregnant with a new child. A woman giving birth is busy. A woman breast-feeding a child is very difficult to get pregnant. Breast-feeding is an effective contraceptive. Today it is possible for a woman on a modern high-fat diet to get pregnant while breast-feeding, but in the Pleistocene it would have been an unusually well-fed woman who managed this feat. Breast-feeding spaces out births in hunter-gatherer populations. Therefore, a pair of breasts was, to an observing ancestral man, a lot like a flashing neon sign saying, “I’m infertile. Don’t bother trying to copulate with me, you’d be wasting your sperm” would be to a modern man.

In short, breasts were once a turn-off, not a turn-on. It is thought that women in the Pleistocene may have spent about half of their fertile years lactating, far more than is typical today. The presence or absence of breasts was therefore a very important signal to the men of the time.

Men will evolve the lustful instincts appropriate to the women of their time. Therefore the evolution of permanent breasts has a huge hurdle to clear. The first women with nulliparous breasts would have had a terrible handicap when it came to attracting good mates. A man’s instinct would warn him not to mate with such a woman, because she would seem pregnant already, so her first child would not be his, and she would not be able to produce a child of his for a long time. He would be mistaken, but if he acted on this belief, then the woman would be less likely to mate.

Once this hurdle is cleared, then breasts are free to evolve either as Fisherian arbitrary traits, or as advertisements of good genes of various sorts. Until it is cleared, all the other explanations are useless.

Women are special

Women are humans, despite what many male observers may tell you, and humans are different from the rest of beasts in many ways. Just as I used the uniqueness of humans to explain why breasts were free from the costs of encumbrance to evolve, so too will I use the specialness of humanity to explain how the first women with permanent breasts managed to overcome their problem.

If the first women with permanent breasts were good examples of womanhood, then surely it is not incredible that they managed to persuade some men to mate with them. Women have sexual desires too (I have this on excellent authority), and so they would have a motive to achieve this feat, especially after being on the shelf for some while.

Women are secretive

Another odd thing about humans is that their females ovulate on the sly. The current accepted theory for this is that women ovulate in secret to keep men guessing. If a man knows that a woman is not fertile, then he is unlikely to make the effort to please her children, please her, provide meat for her, and such like, and he is motivated to seek more fertile mates elsewhere. Male chimpanzees are not very interested in females unless they have big pink rumps which signal that they are fertile. A man kept in the dark has to play the game of developing a long-term bond with a woman, and will mate with her many times before getting her pregnant. Women at their fertile peak are more likely, recent research indicates, to have affairs with sexy men. All this is to the advantage of women, and the disadvantage of men. If women evolved this secrecy for their own benefit, then the fact that they still only grew breasts when they were pregnant would threaten to ruin the whole effect. Breasts meant lactation, and lactation meant children and temporary infertility. This is information men can use to their advantage.

I suggest that one reason that permanent breasts evolved is to maintain the full advantages of secret ovulation. My thinking is that secret ovulation and permanent breasts evolved at about the same time.

How they did it

Those cunning first women with permanent breasts were able to get access to the best sperm in their districts. They were able to do this thanks to several advantages, including any or all of the following:

Women can talk, and can lie more efficiently than other animals. With these powers, they could get access to the men of their dreams, and to their sperm, despite the drawback of having breasts. Humans are social animals that live a long time. A man could have developed a fondness for a woman he knows because of her charming personality, her wit, and her kindness. When presented with an opportunity to mate with such a familiar woman, a scenario he has perhaps imagined a hundred times, he may well be persuaded to copulate with her, even if he is a bit put off by those wobbly things on her chest. He can always close his eyes to them.

The mates of these first women with permanent breasts would probably not have been very vigilant in guarding sexual access to their women, because the women were apparently not very fertile. When a woman produced a child, thus proving herself to be fertile, her mate would only become wary of rivals for her affections when her breasts disappeared. Until, then, he’d think wrongly that he could relax.

Men would not have written-off women with breasts as forever infertile. Eventually, they would feel, a woman with breasts would flatten out and become fertile again. Accordingly, men would often bother to develop relations with the nice women in their area, even during periods when these women were infertile. An established relationship, even a non-sexual one, is an advantage to a woman wanting an affair with a man. This is another human thing. Though male chimps can afford to ignore non-fertile females, men would be more foolish to do so.

The contemporaries of these women would probably have had different instincts from women today. With obvious menstruation, a creature does not evolve to be lusty when infertile. Women would have evolved to be fairly lustful only when in a fertile part of their cycle. Even today, this is when they are at their most lusty. A man seeing a woman with breasts would naturally feel that she would not try to cuckold him, because she would not be lusty. Thus could the first women with hidden menstruation and permanent breasts find it easier to sleep around, thanks to the lower vigilance not only of their male mates, but of their female rivals, whose mates were going through a relatively sex-free period.

Possibly, men of the time had instincts, which may now be lost, that encouraged them to provide food for women with breasts. Women with breasts would be breast-feeding, and so in need of a bit of extra food. A man seeing his mate with breasts would continue to provide for her, to go out hunting for her, leaving her to get sperm from elsewhere, and he might well continue to provide for her as she brought up the resultant bastard child.

The children produced by this cuckoldry, were probably above-averagely good, because the women would have used their powers to get the best sperm, and the fact that they succeeded shows that they were probably fairly high-quality women to start with. Thus did the genes of these women have a good chance to replicate.

That these first women with permanent breasts would have been found less sexy because of their breasts is near certain. It is equally certain, however, that they did overcome this disadvantage. That women today have breasts is proof. If no man ever wanted to mate with a women with breasts, the only women today with them would be lactating.

The evolutionary pressure on men to get rid of an out-dated instinct, that of finding breasts repulsive, would have been very great indeed.

Big bang day

Suddenly, the world for men had changed. Whereas once the instinct to find breasts on women a turn-off was a good thing, in a short while this instinct had become a disadvantage. If one man in the tribe of permanent-breasted women were to be the only one who was not turned off completely by breasts, then he would father the whole of the next generation, and male instincts would change very rapidly indeed. I’m not suggesting that anything so extreme or literally sudden occurred, but my feeling is that the change of male instinct would have been very quick by the general standards of evolution.

Instincts are coded for by genes. Genes are made up of DNA strands which use a sort of language, Gattaca. In English words, a man’s instinct may have been quite simple, and something like the following:

“Look at women, and see if they have a pair of round wobbly things on their chests. If they have such a thing, feel sexually repulsed. Increase this feeling as they are bigger, rounder, wobblier.”

One might argue that a man with such an instinct would do well, in the light of the changes in the bodies of women, to delete this programming entirely. However, a much smaller mutation could alter the programming to:

“Look at women, and see if they have a pair of round wobbly things on their chests. If they have such a thing, feel sexually stimulated. Increase this feeling as they are bigger, rounder, wobblier.”

Only one word of the English has been altered. The instinct is pretty much the same, in that it identifies the breast, and engages an emotion based on some assessment of that identified thing, and an emotion to do with sexual feeling. My guess is that it was much easier to flip the instincts of men from one extreme to the other, than from one extreme to neutrality. Also, if evolution really was moving very quickly, then there may have been a sort of race on to develop a more appropriate instinct for men to have. The simplest solution may have won the race because it was the quickest to implement, not because it was the best solution. In a world where most men found breasts a turn-off, and in which many women had permanent breasts, a man who found breasts positively stimulating might well pass on more genes than men with more moderate opinions of breasts. Men’s lust for breasts could be one of the scars of evolution – a thing which is not optimally efficient, but the result of the particular path which brought us to the present.

Women would have evolved permanent breasts before men evolved to find them sexy. The two changes would have overlapped in time, but the start of the process must have been the breasts, not the lust. If the breasts came first, then an aversion to them would be disastrous for a man. Putting it at its most unrealistically extreme, in a world where all women have breasts, a man averse to them would probably not breed at all. In the rush to avoid this disaster, Nature might over-shoot a bit, and give men an instinct of attraction to breasts which is disproportionate to the true value of them. Men might find breasts so very sexy because they used to find them such a definite turn-off. The aversion was probably pretty strong, as strong as a young man’s aversion today to mating with a very old woman. There really is no point in mating with someone who is infertile, so instincts will go a long way to avoid this.

One reason I have for suspecting that this is true, is the observation that men find almost all breasts sexy. By and large, people make qualitative judgements regarding the attractiveness of those around them. That a woman has skin does not make her sexy. All women have skin. Smooth, healthy, blemish-free, youthfully-taut skin is attractive, while rough, blotchy, sagging skin is positively repulsive. A man is attracted by the quality of the woman’s skin, not its mere presence. While it is true that men do make some qualitative assessments of breasts, it seems that almost all young women have breasts which enhance rather than detract from their sexiness. A reversal of the instinct to find all breasts a turn off would explain this.

Another reason to believe this is that if men’s instincts of lust matched the fertility of women perfectly, then men would find large breasts the least attractive, because a woman’s breasts are at their largest when she is breast-feeding a young baby, and at this point in time she is infertile. Men would find breasts sexier as they got smaller, because as a woman continues to breast feed, her breasts get smaller, and she becomes steadily more and more fertile, as the contraceptive effects of breast feeding fade away. This is clearly not what we find in the real world.

Yet another reason to believe that men’s instincts on breasts flipped from aversion to attraction, is that men find super-normal breasts attractive. Today it is possible for plastic surgeons to create breasts that were impossibly large in the Pleistocene. There are women who make a significant amount of money by having their breasts enlarged to farcical hugeness, and then working as strippers and photographic models. It seems that the instinct that programmes a man’s lust is quite simple on this matter. This reminds me of an experiment I saw demonstrated once in which the young chicks of a species of gull were presented with a variety of things, and it was recorded how often they pecked at them. The adults of this species had a reddish patch on the underside of their beaks. When the chicks pecked at the patch, the adult regurgitated food for the chicks. The chicks pecked hardly at all at the beak of a real live adult, which had had its patch painted out. The chicks pecked most of all at a ping-pong ball which had been painted with glossy scarlet nail polish. It seems that the chicks paid no heed to the fact that a living, moving, familiar, gull-shaped object was next to them, nor to their memories of what had fed them in the past, nor to the smell of a parent just back from hunting in the sea. Instead, their instinct was just “feel the urge to peck at round red things, the rounder and redder the better.” My feeling is that men have a similar instinct when it comes to breasts. True, men prefer real breasts to fake ones, but that they are fascinated by huge breasts is undeniable.

The last hurdle

If the first women to have permanent breasts were poor specimens of womanhood, then the men who chose them for mates might have few descendants today. Only if these first women, with their prototype encumbrances, were at least adequate examples of baby-producing machines, would evolution have favoured men who found such upholstery appealing. I have to be able to show that these first women with ever-present breasts, bizarre mutants though they were, were up to the job of becoming our ancestors.

If you followed my argument in the last section, then you may agree with me that this last hurdle is circumnavigated by this theory. Yes, a lust that attracts men to something other than the most fertile and high-quality women is going to be out-competed by a more optimal lust. But a lust which attracts men to women who are probably no better nor any worse than other women, and which allows those men to cuckold their rivals, will do very well indeed. I have no reason to suppose that the first women who kept their breasts after their first pregnancy were in any other way different from their rivals. If they had given birth once and lived, then they were at least adequate, and these women would get the best sperm, and so the resultant children would be at least as good as the mothers.

Evolution is a process with a strong random element. It could be that the first women with permanent breasts just happened to have been good examples of womanhood, even if they could never run using both arms.

The nulliparas join in

The lusts of men at some point in the past altered to find breasts attractive. Once men had developed this lust, then it would become to the advantage of nulliparas to resemble their more mature rivals. I suspect that women evolved breasts which didn’t disappear after breast feeding, and that this led to the evolution of men’s lust for breasts, and that following this, nulliparous breasts evolved in response to the new lust.

If men find breasts sexy, then a young woman will do better with them than without. This way, she is likely to win a better mate, and start breeding when younger and so produce and raise more children. The breasts she has have no lactation function whatsoever. They are totally useless except for making her sexier in the eyes of men. In the past, young women with no breasts were no less sexy for lack of them, but once some women had permanent breasts, then all women had to be burdened by them.

The situation today

Today men do find breasts attractive, and breasts are permanent features on women. Once human evolution first reached this point, then breasts and the lust for them could start to evolve as Fisherian traits, influenced by sexual selection. One characteristic of a sexually selected trait is that it varies a lot. There is great variation in human breast size. It seems likely that breasts are now sexually selected traits.

The peacock’s tail works largely because it is an honest signal of good genes. If it were a signal, of any kind, of bad genes, then it would not have evolved. To survive and grow a magnificent tail, a peacock must be fit, strong, free from parasites, and well-fed. So when the peahens choose the peacock, they are picking a good one if he has a huge shimmering tail. If peahens preferred peacocks with drab, asymmetrical, damaged tails, then they would probably be picking badly-fed peacocks, who had had several close escapes, and who might be parasite-ridden, and thus they would not be combining their genes with the best available. If the human breast is like a peacock’s tail, then one would expect men to find some breasts more attractive than others, and that these would be the ones most difficult to grow.

J.T. Manning et al. (1997) note that high oestrogen levels in women are associated with a less good immune system. This suggests that only very healthy women can afford high oestrogen levels. Large breasts could therefore act as a peacock’s tail, by advertising a handicap. Just as a peacock can survive and thrive despite its tail, so a woman could survive despite a suppressed immune system.

One reason that a woman with large breasts might be sexy is that she might be advertising the fact that she is easy to impregnate. This might be because large breasts signal a good supply of oestrogen, and one effect of oestrogen is to make more permeable the zona pellucida, the membrane of the egg. It also affects the pH of the endocervical mucus. Sperm therefore would have an easier time fertilising the egg of a woman with large breasts. Note that this reasoning does not explain why women might grow breasts in the first place, but it might explain why men find large breasts sexier than small ones. These last two paragraphs both use the effects of hormones as part of the argument, and so are weakened by what I said above about hormones. However, now I am writing about the variation in modern breasts, not about why breasts evolved in the first place, and in this context the strength of such arguments is much greater.

I wrote earlier that the fact that large breasts sag more quickly is a limitation on the size of breasts. One might also argue that large and pert breasts are therefore proof of youth. It might be in the interests of a woman with a short-term mating strategy to benefit from this proof, even though she will pay the cost later. I consider this to be a very minor factor, if any, in the evolution of breasts, not least because if it were a major factor, then I would expect women to evolve the refinement of breasts which were large when young, and then decreased in size later, so as to minimise the drawbacks of sagging. Also, there are many other reliable cues to youth, such as young-looking skin. It is relevant to note that the careers of women pornographic models seem to be extended by many years, if they have large breast implants. One might argue that these large artificial pert breasts succeed in making the women appear younger. However it could be that the lust inspired by their large breasts merely compensates for the fact that men can see that these women are no longer young. I was considering illustrating this point with a photograph, but have decided against it.

Symmetry is a good guide to health. Many studies have strongly suggested that humans and other creatures judge a symmetrical body to be a good body. Certain parts of a person are particularly tricky to grow symmetrically. The face is one such part, and people judge beauty largely by the face. Small breasts are easy to grow symmetrically, but as breasts become larger, it becomes more and more difficult to grow them symmetrically, and any asymmetry shows more definitely. To grow large and asymmetrical breasts would be to advertise deformity, and be bad for the reproductive potential of the woman. Manning and others (1996 revised 1997) have pointed out that women with large breasts have surprisingly symmetrical breasts, much more than would be typical of body parts in general. This suggests that only women capable of growing symmetrical large breasts, grow large breasts, which fits the idea that they are an advertisement of health. That breast symmetry matters might be reflected in the fact that Manning (1997) found that married women have more symmetrical and slightly smaller breasts than unmarried women. Again, this argument only explains the sexiness of large breasts, not the existence of breasts in the first place.

Miller (2000) points out that a single impressive sexual ornament can suffice. A peacock must be healthy if it has grown a fabulous tail. It does not have to grow two such tails to prove itself. One tail is an honest signal, and an honest signal is proof. Miller contends that the human brain, with its capacity for humour, music, oration and the like, is a sexual ornament. This would mean that a woman who is very intelligent would already have proven to the world with her wit that she has good genes and is healthy. She would not need large breasts to get her man. A woman of lower intelligence would need to do something to convince a man that she was worth wooing. If she had not the wit to make a man stay with her for many years, copulating with her often, and helping raise her children, she could compensate by at least advertising that she was easy to impregnate. Men would find such women especially attractive, since they may appear to offer a man the jackpot: a child without the responsibility of parenting a child.

That women do vary their behaviour according to their own brand of attractiveness is supported by a number of observations. For instance, women who have sexier bodies, rather than pretty faces, are more likely to indulge in casual sex, and lose their virginity three years younger. They also put more emphasis on the physical attractiveness of their partners, and in other ways tended to be more masculine (Mikach and Bailey, 1999). This is possible partly because a woman can be exposed in the womb and during life to testosterone. Some women have high levels of both sex hormones, and testosterone makes people lustful. Women with large breasts achieve menarche (periods) earlier, and get married less often (Manning, J.T. et al., 1997). This suggests that the instincts of women are fine-tuned to their best reproductive interests, and that breasts are sexy. A woman who is likely to succeed in the long term, due to beauty and brains, may grow smaller breasts and wait longer before losing her virginity. Another option for a woman is to become as sexy as possible as quickly as possible, and start breeding as young as possible. This doesn’t mean that they are not fussy about the men they mate with, but it does mean that they would put less weight on how long those men stayed with them. They could get the best sperm by being sexy, rather than being intelligent, but they can only get the best sperm this way when they are young, so they “go for it”. There is supposedly a link between high levels of sex hormones, both oestrogen and testosterone, and lower intelligence, which fits this pattern. This would also fit the frequently noted phenomenon of women thinking little of the intelligence of busty young sexy rivals of easy virtue.

Another benefit of having breasts is that they increase the size of the chest, and so decrease the relative size of the waist. To an observer, a woman’s waist looks thinner if she has breasts. A man will find a fertile-looking woman sexy, and a pregnant woman is not fertile, so a woman’s looking as though she has no bulge at the waist will make her sexier (see my theory on fat thighs).

So, now that women are obliged to carry breasts around with them all the time, it seems that many of them have taken the trouble of growing good ones.

Men, meanwhile, are stuck with a sub-optimal instinct. Their desires are fired by a pair of useless bags of fat. Men cannot afford to lose this instinct, however. Non-lactating breasts are like nuclear missiles – we only need them or want them because other people have them.


1. It is possible, I believe, to raise children on milk from other animals – preferably not cows’, as this is high in sodium. Dog milk is okay, apparently. Babies simply can’t digest solids at first, as the bowel is not fully developed and can’t expel solid material properly or produce all the necessary enzymes. Feeding a baby on blood wouldn’t work. Blood wouldn’t have enough energy in it. It has just a bit of glucose, and very little fat.

2. Suckling causes the release of oxytocin and prolactin in the mother. Oxytocin causes the expression of stored milk; prolactin orders more. The more suckling, the more milk is produced.

3. For menarchy to begin, a girl must acculmulate 17% of her body weight in fat. Up to 27% fat, a woman remains relatively infertile, and after reaching maturity, a drop to below 22% usually results in a halt in menstral function (Gallup 1982).

4. Tovée, Cornelissen et al. (1999, 2001) have shown that people are very accurate indeed in their ability to judge the Body Mass Index (BMI) of their fellows.

5. Cushing’s syndrome is caused by too much cortisol. Cortisol is (amongst other things) the stress hormone, and the excess is typically either due to drugs or a tumour. Cortisol, although related to anabolic steroids, is not the main hormone that causes fat to be laid down post-puberty, which is oestrogen.


Anderson, Judith L. (1988) Breasts, hips, and buttocks revisited – honest fatness for honest fitness. Ethology and Sociobiology vol. 9, pp.319-324.

Anderson Judith L. et al. (1992) Was the Duchess of Windsor right? A cross-cultural review of the socioecology of ideals of female body shape. Ethology and Sociobiology vol. 13, pp 197-227.

Cant, John G.H. (1981) Hypothesis for the evolution of human breasts and buttocks American Naturalist vol. 117, pp. 199-204.

Caro, T.M. (1987) Human breasts: unsupported evidence reviewed Human Evolution vol 2, pp. 271-282

Gallop, Gordon G. Jr. (1982) permanent breast enlargement in human females Journal of Human Evolution vol. 11, pp. 597-601.

Manning, J.T. et al. (1996 revised 1997) Breast asymmetry and phenotypic quality in women Evolution and Human Behaviour vol. 18, p.223.

Mikach, Sarah M. and Bailey, J. Michael (May 1999) What Distinguishes Women with Unusually High Numbers of Sex Partners? In Evolution and Human Behaviour, Vol. 20(3): 141-150.

Miller, Geoffrey (2000) The Mating Mind London.

Pawlowski, B. (1999) Permanent breasts as a side effect of subcutaneous fat tissue increase in human evolution. Homo vol. 50, pp. 149-162.

Polivy, J., Garner, D.M., and Garfinkel, P.E. Causes and consequences of the current preference for thin female physiques. In Physical appearance, stigma, and social behaviour, C.P. Herman, M. Zanna, and E.T. Huggins (eds.). Hillsdale, NJ: Erlbaum, 1986, pp. 89-112.

Singh, D. (1995) Female health, attractiveness, and desirability for relationships: Role of breast asymmetry and waist-to-hip ratio. In Ethology and Sociobiology, 16, 465-481.

Thornhill, Randy and Gangestad, Steven W. (May 1999) The Scent of Symmetry: A Human Sex Pheromone that Signals Fitness? In Evolution and Human Behaviour, Vol 20(3): 175-202.

Tovée, M.J., Maisey, D.M., Emery, J.L. and Cornelissen, P.L. (1999). Visual cues to female attractiveness. Proceedings of The Royal Society, London. B.266: 211-218.

Tovée, M.J., and Cornelissen, P.L. (2001). Female and male perceptions of female physical attractiveness in front view and profile. British Journal of Psychology (in press).