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.

NOTES

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.

很多人可能会认为标题中的这个问题很愚蠢，因为答案很明显：女人有乳房是为了喂养婴儿。实际上，这是一个很好的问题，因为为何几乎所有女性都有乳房是一个谜。大多数女性在目前这个时间并没有分泌乳汁，但是她们拥有乳房。如果乳房仅仅是为了喂养婴儿，那么大多数女性就不会拥有它了。如果真是如此，那么乳房只会在怀孕的时候才发育出来，而且在女人们停止母乳喂养之后又会消失。人类中女性在青春期乳房得到发育，而且在年老之后仍然保留乳房，不管她们是否怀孕，这点在动物世界中是独一无二的。它需要一个解释。

小母牛在到了为喂养自己的小牛而产出牛奶的时候乳房才得到发育。一旦小牛断了奶，小母牛（如今是一只“奶牛”）的奶水枯竭，乳房消失。就像任何身体组织一样，生长乳房是有代价的。生长和维持乳房需要营养，而且乳房本身对于快速移动是一个障碍。确实，突出的乳房完全就是不必要的。这种突起物会毁坏海豹那符合流体动力学的外形，它们能够设法在没有乳房的情况下给自己的幼崽喂奶。一个女人在不需要乳房的情况下长出乳房，需要为此付出营养上和敏捷性上的代价。如果这样做有代价，那么它必须有一种益处，来解释为什么人类进化出这种特征。我们知道这个特征是人类所独有的，所以很有可能对此的解释可以从一些其他同样也是仅为人类所特有的特质中找到。

令人惊讶的是关于这个话题的著作非常少。有少量短篇的论文是很久以前写出来的，而且其中的大多数都被后来的作者所批驳。重复对于许多早期理论的证伪已经没有多少意义了。参阅参考书目以做进一步阅读（特别是Cant, John G.H.1981年发表的那本书）。

素食的女人，肉食的男人

我们的祖先是猎人，他们能生存下来主要依靠吃其他的动物。其他的狩猎物种，像狗、猫和鼬，它们的成年雌性可能只有在自己怀孕后期或是照料年幼的后代的时候才停止狩猎。没有乳房，狩猎也足够困难了。所以乳房的阻碍作用被降低到了最小程度——它们只有在分泌乳汁的时候才发育。食草的兽群，像奶牛、绵羊和鹿必须能够比自己在兽群中的邻居跑得更快才能逃脱捕食者，所以拥有无用的乳房就相当于请别人吃你。大多数哺乳动物依靠速度来逃脱被捕食的命运，所以甚至独居的食草动物最好也是在合适的时候再发育乳房。

然而，人类却不是依赖速度来躲避捕食者的，这是一个非常重要的不同点。很少有会攻击一群人的动物，而且人类会利用团体策略，火、武器、投射物和类似的东西来防御自己。任何强大到足以打倒像成年女性这么大动物的物种，都可以如此轻易地在速度上超过她，以至于脚的速度根本就不是一个问题。人类女性成群地搜寻食物，而在这样做有危险的地方，部落派遣有武装的年轻男孩，偶尔也有更年长的男人，来守卫女人。大多时候女人们都在营地里，在火堆旁边，很少有动物敢到那种地方冒险。因此，人类女性在进化过程中为发育乳房付出的代价相比于其他动物来说小了很多。在更新世（大约起始于两百万年前，终于一万年前，当时地球大多被冰雪覆盖，译者注），很少有怀胎的女性丧命于捕食者口下。她们允许被不分泌乳汁时的乳房妨碍。

换言之，劳动分工使得未生产时的乳房成为可能。我们父辈祖先的行为像食肉动物，需要能够自由地跑、跳来狩猎，而我们母辈祖先的行为更像食草动物，成群地收集食物。一个未产妇（nullipara）是还没有生过孩子的女性。很奇怪的是，我们都知道“处女”这个词，却很少有人知道“未产妇”这个词。

以上我们解释了进化出存在于不分泌乳汁时的乳房是如何成为可能的，但是这距离解释为什么它们最终被进化出来还有很远的距离。

美丽的羽毛，美丽的乳房？

一旦我们确立了进化出乳房的可能，那么持进化论的科学家可以争论，乳房可以进化为性别特征。它们可能就像孔雀的尾巴一样，是对于异性的主观的信号。孔雀的尾巴是很大的、扇状的、绿色的，上面有很多“眼状斑点”。其他的鸟类拥有长的、逐渐变细的尾巴，或是鲜明的红色羽毛。似乎进化出什么类型的羽毛并不重要，只要它可以被辨识，属于每一物种，而且引起异性对它的偏爱。一旦雌孔雀开始认为带有斑点的、扇状的绿色尾巴是迷人的，那么它们就乐于和符合这种描述而且拥有最美丽的尾巴的雄性交配。人类的乳房也可能是以相同的主观的方式进化的。这可能会导致我们相信“费舍尔压倒性优势选择学说（Fisherian run-away selection）”，并迅速地致使一种物种进化出例如颜色鲜亮的羽毛、喉部肉垂和鸡冠，以及或许乳房这样的特征。费舍尔的逻辑是说一旦男人发觉乳房很迷人，那么女人就有选择的压力使自己长出乳房，因此她们变得更加迷人，于是又有更大的压力使男人们认为她们迷人。

经典的费舍尔的例子就是孔雀的尾巴，我要向已经读过这个例子很多遍的进化论者道歉。然而在孔雀中，雄性是更加美丽的。在人类中，这是相反的：女性是更美丽的，而男性则都令人惊讶得丑陋。在大多数物种当中，雌性是生育后代的，它们必须对于掩饰这类行为比较敏感，所以不能过于艳丽。不过即使大多数雄性因为阻碍行动的羽毛而被猎杀，这也没有关系，因为剩下的雄性会对所有雌性受精，而且由于它们设法生存了下来，它们很可能会拥有最好的基因。这种情况最极端的表现形式就是某些物种中雄性很少甚至不参与抚养后代。然而，人类中男性比较多地投入到抚养后代中，因此男人更加有价值。正如之前所说，即使被乳房阻碍，成年女性由于被捕食造成的死亡数目仍然如此之低，以至于上述这条法则在人类中性别是反过来的。

Geoffrey Miller在自己的《The Mating Mind》(2000)一书中令人信服地论述了乳房是由性的选择（sexual selection）来选择的。就像其他类似的特征一样，它们凸显出性别间的差别；它们在骨骼上无从辨认；它们的大小在个体间存在很大差异；它们在青春期后开始变大；它们在性兴奋的时候充血；所有的文化都将其视为性别符号，而且很多文化以切去乳房作为一种性侵犯；在全世界它们都被凸显以示性感，或是被掩饰以减少有关性的注意。乳房大小差异如此之大是表明它们不像手或眼睛那么有实用价值的线索。在分泌乳汁之前乳房主要由脂肪组成。乳房在哺乳期开始大约八个月后产出最多的乳汁。然而，在最初八个月的哺乳期中，乳房的大小是最大的。似乎实际上脂肪阻碍了乳汁的产生，所以需要八个月的时间来改变乳房的构造，来达到最高的效率，并除去装饰性的脂肪。

一只雄孔雀的尾巴似乎有种天生的美，人类因此认为它美丽，不过它们并不是为了让我们观赏而设计的。人类的乳房呈现在观赏者面前一层套一层的圆环。我们很容易想象在这样的设计中也有一种天生的美丽。乳头位于那个位置并不是用来高效地哺乳的。当我们要打破一个桶，放出里面的液体时，我们不会在桶一半高的地方开口。一个更加高效的方法是在桶底开一个口。这样可以更好地得到乳汁（重力喂养），并且让婴儿更容易吮吸。对于乳头位置的一种可能的解释是，当它指向上面时显得更加诱人。同时这样不会显得下垂，因为它几乎从不直接指向下面（看看下文就可以知道为什么这点如此重要）。另一种解释是这种设计在几何学上的完美让观看者可以看到最微小的瑕疵。眼睛可以轻易察觉一个圆是否是一个正圆，它内部的一个圆是否和它同心。如果乳房的外形是用来观赏的，那么它的设计中就应该有些方面是特别难以做到正好合适的。

乳房在不哺乳的时候有非常重要的装饰作用。

所以，越大越好，对吗？

如果费舍尔压倒性优势选择学说能够解释人类乳房的进化，那么可能会有人认为乳房会长得越来越大。不过，这个过程中有许多制约因素。首先，乳房还有另外一个重要的功能：某一天它们需要被用来给婴儿哺乳。拥有很大乳房的女性描述说她们很难用母乳喂养婴儿。婴儿难以吸住乳头，而且他们可能会窒息。被严重拖累的母亲往往需要用双手来喂养婴儿。在我们祖先进化过程中所处的环境（进化适应环境（Environment of Evolutionary Adaption））中，母乳喂养应该是很重要的。如果没有母亲的乳汁，婴儿会死亡（1）。母乳喂养需要很多时间，而更新世的婴儿可能需要比现在的婴儿所需更长的母乳喂养期。低估由于需要用双手来喂养孩子而付出的时间上的代价是不明智的。频繁的少量喂养对于母亲来说远比偶尔的大量喂养更加高效和舒适。乳汁在乳房中贮存了没几个小时后就会导致乳汁分泌的迅速较少，而乳房也会由于充血而感到不适。我们的母辈祖先希望继续采集食物和喂养婴儿的工作，而喂养每个小孩可能需要两年的时间。

另一种错误的“越大越好”的论据是认为男人觉得丰满的乳房性感，因为他们知道这样自己的孩子就不会挨饿了。实际上，未产妇的乳房主要含有的是脂肪，而不是可以分泌乳汁的组织。它们几乎完全无法表明将来这个女人能够分泌多少乳汁。乳房通常在哺乳期的前八个月变大。另外，能够分泌比实际需要量更多的乳汁并不是一种优势。一旦一个女人为了喂养孩子已经分泌了足够多的乳汁，任何剩余的产出都是浪费的废物。几乎所有女人都可以分泌足够多的乳汁来喂养一个孩子。乳汁的分泌会增加，以满足需求，所以一个生育了一对双胞胎的女人会分泌更多的乳汁（2）。

另外一个对于乳房大小的限制是它越是丰满，重力对它的不利影响就越快显露出来。下垂的乳房是年龄的象征，而在所有的文化中，年轻女人都是迷人的。一个年轻的新娘很可能会为男人生下更多的孩子，所以长出大到加速自己衰老的乳房是很不明智的。

还有一个限制则是一对东西长得越大，就越是难以让它们保持对称。身体的对称是健康的表现，而且它似乎在包括人类的多数物种中被用来作为评判配偶的一个重要标准。只有非常健康的女人才可以长出一对丰满而又对称的乳房。

因此，可能费舍尔压倒性优势选择学说在人类乳房进化中得到了体现，使乳房成为了性的装饰，但是一部分限制因素随后又限制了人类外形的变化程度。

它们很性感，你真傻！

对于这个谜团的下一个显而易见的答案是男人觉得乳房性感，所以女人进化出乳房。然而，需要再说一次的是，这还不够。是的，男人的确认为乳房有性的吸引力，但是问题是为什么他们这样认为？可能有人说我们喜欢吃，所以去获取食物。这不是“我们为什么吃东西”的答案。我们吃东西是因为不吃东西会死。我们喜欢吃东西是因为过去那些喜欢吃东西的人更努力地获取食物，于是因此遗传下来了更多基因。愉悦除了自己本身还有一个目的。男人认为乳房性感必定是因为它们是某些值得知晓的东西的信号。男人们尤其认为乳房性感，这表明乳房是女人某样优良的特性的非常可靠的指示物——它们是“诚实的指示物”。

一种对于以上理论的反驳是在一些女人习惯于袒胸的文化中，男人并不认为乳房性感。我要驳斥这种观点：我们不说女人的脸是性感的，多半是因为我们一直都可以看到它。但是如果我给一个典型的英国人看一张Celia Johnson（四十年代英国著名影星）的照片，那么他肯定会说自己在照片中看到的是一个有魅力的女人。女人身上任何向男人传达出其优良特质的身体部位都是性感的。

敌对理论

Wroclaw（波兰一城市）的Boguslaw Pawlowski博士在1999年提出了一种对于永久乳房的解释。他写道当人类不再生活于树上和森林中，而是转移到平原以后，他们进化出了一层皮下脂肪用以隔挡新环境中的寒冷。雌性激素同时负责女孩体内这种脂肪的形成和乳房的发育。乳房是拥有适量脂肪的象征，而男人们认为这样性感，因为胖的女性往往生出更易于生存的胖小孩。这个理论的一个障碍在于，男人肯定也能从这种皮下脂肪中受益，然而男人体内的这种皮下脂肪大大少于女性。他用了雌性激素会“扰乱男性的男性化过程”，以及男性需要变得更加敏捷以进行狩猎和打斗来解释这个问题。我认为即使一层脂肪可以起作用，那么一个敏捷灵活的男人也可能会在夜里冻僵。男人有时候也需要睡觉。

那层脂肪对于女人很重要，这点是可以肯定的。女人体内需要拥有一定量的脂肪才能够生育（3）。以前，要想变胖要困难许多。现代的饮食让它变得容易。如今，多数成功的职业女性田径运动员瘦得无法生育，他们不排卵。进化让男人发觉能够生育的女人迷人，而不能生育的女人就没有吸引力。多数男人自然更喜欢迷人的模特的曲线美，而不是无生育能力的女运动员的精瘦身材（Polivy 1986）。这为女人增加一点点脂肪提供了足够的理由。在我们祖先所生活的过去，女人不用狩猎也意味着她们可以肆意地增加脂肪。而食物的储存和隔离很可能也是有用的，因为为了略显丰满的体态所付出的代价并不高。

对于男人来说乳房可以作为女人拥有了足够脂肪的标志。我同样也不觉得这是个好的解释。人类利用对别人的一瞥来搜集关于对方的信息的能力是很强的。女人可以简单地用拥有多得足以生育的脂肪来向男人发出信号，告诉他们自己拥有足够的脂肪来生育。这样就可以显示了（4）。而且，如果男人把乳房看成是拥有生育需要的足够的脂肪的信号，那么女人可能会进化为有的长乳房，有的不长乳房，以此作为一种欺骗性的信号。她们可以不长乳房，来隐藏自己的脂肪，或者可以利用不足的脂肪长出具有欺骗作用的乳房，来显示自己丰满而能够生育。这和我们的观察不符。如果乳房仅仅是脂肪贮存量的一种信号，那么男人为什么会认为丰满的乳房和苗条的身材的结合是有吸引力的？为什么一个非常胖的女人仍然长着一对乳房？Singh（1995）发现无论在长期还是短期的恋爱中，不管女人其他部位长得怎么样，男人都更喜欢丰满而对称的乳房。

在我看来，Pawlowski的理论太过依赖于激素的作用了。认为激素有一些先天的作用是一个常见的错误。试想在一场战争中，某个男人是炮兵的前进观察员。他被告知如果看到敌人在西面的山谷中就点燃绿色的信号弹，而如果看到敌人在北面的山谷中就点燃白色的信号弹。他在北面山谷看到了敌人，然后点燃了白色的信号弹。几分钟后，他看到了敌军部队中的巨大爆炸。“我的天呐！”他想到，“这枚信号弹的威力真大！”实际上，信号弹对于敌人没有任何作用。真正起作用的是传递给在他身后成百上千炮兵的那个任意的信号。炮兵对于信号做出回应，遵循了事先预定的计划，向地图上的一处网格参考点开火。如果炮兵们不在那里，或者他们没有被告知白色信号弹代表的意义，那么信号弹就不会有任何效果。在另一个星期里，可能又会安排不同的密码，白色信号弹可能会代表完全不同的意义。

在动物中，激素就好像那些信号弹。他们没有先天的作用，对它们的响应是可变的。同样的激素对于某个物种可能有某种作用，而对于另一个物种中又有另一种作用。它可以在某种动物生命中的不同时期对其产生不同的作用。同种激素在同一物种的不同个体间可以产生不同的反应。生物进化出了对于激素的存在的不同反应。人类女性进化出了乳房和皮下脂肪，作为对于雌性激素的响应。她们同时也进化出了对于同种化学物质的其他响应。如果睾丸激素既导致了深沉的嗓音，又导致了胡子的生成，那么它真是一种不同寻常的化学物质。事实是它只是一种信号，是进化让男人对于这种信号做出长出胡子和声音变得低沉的回应。他们可能会进化成以在耳朵上长出很多绿色的小斑点作为对于睾丸激素的回应，但是他们没有那样进化。

同一种激素导致女人长出乳房和皮下脂肪不能解释女人长有乳房这个事实。患有库兴氏综合征的女人由于激素问题变得十分肥胖，但是他们并不长出丰满的乳房。在这里，不是雌性激素，而是皮质醇在起作用（5）。女人的身体对这个信号做出身体变胖的回应，而不是长出巨大的乳房。如果脂肪是用于隔挡寒冷，那么在乳房中增加脂肪并不有效。似乎身体可以产生一种机理，在不生长乳房的情况下长出用以隔离寒冷的脂肪，如果身体可以产生这种机理，那么它就会产生这种机理。导致永久的乳房的应该还有另外原因。

大障碍

我认为多数其他理论失误的地方在于它们在时间上都没有回溯到足够早的年代。我们有充足的理由推测曾经有一段时间我们的母辈祖先在不哺乳的时候是没有乳房的。我们和黑猩猩有着共同的祖先，而它们就没有永久的乳房。在过去的某个时期，原始人只在需要分泌乳汁的时候才长出乳房。如果永久的乳房是作为性的装饰物而得到进化的，那么我们首先必须思考当时的男人的色欲是什么。

如果乳房只在需要乳汁的时候才得到发育，那么当一个男人看到一个女人拥有乳房的时候，他就知道她可能将要生产了，她正在生产，或者她正在给婴儿哺乳。一个孕妇是无法再怀上另一个孩子的。一个正在生产的女人是很忙的。而一个正在用母乳喂养孩子的女人是很难怀孕的。母乳喂养是一种有效的避孕剂。如今通过现代高脂肪的膳食可能可以让正在母乳喂养阶段的女人怀孕，但是在更新世，可以做到这点的必需是一个罕见的营养良好的女人。在以狩猎为生的群居人群中，母乳喂养使分娩不会过于集中。因此，一对乳房对于一个注意观察的远古的男人来说，就很像是现代社会中一块写着“我现在不能生育。不要费力气尝试和我交配，你只会浪费自己的精子”的霓虹灯标牌。

简而言之，乳房曾经并不是催情剂，相反它只会让人“扫性”。人们认为更新世时期的女人花自己生育期的一半时间来分泌乳汁，远多于如今的女性。因此，女性是否有乳房对于当时的男人来说是一个非常重要的信号。

男人会进化出适合于当时的女人的好色本性。因此永久乳房的进化还有一个巨大的障碍有待清除。首先在生育之前就长出乳房的女人肯定在吸引优良配偶的时候遇到了巨大的困难。一个男人的本能会警告他不要和这样的女人交配，因为他看上去已经怀孕了，所以她的第一个孩子不会是自己的，而且她在很长一段时间里都不能产下自己的孩子。他可能会被误会，但是如果他依据自己的信念行事，那么这个女人就只有很小的机会和他交配。

一旦这个障碍被清除，那么乳房就可以自由地作为费舍尔的主观特征进化，或是以各种良好基因的广告的身份得以进化。直到那以前，所有其他的解释都是无用的。

女人是特殊的

女人是人类，尽管男性观察者会告诉你不同的结论，而人类和其他的动物在很多方面是不同的。正如我用人类的独特性来解释为什么乳房可以不用顾忌累赘的代价而得到进化一样，我会用同样的方法通过人类的独特性来解释最早拥有永久的乳房的女人是如何设法克服她们的问题的。

如果最早拥有永久乳房的女人是女人特质的优良实例，那么她们设法说服一些男人和她们交配就显然不是那么难以置信的了。女人也有性欲（我在这方面是杰出的权威），所以她们有实现这个举措的动机，特别是在遭遇了一段时间的白眼之后。

女人是秘密的

另一个关于人类的奇怪的事情是女性是偷偷地排卵的。对此现在广为接受的理论是女人偷偷排卵，以此让男人们猜测。如果一个男人知道一个女人生育能力不强，那么他就不太可能去努力讨好她的的孩子，讨好她，给他提供肉或者类似的东西。相反，他会有动机去别的地方寻找更有生育能力的配偶。雄性黑猩猩只有在看到雌性拥有丰满的粉红色臀部这个表明其生育能力的信号时才会对雌性产生兴趣。一个被蒙在鼓里的男人需要和一个女人有一种长时间的契约，需要在让她怀孕之前和她交配很多次。最近一项研究显示，女人在自己处于生育能力的顶峰时更有可能和性感的男人上演风流韵事。这些都是女人的优势，同时也是男人的劣势。如果女人是为了自己的优势而进化出这种保密的，那么她们仍然只在怀孕的时候发育乳房的事实就会有毁掉这整个效果的威险。乳房意味着哺乳，而哺乳意味着孩子和暂时的不能生育。这是男人可以利用的优势。

我认为进化出永久乳房的一个原因是为了维持秘密排卵的绝对优势。我的想法是秘密排卵和永久乳房大约实在同意时期进化出来的。

她们是怎么做到的？

第一批拥有永久乳房的狡猾的女人能够得到自己地区中最好的精子。她们能够做到这些多亏了许多的优势，包括如下所有：

女人可以说话，而且可以比其他任何动物更加有效地撒谎。虽然她们有乳房这个缺点，这些说话和撒谎的能力使得她们可以得到她们梦想的男人和精子。人类是生存时间很长的社会动物。一个男人可以因为一个女人的人格魅力、智慧和善良而爱上她。当一个和如此熟悉的女人交欢的机会摆在眼前时——这种情节他可能已经想象过一百遍了——他会很容易被说服去和她交欢，即使他对她胸前那对摆动的东西有一点点厌恶。他总是可以闭上眼睛不去看它们。

那些第一批拥有永久乳房的女人的伴侣很可能不会对于自己的女人是否只和自己交欢过于警惕，因为很显然那些女人的生育能力并不是很强。当一个女人生育了一个孩子，因此证明自己有生育能力的时候，她的伴侣只会在她的乳房消失的时候才会堤防自己的情敌。直到那以前，他会错误地以为自己可以放松警惕。

男人们不会认为有乳房的女人是永久没有生育能力并因此放弃她们。他们觉得，最终，一个女人的乳房会变平，她们也会再一次变得有生育能力。于是，男人总是费力气去和自己区域里的最好的女人发展关系，甚至是在这些女人没有生育能力的时期。一个即使是没有性爱的确立的恋情，对于希望和男人有一段风流韵事的女人来说也是有利的。这是另一个人类的特点。虽然雄性黑猩猩可以无视无生育能力的雌性，但是男人这样做会显得更加愚蠢。

与那些女人同时代的人和如今的女人很可能有不同的本能。明显的月经期使一种生物不能进化成在没有生育能力的时候也保持强烈的性欲。女人只有当处于生理周期中有生育能力的时期时才会进化得较为性饥渴。即使在今天，这仍然是她们性欲最强的时期。男人看到女人长着乳房就会自然地觉得她不会给自己戴绿帽子，因为她不会性饥渴。因此，不但要感谢她们放松警惕的男性伴侣，同时还要感谢她们的那些配偶正相对来说经历着缺少性爱时期的女性对手，第一批拥有秘密的经期和永久乳房的女人可以更加容易地和不同的男人做爱。

有可能当时男人拥有的一些本能促使他们为拥有乳房的女人提供食物，这种本能如今可能已经遗失了。有乳房的女人要用母乳喂养孩子，所以需要一些额外的食物。男人看到自己有乳房的伴侣会继续为她提供食物，继续为她外出打猎，她却自己在别处交欢，而且他有可能在她抚养作为果实的私生子的过程中继续赡养她。

由通奸所产下的孩子往往比平均水平的孩子更加优秀，因为那些女人们用自己的力量获得了最好的精子，而且在一开始她们成功证明了自己很可能是高质量的女人。因此这些女人的基因有很大几率得到了复制。

几乎可以肯定，这些拥有永久乳房的女人因为乳房的存在往往不会被认为是迷人的。不过，同样可以肯定的是，她们确实克服了这个劣势。现在的女人都有乳房就是证明。如果没有任何男人愿意和有乳房的女人交配，那么如今唯一拥有乳房的女人就只能分泌乳汁了。

进化的压力迫使男人摆脱了认为乳房令人反感的这个过时的本能。这种进化的压力真的很伟大。