This is one of my most visited posts, so I wanted to make it a little easier to tell what it is suggesting. Toward that end I created this infographic:
[now the original post]
Until very recently, it was a mystery to evolutionary psychology why men prefer women with large breasts, since the size of a woman’s breasts has no relationship to her ability to lactate. But Harvard anthropologist Frank Marlowe contends that larger, and hence heavier, breasts sag more conspicuously with age than do smaller breasts. Thus they make it easier for men to judge a woman’s age (and her reproductive value) by sight—suggesting why men find women with large breasts more attractive.
It isn’t clear to me why finding large breasts attractive isn’t a simple case of a supernormal stimulus at work.
Putting that aside, I can see the point. Ultimately what we find attractive in a potential mate are those features which, in the environment in which we evolved, tended to lead to having lots of progeny. Our entire notion of what is sexually attractive is encoded into our brains today for the very reason that those who had it over the millennia left more copies of their genes behind. Many thousands of years ago, people didn’t live to a ripe old age, so you needed to have kids while the iron was hot, so to speak; you were probably going to be dead at 30 and making babies until it happened. Hence mechanisms in the male brain that made him better at choosing a younger (but still fertile) woman as his primary partner were generally adaptive, because they left more time for baby-making.
On the other hand, they were only adaptive to the extent that you couldn’t already tell a woman’s age — was that a problem? Faces and voices and skin tone and energy level and behavior and peers and so on seem like pretty good indicators too. After all, most women wear bras these days (among many other mechanisms to give the appearance of youth), but we can still generally tell the difference between a 15-year-old, a 20-year-old, and a 25-year-old, can’t we? Would larger but variable breasts on young women really make it easier to judge their age?
Beyond that I have a problem with the quote’s dismissal of a relationship with lactation.
Yes, if we were to measure breast size and lactation today, I doubt that there would be a significant correlation. But that doesn’t mean that there was no such correlation in our ancient past. Imagine women hundreds of thousands of years ago, who almost certainly had much smaller breasts than women today (we are the exception among primates). Picture that, for whatever reasons, some of these women have breasts that are less developed functionally than others, and these are often smaller. Perhaps they have poorer immune systems and have therefore struggled more with parasites, a major problem in those days. Or maybe these women were malnourished during a critical period. Or their hormone levels weren’t right during development. Or perhaps their genes for the construction of mammary glands are just off. In any case, among the small breasts of the day, theirs are smaller, for reasons that will negatively impact their children.
In such an environment, genes that gave men a preference for larger breasts would do well because they would tend to result in healthier children. In those days, “larger” meant something else, perhaps still tiny compared to women of today. But genes encoding such a preference into the male brain would cause average female breast size to increase over the millenia, just as the peahen’s preference for males with longer tails with lots of eyes gradually increased tail size in peacocks. Women who tended to accumulate fat in the breasts (a cheap way to make breasts larger) were considered more attractive, even though the fat had nothing to do with fertility. And just as the peahen’s preference persists even now, when peacocks have enormous tails that make them less able to escape predators, the human male’s preference for larger breasts would persist even though it no longer really means that his children will be better fed or have a better immune system.
In other words, just because you can’t tell anything about how much milk a 600cc breast will produce vs. a 300cc breast — the difference generally being the amount of fat in the breast — that doesn’t mean that preferring a 60cc breast over a 30cc breast wouldn’t have been adaptive way back then. The inflation to today’s numbers could just be a long-term effect of the preference itself, the result of generation after generation of men preferentially selecting women who tended to accumulate fat in the breasts. This kind of effect (if that’s really what’s going on here) is called a Fisherian runaway, after the genius who first suggested it.
In other words, just as women with small breasts today may use padding to trick the male brain into being more attracted to them, women’s DNA may have evolved to pad their breasts on the inside for the same reason.
By the way, you might think that a mutation that eliminated peahens’ preference for long tails with lots of eyes would do quite well, since the resulting males, which would tend to have shorter tails, wouldn’t be so handicapped against predators. But in fact those males would have trouble attracting a mate, so the mutated gene would die out.
If there are in fact genes that cause a preference for larger breasts, then where does it stop? Why not breasts the size of a house? Well, for one thing there are health implications beyond a certain point. But we also seem to have a preference for “normal” — that is, we find anything too far out of the ordinary unattractive. A guy might be attracted to somewhat larger breasts than normal, but he is not attracted to breasts that are so enormous compared to what he has seen before that they seem freakish. Full lips (a sign of female hormones) are also generally considered attractive, but not so full as to seem abnormal.
But today, it is possible and even common for women to simply buy larger breasts (roughly 5% of American women have breast implants now, and if recent trends continue that number will double in the decades to come), or to buy devices that make their breasts look larger, in an effort to make themselves more attractive — ultimately to the genes that construct the male brain. As more and more women do this, it raises the standard for “normal,” and still larger breasts then seem attractive. Then women want larger implants, and so on.
On the computer, of course, it is simply a matter of moving pixels around, and the laws of physics are no obstacle in cyberspace. Images abound on the web of women with breasts that, for most of us, seem ridiculously large. No doubt there is some variation among males in the strength of their preference for large breasts. But it is also true that on the web men can repeatedly find images of women with breasts a little larger than what they are used to seeing, and in so doing continually ratchet up their idea of what is “normal.” Breasts that seem abnormally large or round to one man may be attractive to another man simply because the latter is used to seeing breasts like that — to him, they no longer seem abnormal.
So — couldn’t that be a reasonable hypothesis for why many men have a preference for larger-than-average breasts? In fact, we are generally attracted to somewhat exaggerated sex organs and secondary sex characteristics because they imply good levels of sex hormones. This makes us responsive to supernormal stimuli.
And, by the way, isn’t it possible that a Fisherian runaway is also at work on women, making them generally prefer taller men? Eons ago a man being taller might be an indication of a better immune system or more male hormones, either of which could have represented a real advantage. Children of women with a preference for taller men would have fared better as a result. (Some have suggested that an ancient preference for height might have meant that the woman’s family was better protected, but much better indicators of a man’s fighting ability than his height were surely readily available, and when good indicators are available, adding in bad indicators just makes your judgement worse.)
Fast forward to today, and height has far more to do with where your ancestors lived than it does with the quality of your immune system or your hormones. And height comes at a cost — the taller we are, the greater the stress on our cardiovascular systems. But even though it sounds like a woman might generally be better off with a shorter man because he will live longer, a mutation eliminating the preference for taller men would result in shorter male children who would have less success attracting high-quality mates — the mutation would not flourish. So, like peahens looking for long tails, women may be stuck with a preference for tall men even if it means they are more likely to be left widows.
Of course, that’s ignoring the fact that a woman’s genes might have benefited from her choosing taller men because of wider opportunities or bigger rewards that he might receive simply for being taller. Nuances upon nuances — fun stuff.
[I'd love to hear from a professional in the field of evolutionary psychology on these thoughts. I am *not* one.]