The Scientific Fundamentalist

A Look at the Hard Truths About Human Nature

Satoshi Kanazawa is an evolutionary psychologist at LSE and the coauthor (with the late Alan S. Miller) of Why Beautiful People Have More Daughters. See full bio

September 28, 2008, Evolutionary Psychology

Barbie: Manufactured by Mattel, designed by evolution V

Why do men prefer women with large breasts?

Large breasts

Why men prefer women with large breasts had long been a mystery in evolutionary psychology, especially since the size of a woman’s breasts has no relationship with her ability to lactate; women with small breasts can produce as much milk for their infants as those with large breasts. So women with large breasts do not necessarily make better mothers than women with small breasts. Why, then, do men prefer women with large breasts? There was no satisfactory answer to this question until recently.

The then Harvard anthropologist Frank Marlowe suggested a solution to this puzzle in the late 1990s, although with hindsight it is another mystery why nobody else thought of the idea sooner. Marlowe makes the simple observation that larger, and hence heavier, breasts sag more conspicuously with age than do smaller breasts. Thus, it is much easier for men to judge a woman’s age (and her reproductive value) by sight if she has larger breasts than if she has smaller breasts, which do not change in shape as much with age. Recall that there were no driver’s licenses or birth certificates that men could check to learn how old women were in the ancestral environment. There was no calendar and thus no concept of birthdays in the ancestral environment, so women themselves didn’t know exactly how old they were. The ancestral men needed to infer a woman’s age and reproductive value from some physical signs, and the state of her breasts provided a pretty good clue, but only if they were large enough to change their shape conspicuously with age. Men could tell women’s age more accurately, and attempt to mate with only young women, if they had larger breasts. Marlowe hypothesizes that this is why men find women with large breasts more attractive.

More recently, there has been a competing evolutionary psychological explanation for why men prefer women with large breasts. A study of Polish women shows that women who simultaneously have large breasts and a tight waist have the greatest fecundity, indicated by their levels of two reproductive hormones (17-β-estradiol and progesterone). So men may prefer women with large breasts for the same reason as they prefer women with small waists. Further empirical evidence is necessary to evaluate which of these two competing evolutionary psychological explanations is more accurate. This is just one of many areas where there are competing hypotheses in evolutionary psychology -- a sign of active, healthy science and clear evidence that critics of evolutionary psychology who claim that it consists of empirically untestable “just-so stories” are simply ignorant of the field.

Men can accurately infer a woman’s age and reproductive value if they can directly observe their breasts and other physical features (such as the fat content and distribution of the body, evidenced by the small waist, as I explain in the previous post). But what would men do if they could not directly observe women’s bodies? What if the woman’s body is concealed, by heavy clothing, for example? Men need another way to determine a woman’s age: her hair color. That’s the topic of my next post.