Homo Mysterious: Evolutionary Puzzles of Human Nature (7 page)

A variety of hypotheses revolve around the concept that breasts evolved because they accurately signal the genetic quality of the woman bearing them (or, baring them). For one, they are prominently displayed, left and right, and thus readily judged as more or less symmetric. A spate of research has shown that among many nonhuman animals and human beings as well, sexual and romantic desirability correlates with degree of body symmetry: Attractive bodies and faces are those in which the left and right sides match closely.
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This makes sense, because body symmetry itself correlates with a low level of deleterious mutations and parasites, raising the possibility that pronounced breast development was favored as a kind of “honest signal” whereby women displayed their symmetry (and hence, their health) to men.

Recent studies, interestingly, have lent support to this conjecture: For example, women with symmetrical breasts have greater
fertility than do their less balanced counterparts.
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Men who preferred women with symmetrical breasts could well have experienced an evolutionary benefit as a result, and such a male preference could have impacted female anatomy, especially if the preference was manifested by men who were themselves likely to confer added fitness upon their preferred sexual partners.

An interesting wrinkle here is that larger breasts are more likely to be asymmetric than are smaller ones,
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a finding that is difficult to interpret: Is it because whatever causes enhanced asymmetry also generates increased size? Or might it be that greater size is simply more likely to reveal any underlying differences? Either way, just as men frequently worry about penis size, women often obsess about whether their breasts are suitably symmetrical. Indeed, one typically unmentioned but nonetheless genuine cosmetic payoff of brassieres is to obscure underlying breast asymmetries.

But why might women have gone along with being judged and evaluated in this way? Wouldn’t they be better off obscuring any imperfections rather than presenting them conspicuously and literally right up front, side-by-side for everyone to see? For one thing, if attractively symmetrical women displayed their charms, whereupon men insisted on making a similar assessment when it comes to making a sexual choice, then other women may have had little alternative but to go along, at least if they wanted a chance with the more desirable men.

Even then, increased breast size could have evolved as part of a female counterstrategy. Consider two very small breasts on the same woman; one has a volume of 75 ml and the other, 100 ml. Such a 25% disparity would be readily apparent. By contrast, if the breasts in question were expanded to, say, 475 and 500 ml, the asymmetry between them would be significantly obscured. The evolution of women’s breasts may accordingly have involved making it more difficult for men to assess their breasts by, paradoxically, making them
more
prominent.

It is also possible that breasts evolved as a way of signaling female quality and desirability, independent of symmetry. At issue in this approach is what the great evolutionary theorist R. A. Fisher called “residual reproductive value,”
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which is essentially an individual’s future breeding potential: For women, residual reproductive value
is highest in the early stages of reproductive capability. It then declines with age and eventually approaches zero at menopause. Not coincidentally, sexual attractiveness as subjectively measured by people’s judgment closely tracks objective measures of residual reproductive value.

It can be predicted with some confidence that when it comes to a one-night stand or equivalent, men would prefer females at peak immediate fertility, as is the case among most mammals. (Among human beings, this would be at about age 21.) On the other hand, with the prospect of long-term bonding, we would expect males to prefer females with maximum reproductive value, that is, who are just entering maturity and have the largest possible reproductive future ahead of them. Too young? She isn’t yet ovulating. Too old? She’s no longer ovulating. The ideal is just right.
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Given that women conceal their ovulation, how are men to know when a prospective romantic partner is old enough to be a reproductive prospect? Maybe by her breasts, which are as anatomically prominent as her ovulation is hidden. And how are men to know when a prospective romantic partner is probably too old? Once again, maybe by her breasts: After all, mature women ruefully acknowledge that feminine aging mandates a progressively failing battle against gravity.

It seems a bit strange that men should be so obtuse as to need ripening breasts to announce reproductive competence. Among other species, males have little or no difficulty telling who is and who is not sexually mature. But then again, our sense of smell is retarded as mammals go, and given that ovulation itself is notoriously concealed in our species, perhaps breast development has evolved to fill the gap and provide information not otherwise available.

Crucially important for what I dub the Goldilocks hypothesis is that such a signal would be difficult to fake
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: Undeveloped breasts clearly indicate sexual immaturity, whereas sagging indicates age. If so, then men should prefer breasts that are relatively plump and that therefore provide comparatively honest information as to sexual maturity. As with the symmetry situation, women would to some extent be constrained to go along, especially insofar as the
most desirable men (those likely to contribute positively to a woman’s reproductive success) were turned on by breasts—and, by extension, their owners—that are not too young, not too old, but just right, prominent but not too droopy.

Larger breasts will naturally sag over time. Hence, it is easier to judge the age of a large-breasted woman than one whose bosom is less “developed.” This in itself could have induced men to prefer larger breasts, since they provide more reliable information. Breasts that were protruding but also firm would be one way that women could advertise their youth, thereby attracting a larger number of admiring males from which they could then choose. Older women would accordingly be better off having smaller breasts, since this would help them obscure their age. But there is no getting around the fact that connective tissue stretches and weakens with the years, and, moreover, it appears to be anatomically impossible to go from large firm breasts when young to smaller, equally firm and perky ones in old age. In addition, selection wouldn’t work against a trait that conveys substantial benefit early in one’s reproductive career even if it becomes costly later, when an individual wouldn’t be able to breed in any event.

Finally, my favorite hypothesis, which I confess to admiring for reasons that go beyond scientific plausibility. For one thing, it is my own. And for another, it makes use of two important ideas in evolutionary biology—ideas that so far have been used to explain certain male traits—and turns them around, applying them to women.

The first is known as the “sexy son hypothesis.”
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It has emerged as a cornerstone of sexual selection theory, which—ever since Darwin—has struggled to explain the existence of bizarre, exaggerated male traits. The problem is that such characteristics as the elaborate, gaudy tail feathers of a peacock appear to constitute an evolutionary liability rather than an asset. Feathered finery requires a lot of metabolic energy to grow and maintain, while also subjecting its owners to such risks as getting tangled in vegetation and making their possessors more apparent to predators.

Sexual selection theory was developed to explain the peacock’s tail and other comparable anomalies; its key concept is that if a trait makes its owner sufficiently attractive to members of the opposite sex, this can more than make up for any detriment to survival, so long as the bottom line—genetic representation in future generations—remains positive.

But biologists still needed to explain why females should be endowed with such preferences. It isn’t sufficient simply to claim, as Darwin did, that members of the “fair sex” are naturally possessed of greater aesthetic perceptiveness. Enter the sexy son hypothesis, which says that the peahen was selected to prefer fancy-plumed peacocks because this would increase the chances that her male offspring would inherit a comparably impressive tail and would therefore be especially attractive to the next generation of peahens. Females who mate preferentially with sexy males would become the mothers of sexy sons, who would reward their mothers’ preferences by providing more grandchildren.

What about applying the sexy son hypothesis, inverted, to human beings? Instead of peahens choosing peacocks, substitute men choosing women, and instead of fancy tails, make it prominent nonlactating breasts. Instead of sexy sons, think doughty daughters.

The original sexy son hypothesis did not speak to what originally started the process, what first induced females of a given species to prefer a particular plumage, wattle, or bright color pattern among males. It simply emphasized that once such a preference developed, whatever its source, it could be maintained and even enhanced via sexual selection. On the other hand, I have already suggested several different factors that might have initiated the evolution of at least slightly pronounced nonlactating breasts. Whatever started things off, male preference for ample-breasted women could in theory have maintained and even enhanced female breast size if the doughty daughters thereby produced would themselves have been somewhat more attractive to the next generation of men. Inverting the sexy son hypothesis, men who preferred women with conspicuous nonlactating breasts would have fathered daughters who also had prominent breasts; if the succeeding generation of men exhibited preferences like their fathers, such doughty daughters would have rewarded their fathers’
preferences via increased numbers of grandchildren produced by those daughters once they grew up to be sexually enticing and thus reproductively successful young women.

Enter, now, the second relevant idea from evolutionary behavioral biology. Called the “handicap principle,” it suggests a particular reason why males—of any generation—might be selected to include prominent breasts among those female traits found to be sexually enticing. The handicap principle is a powerful and important concept, which speaks to one of the often-unappreciated complications of animal communication: the problem of truth versus lies, honesty versus deceit.
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In the past, biologists assumed that animal communication was simply a matter of one individual—the sender—attempting to convey accurate information, and another—the receiver—figuring it out. Fair enough, it seemed, until biologists began unraveling the evolutionary process and came to realize that natural selection operates most powerfully at the level of individuals and their constituent genes. Although there might be a payoff for all concerned if communication proceeds smoothly, there is no inherent reason for a sender to tell the truth. Instead, his or her goal is to do whatever it takes to enhance the success of his or her genes, which might well involve manipulating the receiver rather than providing accurate information.

This, in turn, would place a premium on communication that cannot be faked, that possesses a “reliability component.” For example, an additional reason for peahens to preferentially choose peacocks with fancy tails—besides the prospect of thereby begetting sexy sons—might well be, ironically, that these tails are such a handicap. In order to thrive
despite
so much feathered finery, according to the handicap principle, the male in question must be quite a guy! He might simply claim to be unusually sturdy or mutation-free, but talk is cheap. By functioning effectively despite being encumbered by this handicap, the elaborately ornamented peacock proves his quality.

The handicap principle could help explain the evolution of prominent breasts if we add it to the doughty daughter concept. The idea is that when not lactating, developed breasts may in fact be a handicap, which might be just the point. Like the peacock’s tail, perhaps breasts signal an ability to function effectively
in spite of
having to grow them and then carry them around. A woman who shows herself capable of surviving and prospering despite her mam-mary handicap—who was, in a sense, so genetically “wealthy” that she could grow wasteful and troublesome breasts for no survival benefit at all—must be a quality individual indeed. If chosen by high-quality males, her daughters would then likely be similarly endowed, not just with breasts, but also with the ability to flourish in spite of them.

Here is a further consideration, something that initially appears to be a problem with the doughty daughter hypothesis, but which can perhaps be reconciled after all. In the original sexy son version, these sons don’t only inherit their fathers’ presumed greater viability, but they also get the handicap. If so, and if along with a viable genotype, one’s offspring is also encumbered by some sort of elaborate secondary sexual trait (such as the peacock’s tail), where is the payoff? One way around this difficulty is if the immediate beneficiaries of a sexy son process aren’t those sons, but the daughters, who are likely to gain their father’s superior genes without having to cope with his sexually selected handicap (a weirdly fancy tail, or whatever, which only shows up in males). By the same token, it is possible that when men chose elaborately ornamented women (i.e., those with conspicuous nonlactating breast tissue), their sons would have been the real beneficiaries since they would have inherited their mother’s ability to flourish despite those awkward mammaries, without the handicap of actually having to produce or cart them around.

There is yet another problem with converting the sexy son hypothesis into its doughty daughter equivalent: Those sexy sons are especially likely to convey a fitness reward to their parents because in most species, males have a particularly high variance in reproductive success. A small number of successful sons can have a disproportionate effect on their parents’ fitness since some males have more than their share of offspring, while others have less. By contrast, there is less difference between the most successful females and the least. As far as we know, this is why males are overwhelmingly the more sexually selected sex.