Are Women Evolutionary Sex Objects?:
Why Women Have Breasts

Frances E. Mascia-Lees, Sarah Lawrence College

“Increase Breast Size…Guaranteed! #1 Seller in America” announces the advertisement for Bioussant Breast Enhancing Tablets in the September 2002 issue of Vogue magazine.

“Breast Enlargement through Hypnosis” claims The Body Contouring Programme^tm offered on the internet.

These ads, like hundreds of others in popular magazines and on the web, have hit a real nerve with U.S. women. Dissatisfied with their breasts, women in the United States have spent millions of dollars on creams, lotions, devices, and techniques for breast enlargement in the last few decades. For those women suspicious of such claims, Victoria Secret has another answer. It offers a new twist on the old push-up bra: the Miracle Bra® has silicone-filled cups meant to mimic the shape and feel of breast tissue. Until recently, silicone was also used for surgically inserted breast implants, still the only way to ensure a permanent increase in breast size. Over 2 million U.S. women opted for surgical breast enlargement last year, according to the American Society for Aesthetic Plastic Surgery (http://surgery.org/stats). This number has increased by 144 percent since 1997, despite the health risks the procedure poses and its considerable expense: it frequently costs over $3,000 for the surgeon’s fee alone.

Perhaps we should not be surprised by such statistics: after all, men seem to have an overwhelming attraction to breasts. Isn’t a woman’s wish for an enhanced bust line just a natural response to a primal desire to attract a mate? Many contemporary thinkers would suggest that this is the case. They invoke the notion of sexual selection in their arguments, arguing that some time long, long ago in the human evolutionary past, some males became erotically aroused by females with visibly enlarged breasts, choosing them more often as sex partners than their “flat-chested” sisters, thus maintaining this trait in human populations. Some writers even argue that men’s attraction to breasts was a key to the survival of early humans. Given the putative significance of breasts to the human species, is it any wonder that women in the twenty-first century spend millions of dollars, and take medical risks, to enhance theirs?

Although sexual selection arguments are extremely popular, there is another, more plausible explanation for why enlarged breasts evolved. As I will argue, females with visible breast enlargement would have been better able to support themselves and their infants in the environment in which our early human ancestors lived. Indeed, I suggest that the more robust notion of natural selection is the key to understanding why women have breasts, not the problematic idea of sexual selection. But first, I want to turn to sexual selection explanations and analyze their claims, assess the evidence they invoke, and identify their limitations. Doing so reveals that women may not be evolutionary sex objects after all.

“Big” Breasted Women

Since all mammals have breasts to nurse their young, it might seem odd to search the unique evolutionary past of humans for an explanation of their existence. Yet, there is a reason to do so. There is something different about human female breasts, leading scientists to develop theories about them: after the age of puberty, the breasts of a human female become “permanently enlarged,” although this is not the case in other primates, our closest evolutionary relatives.

If you were to look at a chimpanzee or a gorilla, or any other ape or monkey for that matter, the breasts of a non-pregnant, non-nursing female would not be enlarged. Although the breast tissue of a female ape or monkey swells when she is pregnant and nurses her young, once she is done lactating, it typically recedes back to a flattened form. Most of the time, you would not be able to distinguish a male from a female by the size and shape of breasts alone; breasts in non-human primates are not what scientists call “a sexually dimorphic trait,” one that exists in two different forms in the males and females of the same species.  

Examples of sexually dimorphic traits in non-human primates include the large canine teeth of male, but not female, baboons, and the large size of male gorillas in comparison to females. 

There is also a wide range of sexually dimorphic traits in other animals. Perhaps the most well known example of sexual dimorphism is the beautiful plumage of the peacock in comparison to the dull feathers of the peahen. In humans, breasts are most certainly sexually dimorphic. When contemporary thinkers offer explanations for the evolution of human breasts, it is their permanently enlarged condition in human females that is under consideration.

It’s Sex Stupid, Not the Environment

The existence of sexually dimorphic traits has been a concern to scientists for quite some time. Indeed, Charles Darwin, the founder of modern evolutionary studies, was quite puzzled by them. His theory of natural selection seemed inadequate to explain characteristics that existed in only one-half of each species. If a trait aided certain individuals’ ability to adapt to their environments, increasing their chances of survival and passing on their genes to subsequent generations, why would it not appear in both males and females? Looking at it another way, we might ask, if peahens survive without spectacular feathers, and female baboons without deadly canines, what accounts for their existence in the males of these species?

Darwin proposed that natural selection might not be the only evolutionary force at work. Instead, he claimed sexually dimorphic traits arose because the individuals who possessed them had an increased reproductive advantage stemming from another source: the advantage it conferred on them in terms of increased access to mates. Darwin argued that many sexually dimorphic traits were not advantageous to an individual in the struggle for survival but in the struggle over mating opportunities. Darwin called this differential access to mates “sexual selection,” and outlined how it worked in animals, including humans, in his book The Descent of Man (1871). 

Darwin focused on two types of traits that increased access to mates: the weapons and other characteristics used and displayed by males when fighting among themselves over access to females, such as the large canines of baboons, and those traits that make males more attractive to females, such as peacock plumage. Interestingly, Darwin posited that females choose males, not that males choose females, suggesting sexual selection operated on males alone. This may be because he recognized that from a species standpoint, it makes good sense for all females to mate, while this may not be equally true of males: theoretically, it takes only one “good” male to impregnate a large number of females. Whatever the reasons for Darwin’s view that sexual selection did not directly modify females, it is clear he did not think that male choice among more or less desirable females was an important evolutionary force.

Yet, this is exactly what contemporary scientists argue in their attempts to explain the difference between the breast of humans and other primates. They claim that human female breasts became sexual signals to attract males. Females with this trait would have increased mating opportunities, thereby passing the genes for permanently enlarged breasts on to subsequent generations. A prime example of this way of thinking is the following statement by anthropologist, Bernard Campbell:

Since in the primates, and particularly among men, the choice of partners lies with the male, it follows that only the physical characteristics of the female are subject to sexual selection of this kind… breasts have always been attractive to men and have no doubt been subject to sexual selection (1970:304).

But why should this particular body trait have become the object of sexual desire?  

Breasts and Butts

Desmond Morris, a curator of mammals at the London Zoo, was among the first to offer an explanation. In his widely read book, The Naked Ape (1967), he suggested that permanently enlarged breasts in human females resulted from hominid bipedalism. It is widely believed among scientists that our earliest ancestors survived by becoming bipedal, that is, by walking on two feet in an erect position. Although there are a number of specific explanations for why this trait might have conferred an adaptive advantage on some ancient hominid population, scientists agree that bipedalism is the defining characteristic of the human species. Indeed, when a fossil primate is unearthed, the only sure way to determine whether it is a hominid is to figure out whether the individual in question walked upright.

The link between bipedalism and permanent breast enlargement, according to Morris, has to do with the erotic nature of breasts. He argues that as early humans (hominids) began walking upright, face-to-face encounters between the sexes became the norm, affecting the position used in sexual intercourse: males would no longer mount females from behind as they do among non-human primates. In the non-human primate position, presentation of the female buttocks to the male is an erotic display that stimulates male interest and excitement. With the advent of bipedalism, Morris argues, if females were to be successful in shifting male interest around to the front, evolution would have to do something to make the female frontal region more stimulating to males. This was accomplished, Morris says, through self-mimicry in which female breasts came to look like rounded buttocks: female breasts became mimics of “the ancient genital display of [the] hemispherical buttocks” (1967:75). Szalay and Costello (1991) have continued this line of thinking, but argue that permanently enlarged breasts sexually arouse males not because they look like buttocks, but because they mimic the appearance of female genitalia. 

Your Cheatin’ Heart

Morris has not been alone in arguing that women’s breasts arose because of their erotic appeal, but more recent arguments do not focus on breasts as misplaced buttocks, although they continue to see breast as sexual signals developed and maintained through sexual, not natural, selection.  Instead, these theories focus on the role breasts played thousands of years ago in enticing a male to stick around after the birth of his children, ensuring his parental investment in them.

Parental investment is a key concept in the field of sociobiology and its recent descendent, evolutionary psychology. Proponents of these fields draw on the principles formulated by entomologist, E.O. Wilson in his influential book, Sociobiology: The New Synthesis (1975). Drawing on his study of insects, Wilson claims that many of the traits and behaviors of animals, including humans, are genetically programmed to increase genetic fitness. To understand the relationship between the evolution of permanently enlarged breasts and male parental investment, it is necessary to take a short excursion through the model underlying all sociobiological and evolutionary psychology explanations of sex and gender differences.

Rather than understanding human reproductive strategies as ones that benefited the human species as a whole, sociobiolo­gists see male-female sexual relations as a contest between men and women. This “battle of the sexes,” as they call it, has arisen over the course of evolution because men and women have to develop competing reproductive strategies to ensure that their own genes would be passed on. As sociobiologist Richard Dawkins explains, selfish genes motivate male and female gender behavior. That is, females and males have a genetic propensity to exploit each other, trying to force the member of the opposite sex to invest more in their offspring in order to optimize the chances that their own genes will be passed down to future genera­tions (Dawkins 1976:150).

According to sociobiologists, these strategies are the result of the differences in the biology of males and females, in particular the disparity in the investment men and women must make in the rearing of children to ensure the child’s survival and the survival of their own genes. Differing degrees of this parental investment arise, sociobiologists and evolutionary psychologists argue, from the fact that a female’s egg, or ovum, is much larger than a male’s sperm, requiring more energy investment for its production, and because women get pregnant and men do not. Dawkins has made the importance of the different size of ova and sperm clear, stating that it is possible to interpret all the other differences between the sexes as stemming from this one basic dif­ference (1976:152).

Men and women, he argues, must develop different reproductive strate­gies. Sociobiology and evolutionary psychology propose that males can optimize their genetic fitness, the chances of passing on their genes, by impregnating as many females as possible. Males are therefore genetically programmed, according to sociobiologists, to be hyper-sexual, to cheat on their mates, and to have frequent sex with multiple partners.

Women, so this scenario suggests, are chaste by comparison. Once pregnant, more frequent sexual relations will not increase a woman’s chances of successfully passing on her genes, since she can only be preg­nant once at a time. After her child is born, according to sociobiologists and evolutionary psychologists, a female increases the chances of her prior investment in her ovum and in pregnancy paying off by investing even more time and energy in the rearing of her child. To relieve some of this burden, they argue, a woman attempts to entice a man to help her care for her dependent child. Even with this help, it is not in the woman’s self-interest to abandon her child to a male caretaker in order to be impregnated again by a different male. For a female must fear that if she leaves her baby in a man’s care, his relatively small initial investment in the child might cause him to abandon it completely. She would risk losing her investment entirely. According to sociobiologists and evolutionary psychologists, women benefit from male help but must develop a means to keep the male interested in her so that he does not merely leave her in order to spread more of his own genes around.

Sociobiologists and evolutionary psychologists claim that to keep the male around, women have developed a number of traits and behaviors. One set of behaviors, for example, is known as the “domestic-bliss strategy,” which

involves a woman playing “hard to get” in the mating game. By being “coy” and holding out on sex, sociobiologists and evolutionary psychologists claim, the female is likely to attract a male willing to wait for sex. It is in the female’s interest to find such a man, since he might also be more willing to stick around after the birth of a child than a man who wants sex without commitment. Long courtship periods also aid the male: he is in less danger of being “duped” into believing that a child is his when another male has really impregnated the woman. Having invested so heavily in courtship, the male optimizes the return on his investment by remaining committed to his coy female partner.

Coy females, and more-or-less faithful males, are the ones most likely to rear children successfully and pass on their genes, according to sociobiologists and evolutionary psychologists. What may have begun as an idiosyncratic reproductive pattern becomes stabilized in the population once enough females play the game. As Dawkins argues,

when coy females increase in numbers so much that they predomi­nate, the philanderer males, who had such an easy time with fast fe­males, start to feel the pinch. Female after female insists on a long and arduous courtship. The philanderers flit from female to female, and always the story is the same. The net pay-off for the philanderer male when all the females are coy is zero. (1976:164)

Nevertheless, since no system is perfect, “loose, fast females” will always exist, tempting men to increase their genetic fitness by copulat­ing with them, even if the men are monogamously bonded to other women. Faithful wives and less faithful husbands are thus the product of natural selection. According to sociobiologists and evolutionary psychologists, the genes giving rise to these behaviors have been selected for during the course of human evolutionary history.

Breasts as Weapons for “Stay-at-Home Moms”

For sociobiologists and evolutionary psychologists, enlarged breasts are just another weapon in a female’s arsenal for attracting a man and keeping him around. For example, J. Cant (1981) suggests that breasts became signals alerting a male to which female would be more likely to raise his offspring successfully. Cant argues that females better able to build up fat and maintain it would have more reserves to convert to parental investment during pregnancy and lactation. Since breasts are primarily composed of fat, Cant argues that they became an indicator of the nutritional status of the female.

Roger Short (1976) also argues that females with shapely breasts were more likely to attract a male and keep him around, ensuring that he would take care of her and their offspring. Owen Lovejoy (1981) has come to a similar conclusion: he argues that females who could use the permanent features of her body to attract a male would have a distinct advantage over those women who relied on behavioral strategies alone.

These theorists stress how keeping a man around saved the human species from extinction.  Short, for instance, argues that females became dependent on males for food in the far reaches of the human evolutionary past because females became restricted in their movements due to the increased dependency of infants on her care. Unable to support herself and her children alone, a woman would need male parental investment in children became increasingly necessary for survival. According to him, breasts became objects of attraction that ensured the pair bonding between a male and a female, and thus the survival of children.

Lovejoy not only focuses on the role male provisioning of dependent females played in the development of permanent breast enlargement, he also puts it at the very center of his explanation of bipedalism and the evolution of humans. Cant thinks bipedalism arose to solve what he calls “the demographic dilemma of apes.” This dilemma resulted from the reproductive strategy used by apes, one referred to as K-selec­tion. As paleoanthropologist, Donald Johanson and science writer, Maitland Edey explain:

There are two fundamentally different ways in which an animal can function sexually: It can produce a great many eggs with an invest­ment of very little energy in any one egg, or it can produce very few eggs with a large investment in each. These are known as the “r” strategy and the “K” strategy respectively . . . “K” is obviously far more efficient than “r,” but it too has its limits. Accidents, predation, seasonal food failure, illness-all take their toll on animals. Losing an infant to one of these hazards after an investment of five or six years is hideously costly compared with the loss of an egg by [an r strategy animal]. (1981:46)

Lovejoy hypothesizes that apes, including those which were the ancestors of humans, were dangerously “K”-selected; that is, they were perilously close to extinction because they invested a large amount of energy in the rearing of a very few children. Such a focused investment could be disastrous if only a few infants died each year. According to Lovejoy, hominids were able to avoid this fate, and to become the most successful primate, by speeding up their birthrate. This resulted in more overlap among children and required that mothers care for more than one baby at a time. To do so, females needed to restrict their movements and direct the energy saved by staying put into caring for children. Rearing more children required females to become dependent on males to provide food for them and their offspring. The reward for such shar­ing, Lovejoy argues, was sex. But sex can be dangerous.

When males and females come together in primate species to cop­ulate, at least according to Lovejoy, competition and fighting among males often occur. Thus, he reasons, mechanisms must have evolved to reduce conflict. This reduction in male competition, Lovejoy thinks, was accomplished through a characteristic often thought unique among human females: the loss of estrus. Non-human primate females have a moment of height­ened fertility when they ovulate once or twice a year. During these moments, sexual intercourse is more likely to result in conception. External signs, such as enlarged genitalia, that cue male partners to a female’s readiness to mate often signal this period, known as estrus. In some animals, this time is called “heat.” Human females do not have these discrete moments and have lost the signs that signal ovula­tion, allowing males and females to have sex at any time during a female’s reproductive cycle. Loss of estrus, according to this scenario, allowed for permanent relationships between hominid males and females. Johanson and Edey summarize Lovejoy’s argument this way:

There are always ways of defusing male [aggression due to competi­tion for mates]….One is to lower the competition for sex. If each male has his own female, his own private gene receptacle, he doesn’t have to fight with other males for representation in succeeding gen­erations. More parental care and food sharing become possible. As a result, the females can afford to become less mobile. If the primate becomes less mobile . . . it can become more bipedal….Because it doesn’t have to run as much, it can afford to be less efficient in order to do other things that begin to have survival value—like carrying the extra food needed to nurture extra children. If a male is now walking upright, he’s better equipped to carry food and more likely to bring it to the female (Johanson and Edey 1981:47–48, emphasis added)

And what features, besides “continuous sexual receptivity,” became the cement binding well-providing males to these new stay-at-home moms? Large breasts, beautiful skin, shiny hair, and a shapely body.

What you Can’t See, Can Hurt You

Gordon Gallup (1982) focuses on the danger that the loss of estrus might have posed to hominid males in his explanation for permanent breast enlargement. Able to have sexual intercourse at any time, men might waste their sexual energy on non-fertile women. According to him, with the disappearance of obvious external signals of ovulation, males who could synchronize copulation with ovulation would be favored: they would be more likely to pass on their genes to subsequent generations. For Gallup, breasts became the reliable cue for detecting periods when a female was or was not ovulating. He argues that over the course of a female’s lifetime, there is a change in the size and contour of her breasts that signals to males her desirability as a sexual partner: young girls have little, if any, breast development, while the breast of menopausal women lose their firmness. Since neither the young girl nor the older woman can become pregnant, the shape of their breasts tell the male that he should not waste his time and energy having sexual intercourse with her: each is an inappropriate choice of  sexual partner because a male will not be able to pass on his genes by copulating with her. Hominid males who were able to recognize “the wrong shape” of the flat breasts of prepubescent girls or the “sagging” breast of menopausal women, Gallup theorizes, would avoid copulation with these non-fertile women and instead be attracted to fertile hominid females with the “right shape” breasts: large, round, and firm.

What’s Wrong With This Picture?

European culture in general, and U.S. culture in particular, is obsessed with women’s breasts: they are deeply interwoven with standards of beauty, conceptions of sexuality, even women’s own sense of self worth. In the United States, breasts have been fetishistically represented in art for hundreds of years. Today images of breasts permeate U.S. culture whether in museums, on book covers, in fashion ads, or on movie screens. They are most frequently depicted as alluring symbols of womanhood and desire.

Since in many European societies, as well as in the United States, women’s breasts are nearly fanatically eroticized, it is not hard to understand why sexual selection explanations appear to make sense and get so much attention. What better way to explain the human/non-human primate distinction in breast size and shape than by arguing that like many men today, our early ancestors must have found large, rounded female breasts attractive, mating with “big-chested” women more often than flat-chested ones, ensuring that this trait would be maintained in the human gene pool? Because sexual attraction is emphasized so widely in many contemporary societies, and traits like breasts and beautiful bodies are so frequently touted as the key to such attraction, it seems obvious that sexual selection must have been at work in our evolutionary past.

However, sexual selection does not simply mean that males and females have a sexual attraction to one another. It suggests that certain traits are the basis of increased sexual opportunities for some individuals over others, conferring on them increased reproductive potential. But is this a legitimate claim? In a recent article, B. Pawlowski (1999) suggests not. He asks, “Was the reproductive value of early hominine males so diverse that females had to compete over them and elaborate some erotic or informative signal [such as permanently enlarged breast] about their reproductive value” (1999:150)?   This seems unlikely given that, as Darwin pointed out years ago, from a species standpoint, it makes good sense for all females to mate. And this has indeed been the case for most of human history: until recently, it was rare to find unmarried women in most societies.  Moreover, women with small amounts of breast enlargement, who in contemporary U.S. society are often referred to as “flat-chested,” have no more difficulty finding partners than well-endowed women. Neither do they have fewer children than women with larger breasts, indicating that there is no evolutionary advantage to having big breasts in and of themselves. In other words, when it comes to finding a mate and having children, breast size does not matter, even though many advertisers and plastic surgeons might love us all to think so.

One television show after the other on  “The Learning Channel” and “PBS” claims that “beautiful” faces, the practice of wearing lipstick, or fresh, clean skin have been sexually selected for in the course of human evolution. In actuality, however, there are no data to support the claim that particular traits such as these granted some females a reproductive advantage over others thousands, if not millions, of years ago. Unfortunately, theories purporting sexual selection as the key factor in the evolution of permanent breast enlargement in human females cannot be directly tested. Instead, they must be assessed in terms of the validity of their underlying reasoning and assumptions and the soundness of the indirect evidence marshaled in support of them. How do the theories outlined above hold up to such scrutiny? Not well, as the following discussion suggests.

Problem #1: Unwarranted Generalizations

Sociobiologists and evolutionary psychologists make unwarranted generalizations. Most sociobiologists and evolutionary psychologists writing today are from Britain and the United States. These authors, drawing on knowledge of their own societies, tend to claim that certain behaviors and traits are universal human characteristics, when in actuality they are quite specific to time and place. This is true of their claim that breasts have erotic appeal to men. Which men, we might ask? Sexual attraction to breasts is not universal; not all men or societies eroticize them. Yet, claiming that they do allows sociobiologists and evolutionary psychologists to look for explanations in a unique hominid evolutionary past.

That sociobiologists and evolutionary psychologists use themselves and the behaviors found in their own societies as a universal reference point is revealed by how strikingly similar the characteristics attributed to all males and females in their accounts are to those that exist today in Britain and the United States. Until the sexual revolution of the 1960s in the United States, for example, women were expected to be virgins at the time of marriage, a trait that was highly valued. Male indiscretions, by contrast, were not cause for alarm or disparagement. This has been rationalized through the belief that men by nature have stronger sex drives than women. If we look cross-culturally, it is clear that not all societies think men are highly sexual and women have evolved to be chaster.

 The claim that men have a more active sex drive than women, mating more promiscuously than females is a stereotype, contradicted by researchers such as Fatima Mernissi (1987). She points out that in Morocco, women are viewed as more highly sexed than men. There, men fear the intensity of female sexuality, seeing it as capable of distracting them from their ded­ication to God. The existence of societies in which men have sex more often than women does not provide proof of the naturalness of an intense male sex drive and a passive female one, as sociobiologists and evolutionary psychologists claim. Because women experience the consequences of pregnancy more directly, they may have stronger societal sanctions against frequent sex, not internal genetic mechanisms regulating it. Indeed, in most primate groups females initiate sexual inter­course, not males.

The idea that males and females are natural rivals and play a continuous game of cat and mouse is another unwarranted generalization; it is questionable both inside and outside the context of European societies and the United States. It may adequately capture the dating behavior of singles in New York City bars in the 1980s and 90s or on “Sex and the City” on Sunday night, but not necessarily the behavior of dating couples in Salt Lake City or that of young people in love in Bulgaria or of Inuit men and women bonded together in Alaska.

The ethnocentrism of sociobiological and evolutionary psychology accounts has recently been analyzed by paleoanthropologist, Dean Falk. She points out that the female dependency/male provisioning model, for example, may not only reflect European and U.S. attitudes about gender roles, it may also say more about present-day male worries than about early hominid behavior and evolution (1997). Interestingly, many sociobiological explanations of the sexual and gender behaviors of men and women became prominent in the 1970s and 80s in the United States and Britain. It was at this same time that the women’s movement called for ending the sexual double standard, and many women began to become both more sexually and economically independent than they had been, especially in the 1950s. Increased male anxiety was everywhere: discussed on T.V. talk shows, written about in magazines and novels, and portrayed on movie screens across the country. Falk’s suggestion indicates that it also found its way into the scientific literature. According to her, Lovejoy’s model assuages contemporary male sexual anxieties. It tells men that they need not worry about whether they will have children, if a woman’s children belong to him, or whether “his woman” will leave him for another man. A woman, this model suggests, will happily have sex with a man and form a life-long pair bond with him, as long as he supports her sufficiently (see Falk 1997:114-115). Contemporary gender roles, behaviors, expectations and anxieties are thus projected back into the evolutionary past and seen as being at the very foundation of the survival and evolution of humans.

Problem #2: Taking Traits out of Context

Another problem related to unwarranted generalization is that sociobiologists and evolutionary psychologists take behaviors out of context. This makes it appear that a behavior or trait is actually more widespread than it actually is. For example, in his attempt to make claims about the evolutionary basis for rape, evolutionary psychologists Randy Thornhill and Craig Palmer (2000) compare behaviors among insects, animals, and humans, without providing evidence that the behavior of ants or of spiders has any direct relationship to human activity. For example, is the act among scorpion flies of stealing a female’s eggs when he is not the father equivalent to the rape of women by men in human societies, as Thornhill and others would have us believe? 

These theorists also take behaviors out of cultural context: they often assume, for example, that if a certain behavior in another culture looks the same as one in their own, it must mean the same thing. Anthropologists have repeatedly shown the pit­falls of this thinking. Even something as simple as a wink can have many different meanings associated with it; a wink, in many societies, for instance, does not necessarily carry the meaning of flirtation that many people in the United States attribute to it (see Geertz 1973). Generalizing from insects and non-human animals to humans and from a small number of societies to all others, without providing actual evidence for the widespread nature of the trait of behavior in question, renders many of the claims of sociobiology and evolutionary psychology highly suspect. 

Problem #3: Problematic Assumptions

A number of assumptions underlying sociobiology and evolutionary psychology models do not hold up under close scrutiny. Claims about differential parental investment, female dependency, and the prevalence and importance of pair-bonding to human survival are particularly problematic. 

a)      Assumption #1: there is differential investment in a child from the moment of its conception since female ova are bigger than male sperm, requiring different mating and parental investment strategies on the part of men and women. 

This assump­tion may be warranted for species in which the size of the female gamete is large in relation to total body size, but not necessarily in humans where the energy needed to produce male and female gametes is mini­mal. In addition, human females are born with all of their ova, while men must continuously produce sperm. Sociobiologists and evolutionary psychologists have not yet produced studies measuring the differential energy required in the matu­ration of eggs versus the production of sperm, even though, as we have seen, they base their entire explanation of the evolution of gender roles and behaviors on this assumption.

b)     Assumption #2:  female hominids became dependent on males for their survival sometime in the evolutionary past, requiring them to find ways to attract a male, keep him around, and induce him to invest in her children. 

The assumption of female dependency on males for resources is highly questionable given ethnographic evidence of female mobility in many societies. Among foragers, women with infants often range over large areas in search of vegetable foods, which make up the largest proportion of the diet for most hunting and gathering groups. In fact, ethnographic evidence indicates that the foraging pattern, however, that prevailed over most of human evolution involves no universal pattern of female dependency. Evidence from a range of societies indicates that women, including those with children, have undertaken practically ever subsistence activity known to humans.

To assume that some female pre-hominids decided to stay home to raise the kids, depending on a man to bring home the bacon, is not only ethnocentric, but is also inconsistent with data from studies of non-human primates. For example, among chimpanzees, who also have a protracted period of infant dependency, surrogates such as aunts and older siblings often care for infants, allowing mothers to continue their subsistence activates. Other studies reveal that female primates remain mobile throughout all reproductive stages (see Zihlman 1997:102)

 Lovejoy would argue, of course, that early hominid mothers survived by curtailing this primate activity pattern, thus increasing their ability to rear several children at a time, something necessary given the demographic dilemma he hypothesizes. But many anthropologists have questioned whether early homi­nids were in a K-trap, experiencing a demographic dilemma requiring an increase in birth rate and infant survivorship. They suggest that there is no evidence to support this contention (see, for example, Harley 1982; Isaac 1982; Wood 1982).

c)      Assumption #3:  human females are unique in being “continuously sexually receptive” due to concealed ovulation, leading to long-term pair bonding based on a woman’s sexual attractiveness.

Theorists who argue that the uniquely human trait of concealed ovulation enticed males to permanently bond with a female because he could now be ensured of continuous sexually activity, neglect a good deal of evidence showing that sexual activity outside a female’s period of maximum fertility is not an exclusive characteristic of human sexuality (see Burton 1972; Hrdy 1979; Manson 1986; and Rowell 1972). Primates may have periods of estrus, but this does not mean that sexual activity is confined to these periods. Theorists like Lovejoy, Cant, and Short argue that pair bonding saved the human species from extinction, since it ensured that a man would stick around to take care of “his woman” and their children. The presumption that the long-term commitment of one male to one female is an old, established pattern is contradicted by evidence that shows that long-term monogamy is a relatively rare pattern even among modern humans (see Slocum 1975:43). It is also relatively rare among non-human primates.

As an audience member at a public lecture given by Owen Lovejoy on his theory of human evolution, I had a first-hand opportunity to ask him about his assumption that pair bonding was essential to human evolution when is was contradicted by both cross-cultural and primatological data. His response? He had no evidence, but he did have years of thinking about the scenario he was offering, which had allowed him to “get inside the heads of these animals,” our hominid ancestors, leading him to near certainty about his conclusions. Creative thinking always has a place in science, especially for the building of hypotheses. However, to be taken seriously, it must ultimately be supported by evidence. Lovejoy has yet to provide it.

And what about Gordon Gallup’s claim about concealed ovulation: that in the absence of clear markers of estrus, permanently enlarged breasts signaled males to a female’s ovulatory condition? This claim is not only unsupported by evidence, but also makes little sense. Breasts enlarge during pregnancy and lactation. It is hard to explain why some hominid males would have come to prefer females with permanent breast swelling if enlargement is actually associated with these periods of non-fertility (Pawlowski 1999:150). And there are a number of clues that could have served this purpose equally well. Gray hair, for example, or wrinkled skin.

It’s the Environment (After All) Stupid

How then can permanently enlarged breasts be explained? If not favored by males because of their erotic appeal, what does account for a trait that so clearly differentiates human women from all other primate females? Is there a way to rid ourselves of sexual selection theories and develop a model without its problematic assumptions? 

In 1986, I began an effort to do just this, along with my colleagues biological anthropologist, John Relethford and endocrinologist, Tim Sorger. We were struck with how often sexual selection was invoked to explain permanent breast enlargement even though its status among evolutionary theorists has always been much more tenuous than that of the concept of natural selection, the keystone of Darwin’s contribution to evolutionary studies.  Darwin’s contemporary, Alfred Russell Wallace who had independently developed the idea of evolution by natural selection, for example, suggested that natural selection could just as easily explain the sexually dimorphic characteristics associated with male fighting, as could sexual selection: the biologically weaker animal would lose the fight, being wounded or killed in the process, reducing the likelihood or even possibility that his genes would be passed on. Recognizing some of the inherent problems with the idea of sexual selection, Darwin himself tried to shore it up by combining his understanding of it with his firmer belief in natural selection. He suggested, for example, that females might choose males for certain “attractive” traits (sexual selection) because they were really indicators of some general, underlying fitness that would increase the male’s chances of survival (natural selection). Unfortunately, as several renowned evolutionary biologists have pointed out, there is no evidence that such a thing or characteristic as overt vigor or fitness that correlates with attractiveness exists.

Why, then, not look to natural selection, rather than sexual selection, as a possible explanation for permanent breast enlargement in human females? This is exactly what my colleagues and I did: we developed a scenario for the evolution of human female breasts in terms of the selective advantage they may have conferred given the environmental conditions under which this feature may have developed. Our guiding questions were thus these: 

• what were the environmental conditions that may have favored the development of permanently enlarged breasts in our hominid ancestors?, and

• how might those individuals possessing this trait have been better able to survive under these conditions?

In order to answer these questions, we drew on current ideas in evolutionary theory, studies of the early hominid environment, and endocrine studies of breast development. We considered evidence of fat ratios in primates and humans and of energy expenditures in contemporary hunter and gatherers. Our intent was not to present the be-all-and-end-all of explanations, but to show that a tenable model could be devised without resorting to the problematic assumptions of sexual selection, without, in other words, treating women as evolutionary sex objects. Indeed, we argue that breasts were not selected for at all.

Such a claim must surely seem odd. Doesn’t evolution explain every trait, at least every physical trait that individuals possess? Not really. Evolutionary biologists today, such as Roger Lewontin and Stephen J. Gould (1979), have argued that the production and development of some anatomical structures may best be explained as by-products of other selected features through such processes as allometry (changes in proportions of an organism due to growth) or pleiotropy (the ability of one gene to influence more than one trait). Following this line of argument, my colleagues and I proposed that permanently enlarged breasts in human females were not the actual unit of adaptation. We argue that natural selection pressures were not acting directly on breasts but on another feature related to them: fat deposition.

Our argument can be summarized this way: Females with permanent breast enlargement, reflecting increased fat reserves, would have been better able to support themselves and their infants under the changing, tenuous ecological conditions of resource fluctuation characterizing the early hominid environment. This would have increased their chances of bringing their infants to reproductive maturity and having their genes maintained in subsequent generations. In other words, our model contends that breasts are a by-product for the selection of fat.

            The questions that still need answering are these: Why did fat confer a selective advantage to female hominids? How are breasts a by-product of this selection for fat? And what were the tenuous ecological conditions exerting this selective pressure?

The Mascia-Lees, Relethford, Sorger Model

What’s Fat Got to Do With It?

Cant, too, as you may remember, argues that fat storage was critical in terms of the selective advantage of permanently enlarged breasts. Cant’s scenario, however, assumed males sexually selected breasts because they signaled to males that a well-endowed female would be a good choice as the mother of his children. Besides presenting a sexual selection argument, with all its drawbacks, Cant does not make a very compelling case for why fat deposition would have occurred in the breasts. His argument is that breasts and buttocks are highly visible concentrations of fat, and are less ambiguous signs of nutritional status than a thin layer of fat over the entire body. It thus makes sense, he says, “for an upright biped to accumulate fat both dorsally [on the behind] and ventrally [on the front]” (Cant 1982:201). Why this “makes sense” is unclear unless Cant is offering a gyroscopic explanation: fat in front and back acts to balance the upright biped. But there is no evidence for this; and it’s just not necessary to hypothesize some flimsy link between breast fat and bipedalism when there is better evidence of the relationship between fat storage and breast development. This is the known capacity of fat, or adipose tissue, to store estrogen, or more specifically, of fat to aromatize androgen, thereby producing estrogen (Huss-Ashmore 1980:69).

Breast development at puberty is related to two types of growth:  cells proliferate, increasing the volume of the lactiferous ducts, and adipose tissue accumulates in the connective tissue of the breasts. Both processes are due to an increase in the amount of estrogen. Progesterone, another hormone, is also associated with fat deposition in female breasts, but estrogen is the chief regulator. Given this relationship of estrogen to breast development, it is very possible that permanently enlarged breasts came about because of high concentrations of estrogen in the human female body during and after puberty due to increased fat. The sagging breasts of pregnant and lactating apes are important in this regard: it shows that they have the receptor or target tissues that can react to increased levels of estrogen, suggesting our shared ancestor had them as well. With the higher levels of estrogen present in human females, due to increased fat deposition, the potential for permanent breast enlargement found in the higher primates could have become an actuality in our hominid ancestors.

To make the case for natural selection operating on these fat stores, it is necessary to explain how fat deposition might increase the capacity of individuals with this trait to reproduce more successfully or produce more viable offspring than those without fat deposition. What, in other words, accounts for the selection of increased fat stores? And why did they grant some hominid females increased chances of survival?

Rose Frisch has demonstrated that it takes a certain amount of fat in relationship to lean body weight for the onset and maintenance of regular menstrual cycles (Frisch and McArthur 1974). In addition, studies show that there is a complex interaction between energy balance, body composition, and reproductive function (Huss-Ashmore 1980:82). The ability to store energy as fat has adaptive consequences both for maintaining physiological reproductive ability and for the successful rearing of offspring: both confer enhanced fitness (Huss-Ashmore 1980:84).

Females with greater reserves of adipose tissue were better able to satisfy the energy needs of their offspring through the fat laid down in the fetus, which could serve as a metabolic fuel in the newborn. Increased fat could allow a mother to produce a more adequate and abundant supply of milk for infants up to three years after birth. This may have been necessary as the period in which infants became dependent on their mothers for food increased.  

Although we do not completely understand the actual mechanisms of converting fat reserves to milk, there is indirect evidence that suggests that there is a conversion of internal fat stores to milk during nursing as well as the replenishment of these stores from subcutaneous deposits during childbearing years (Cant 1981:201). If increased infant dependency and scarce resources occurred in tandem, the contribution a mother would have to make to her child’s diet would have become proportionately more important. The evidence from contemporary hunter-gathers, such as the !Kung San, is that mothers often nurse their infants for up to three years after birth, significantly longer than among any other primate. Such long periods of nursing in our evolutionary past would have placed a considerable energy drain on the lactating mother.

 Women in hunting and gathering societies today often have high workloads. If hominid females did as well, as is likely,  the pressure for selection of fat storage would have increased.  Studies indicate that during periods of high workload, fat individuals use adipose stores while lean individuals are forced to sacrifice muscle just at the time when muscle is most needed for work. Sparing muscle during periods of high workload would enhance the work performance of an individual and the productive capacity of the entire society. The combination of a female’s high workload and increased energy demands from prolonged lactation could have exerted considerable stress for the selection of increased fat deposition.  

To Every Season

      Selection for fat deposition would have been most intense where energy resource fluctuation was large, regularly recurring, and of sufficient duration to invoke a physiological response. Such conditions are commonly encountered in seasonal habitats (Huss Ashmore 1980:87). A study by John Speth (1984), using geologic and ethnographic data, shows that the early hominid environment of east and south Africa exhibited just such seasonality. One indicator of seasonal stress would be loss of body weight. Speth points out that Gombe male chimpanzees experience up to 15% weight loss in the dry season and contemporary !Kung have been documented as experiencing approximately 6% weight loss during the late spring. It is highly probable that early hominids who possessed a much less well-developed technology than the !Kung, would also have faced periodic seasonal food stress and weight loss.

These conditions may have made fat reserves critical for hominid survival by acting as a buffer against caloric and nutritional deficiencies. And indeed, both human males and females exhibit exaggerated deposition of adipose tissue in comparison to the other primates. This fat deposition accounts for the great difference in birth weight between newborn humans and newborn living great apes. Nursing infants for longer than six months, the length of one plentiful season, would have placed stress on females, and it is likely that increased adipose tissue in the female, as compared to the male, is related to their role in child bearing and lactation.

     Given the adaptation of early hominids to seasonal environmental conditions, it is extremely likely that the potential for some males and females to store fat increased their reproductive success. In other words, they would have been more likely to survive as well as bring their offspring to reproductive maturity. This is clearly natural selection at work, not sexual selection.

One last note of interest, which is in need of further investigation, is the relationship between estrogen and loss of estrus. Females with higher levels of estrogen reflecting increased fat deposition may have lost a discrete period of estrus. Although the physiological processes associated with loss of estrus are complex, experiments have indicated that the female motivation to mate is influenced by circulating levels of estrogen and testosterone. With increased levels of circulating estrogen, due to increased fat deposition, hominid females may have become willing to mate more often. It would become increasingly possible for females, lacking a discrete breeding season, to be pregnant at times of the year not necessarily correlated with plentiful environmental resources. This would have increased the selective pressure for fat stores even more.   

Alternative Scenarios

Since the publication of our article outlining our natural selection scenario, other researchers have built upon our ideas and expanded our model. For example, Pawlowski has recently argued that fluctuating food resources were not the only selective pressure operating on early hominids. In addition, he suggests that the large difference between daytime and nighttime temperatures characteristic of the open environment of early hominids might also account for the importance of fat storage to early hominid survival. Those individuals with decreased amounts of fur were able to withstand very high daytime temperatures, but would be vulnerable to the very cold nights of this environment. Those individuals with more fat were better able to withstand these hypothermic conditions. Pawlowski, too, argues that permanently enlarged breasts are a side effect of fat storage. Under conditions of early hominid development, fat storage would increase the likelihood of the survival of individuals possessing this trait. 

Conclusions

Whatever the exact selective advantage of fat, it is clear that the evolution of permanent breast enlargement in human females need not be explained through their erotic appeal to men. What my colleagues and I hoped to show by presenting our explanation is that a reasonable argument based on natural selection could be developed. Our model is not as “sexy” as the explanations that see breasts exclusively as erotic attractors of men. But it avoids relying on such poorly substantiated concepts as differential parental investment, female dependency, and sexual selection, ideas that may reinforce twenty-first century notions about women and gender roles but have little, if no, empirical evidence to support them. The idea that female breasts are little more than objects of sexual attraction for men is a popular one in many European societies, and certainly in the United States, among not only producers and audiences of slick programs on “The Learning Channel,” but also quite obviously among many scientists. But, it seems, they may be indulging more in sexual fantasy than scientific fact.

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Posted:  December 05, 2002
F. E. Mascia-Lees, Sarah Lawrence College 
Copyright 2002
All rights reserved.

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