Women Evolutionary Sex Objects?:
PHOTO OF CLASSIC 1950s
“leave it to Beaver” MOM
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.
you Can’t See, Can Hurt You
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.
Wrong With This Picture?
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.
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.
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)
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?
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
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.
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:
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,
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
PHOTO OF lactating FEMALE
CHIMP WITH SAGGING BREASTS
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
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
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.
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.
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
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.
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.
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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December 05, 2002
F. E. Mascia-Lees
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,Sarah Lawrence College