Steven W. Gangestad. Encyclopedia of Human Relationships. Editor: Harry T Reis & Susan Sprecher. 2009. Sage Publication.
“I should say that the majority of women (happily for them) are not very much troubled with sexual feeling of any kind.” So concluded William Acton, a physician of Victorian England, in Functions and Disorders of the Reproductive Organs in Youth, in Adult Age, and in Advanced Life, which appeared in eight editions in the 19th century. Two years after Acton’s first edition was published, in 1859, Charles Darwin’s Origin of Species appeared. A dozen years later, in 1871, Darwin authored The Descent of Man. In the latter, Darwin elaborated his theory of evolution driven by female choice, as pushed along by female sexual preferences. Aside from a few notable exceptions, this idea was largely ignored by evolutionary biologists for the century that followed. Robert Trivers’ highly influential paper of 1971 revived interest in it. Currently, this concept of “sexual selection” is widely regarded as one of the most important forces giving rise to evolution and, hence, one of the fundamental subjects within evolutionary biology.
More particularly, female “sexual feeling”—patterns of sexual interest, preference, and motivation—is considered central to an understanding of the social nature of a species. Specifically with respect to human sexuality, evolutionary biologists and psychologists have documented a variety of patterns of women’s sexual interests and preferences. Though much more work is needed, the field now offers an outline of what selective pressures shaped these patterns as well as the impact of these patterns on broader relationship phenomena. Perhaps not surprisingly, the idea that humans have evolved as a pair-bonding species in which males and females cooperatively care for offspring (though not without conflicts between them) has been a central thesis. Views about how a history of pairbonding and care of offspring by two parents (rather than almost exclusively mothers, as is characteristic of most species of mammals) is revealed by patterns of female romantic interests, however, have varied across time. This entry discusses evolutionary perspectives on women’s romantic interests, their historical roots, and their current status.
The Search for Women’s Estrus
From a historical perspective, evolutionary perspectives of women’s sexual and romantic interests arose in three distinguishable waves of theory and research. The first began in the late 1800s, when reproductive biology became a serious scientific field. A key concept introduced at that time was estrus. Estrus was defined as a period of relatively intense mammalian female sexual motivation as well as attractivity to males. It is largely confined to a phase of female reproductive cycles around the time of ovulation, when females are fertile (that is, when sex can potentially lead to conception and, ultimately, birth of an offspring). The term derives from the Greek word for botfly. When botflies lay eggs on the hides of cows, cows become excited and, accordingly, even in classical times Greeks used the term for botflies to describe a frenzied state. Reproductive biologists co-opted the term to refer to the frenzied state of a fertile mammalian female (also referred to as “heat”).
In the 1920s, reproductive biologists began discovering hormones that play important roles in the regulation of reproductive function, the first of which was estrogen. In most all mammalian females, estrogen levels peak just before ovulation, and, indeed, this rise plays key roles in orchestrating changes in reproductive physiology that, for a few days of the reproductive cycle, permit conception. The name estrogen was derived from the term estrus: Estrogen is purportedly the “gen” or generator of estrus. (Other reproductive hormones, notably progesterone and testosterone, play important roles as well.)
Women possess estrogen and, just like most other mammalian females, their estrogen levels surge just before ovulation. When reproductive biologists discovered estrogen, then, it made sense to look for evidence of estrus in women. Beginning in the 1930s, many studies looked for evidence that women exhibit marked sexual desire (as well as greater attractivity to men) when fertile in their cycles. Though evidence has been mixed, researchers have not found consistent evidence of these changes across women’s cycles. For instance, in a 2005 study, Alexandra Brewis and Mary Meyer analyzed reports of recent sex intercourse with a primary partner provided by about 20,000 normally ovulating women in 13 developing countries. No evidence was found that the frequency with which women have sex with their primary partners changes across the cycle, aside from a drop during menses. Although this study is limited because intercourse is hardly a straightforward measure of sexual desire, studies using other measures (self-reported desire, female-initiated intercourse) have yielded mixed results as well.
Even by about 1960, then, a number of scholars concluded that, evolutionarily, women had “lost” estrus—any marked changes in sexual or romantic interests across the cycle. It was replaced by continuous sexual receptivity across the cycle. As women’s hormones change in ways similar to other primate species, this conclusion was tantamount to the claim that hormonal control of women’s sexual and romantic interests had, over the course of human evolution, been relaxed.
The Search for Why Women “Lost” Estrus
This conclusion having been drawn, a second wave of scientific investigation into women’s sexuality attempted to address the question of why women had lost estrus. What selective forces led to the disappearance, in humans, of marked female sexual motivation and attractivity to males during the fertile phase of women’s cycles? By the late 1970s, this question became a salient one for scholars interested in understanding the unique social nature of the human species. Indeed, in 1979 alone, at least four major papers proposing answers to this question appeared (though the answers offered by these papers are but a subset of all that have been offered).
One particularly prominent and influential view was offered, separately by Donald Symons and by Richard Alexander and Elizabeth Noonan. It proposed that loss of estrus and the purportedly crucial feature that resulted, the concealment of ovulation (lack of signs of ovulation evident to either females themselves or to observers), played an important role leading to pair-bonding in humans. As long as a male can discriminate females who are fertile from those who are infertile in their cycles, it may benefit him to attend particularly to those who are fertile—that is, to those females who are able to conceive an offspring at that point in time. If a male cannot discriminate fertile from infertile females, however, it may benefit him to attend particularly to one female whose offspring he has high probability of having sired, particularly if the assistance he offers to care for those offspring can substantively contribute to their well-being and fitness. Loss of estrus and concealed ovulation, in this view, was not the sole reason why human males invest in offspring and why males and females pair-bond. But in the context of other factors facilitating biparental care (care of offspring by both parents) in humans, loss of estrus and concealed ovulation arose in females as tactics to enhance the care that their offspring received from putative fathers.
One contrasting theory was proposed by Lee Benshoof and Randy Thornhill (and mentioned by Symons as well). They argued that loss of estrus and concealed ovulation evolved after pairbonding in humans had already arisen. Rather than functioning to facilitate pair-bonding, concealed ovulation in this view functioned to facilitate female choice of sires of their offspring. Sometimes, it may not pay a female (in the currency of her evolutionary fitness) to have her offspring fathered by her primary mate. Rather, for a variety of reasons (better genes, genes more compatible with hers, other material advantages), it may be best for her to choose a sire of her offspring other than her primary partner. By hiding the period she is fertile and can potentially conceive, concealed ovulation, in this view, interferes with the ability of her partner to prevent “extra-pair copulation” (sex with a partner other than a primary partner in a pairbonding species) through “mate-guarding” and thereby fosters female sire choice.
No one theory of concealed ovulation clearly won out over all competitors through the 1980s and 1990s. Much research during this period, however, did provide evidence for the importance of pair-bonding in shaping human mating and romantic preferences.
Women’s Estrus Found?
The third phase of research speaking to the evolution of women’s sexuality began in the mid-1990s. Though it didn’t begin with this intention, viewed in retrospect a major thrust of this most recent phase has been to question the conclusion reached almost half a century ago, that women have lost estrus. A wealth of research now indicates that women’s sexual interests and motivations do systematically change across their cycles. When fertile in their cycles, normally ovulating women particularly prefer a variety of masculine male traits perceived through multiple sensory modalities (e.g., men whose scent is associated with social dominance, facially masculine men, men whose bodies are masculine, masculine vocal qualities, masculine behavioral displays, relatively tall men). As well, women prefer traits associated with body symmetry (an indicator of developmental instability), more so when fertile than when infertile in their cycles (e.g., scents associated with symmetry, facial features associated with body symmetry). Masculine traits and traits associated with symmetry may have been indicators, in harsh ancestral environments, that males could pass onto offspring genes that would promote fitness and success. Women don’t particularly prefer all positive male features when fertile (e.g., those particularly favored in long-term mates, such as kindness or faithfulness, are not particularly preferred then). And even with respect to traits women do prefer more when fertile, they only find these traits more sexually attractive; women do not rate these traits as being particularly attractive in their long-term mates.
Although the evidence that women’s preferences do shift across their cycles is now compelling, it is worth noting that women may not be conscious of these preference shifts. The methodology used in these studies has not typically asked women to report how much they desire or prefer various partner qualities in mate. Rather, women in these studies typically have been asked to rate a series of men who differ on various qualities (as manipulated by researchers, or as a function of variation in representative samples of men). By examining how male features predict women’s stated attraction to men (their sexual attraction or their attraction to men as long-term mates), researchers infer the features that women find particularly attractive, and then compare the inferred preferences across phases of women’s cycles. It is quite possible that most women are not aware of how their preferences, as assessed in these studies, vary across their cycles.
One interpretation of this evidence is that estrus was never lost during women’s evolution, and that women’s sexual motivations and interests have never become fully divorced from hormonal influence. Rather, women retain a distinctive phase of sexual interests and pattern of attraction when fertile in their cycles, one influenced by hormones such as estrogen. This phase may have been modified during human evolution. For instance, overt signs of women’s fertile phase appear to have become suppressed during human evolution, even if not fully eliminated and even if women do retain distinct patterns of interest when fertile. “Concealed estrus”—a lack of overt signs of women’s estrus to observers—may have been selected in the context of pair-bonding, along lines similar to earlier theories of concealed ovulation. In particular, concealed estrus may have permitted ancestral women to maintain support from pair-bond partners while increasing their latitude for choosing sires for offspring. Moreover, women may have evolved a phase of “extended sexuality”—sexual receptivity and interests outside of the fertile phase—in combination with estrus. The primary benefits that may have led this sexuality across the cycle purportedly would have been derived through pairbonding, which it could have promoted along lines suggested by Symons, Alexander, and Noonan (albeit regarding concealed ovulation rather than extended sexuality per se).
In this view, features of women’s sexual motivations and their changes across the cycle constitute telltale signs of ancestral selection pressures that forged patterns of human mating. In particular, patterns do contain signatures of humans having evolved as a pair-bonded species. At the same time, however, they also reveal that conflicts of interest between male and female pair-bonded partners (e.g., those surrounding sexual fidelity to the relationship) have played a hand in shaping female sexuality as well.
It should nonetheless be emphasized that these recent views of female estrus, though garnering a good deal of empirical support, have not been fully vetted by a scientific process. Current views of the evolution of women’s sexuality markedly differ from dominant views of half a century ago. Quite possibly, future generations of scholars will view current interpretations of the extant literature as incomplete or flawed in important ways.