Research over the last three decades in the field which has become known as evolutionary psychology has focussed disproportionately on mating behaviour. Geoffrey Miller (1998) has even argued that it is the theory of sexual selection rather than that of natural selection which, in practice, guides most research in the field.
This does not reflect merely the prurience of researchers. Rather, given that reproductive success is the ultimate currency of natural selection, mating behaviour is, along perhaps with parental investment, the form of behaviour most directly subject to selective pressures.
Almost all of this research ultimately traces its ancestry to 'The Evolution of Human Sexuality' by Donald Symons. Indeed, much of it was explicitly designed to test claims and predictions formulated by Symons himself.
For example, in his discussion of the age at which women are perceived as most attractive by men, Symons argues that, if human evolutionary history were characterised by fleeting one-off sexual encounters, this would be the age of greatest female fertility (i.e. mid-twenties). However, if men evaluate women for the purposes of more lasting long-term unions, then men should be maximally attracted to women of the highest 'reproductive value' (a term borrowed from demography and population genetics) – in other words, those at the beginning of their reproductive careers, such that, by entering a long-term relationship with the woman at this time, a male is able to monopolise her entire lifetime reproductive output (p189).
Evidence subsequently collected from sources such as 'lonely heats advertisements' and surveys (e.g. Kenrick and Keefe 1992) has found that men perceive women in their late-adolescence (e.g. mid- to late-teens) as most attractive, confirming Symons' intuitive impression that it was women of maximal 'reproductive value' who were perceived as most attractive.
Support has even emerged for some of Symons' more speculative hunches.
For example, one of Symons' two scenarios for the evolution of concealed ovulation, in which he professed "little confidence" (p141), was that this had evolved so as to impede male mate-guarding and enable females select a biological father for their offspring different from their husbands (p139-141).
Consistent with this theory, studies have found that women's mate preferences vary throughout their menstrual cycle in a manner compatible with a 'dual' or 'mixed' mating strategy, preferring males evidencing a willingness to invest in children at most times, but, when at their most fertile, preferring characteristics indicative of genetic quality (e.g. Penton-Voak et al 1999). Meanwhile, in a questionnaire distributed via a women's magazine, Bellis and Baker (1990) found that women engaged in affairs do indeed report engaging in 'extra-pair copulations' (EPCs) at times likely to coincide with ovulation.
Interestingly, Symons even anticipated some of the mistakes evolutionary psychologists would be led into.
For example, he warns that researchers in modern western societies may be prone to overestimate the importance of female choice as a factor in human evolution, because, in their own societies, this is a major factor, if not the major factor, in determining marriage and sexual and romantic relationships (p203).
However, in ancestral environments (i.e. what evolutionary psychologists now call the 'EEA') arranged marriages were likely the norm, as they are in most cultures across the world (p168).
Thus, Symons concludes, "there is no evidence that any features of human anatomy were produced by intersexual selection" (p203) – i.e. by female mate choice, like, for example, the peacock's tail.
[Indeed, the human anatomical trait in humans that perhaps shows the most evidence of being a product of intersexual selection is a female one, namely the female breasts, since the latter are, unlike the mammary glands of most other mammals, permanently present from puberty on, not only during lactation, and composed primarily of fatty tissues, not milk: Møller 1995; Manning et al 1997; Havlíček et al 2016.]
Instead, "human physical sex differences are explained most parsimoniously as the outcome of intrasexual selection" – i.e. male-male competition, especially fighting, as evidenced by the substantially greater upper-body muscle mass of adult males as compared to females.
This was, however, a warning almost entirely ignored by a subsequent generation of researchers before being forcefully reiterated by Puts (2010).
Homosexuality as a 'Test-Case'
An idea of the importance of Symons's work can be ascertained by comparing it with contemporaneous works addressing the same subject-matter.
Edward O Wilson's
On Human Nature
was first published only a year before 'The Evolution of Human Sexuality'. Yet Wilson's chapter on sex bears little resemblance to the subject matter of modern evolutionary psychology and the latter portion of the chapter is devoted to introducing a now faintly embarrassing theory of the evolution of homosexuality which has subsequently received no empirical support (see Bobrow & Bailey 2001).
In contrast, Symons's own treatment of homosexuality is innovative. It is also characteristic of his whole approach and illustrates why 'The Evolution of Human Sexuality' has been described by David Buss as "the first modern treatise on evolutionary psychology proper" (
Handbook of Evolutionary Psychology
: p251).
Rather than viewing all behaviours as necessarily themselves adaptive, Symons instead focuses on admittedly non-adaptive (or even maladaptive) behaviours, not because he believes them to be adaptive, but rather because they provide a unique window on the evolved psychology underlying them.
Accordingly, Symons does not concern himself with how homosexuality evolved, implicitly viewing it as a rare and maladaptive malfunctioning of normal sexuality. Instead, he uses homosexuality as a case-study and window on the nature of male and female sexuality as it manifests itself when freed from the constraints imposed by the conflicting demands of the opposite sex.
On this view, the rampant promiscuity manifested by many homosexual men (e.g. 'cruising' and 'cottaging' in bathhouses and public lavatories) reflects the universal male desire for sexual variety when freed from the constraints imposed by the conflicting desires of women – a desire that, although obviously reproductively unproductive among homosexuals themselves, nevertheless evolved because it enhanced the reproductive success of heterosexual men by motivating them to attempt to mate with multiple females and thereby father multiple offspring.
In contrast, burdened with pregnancy and lactation, women's 'potential reproductive rate' is more tightly constrained than that of men. They therefore have little to gain reproductively by mating with multiple males, since they can generally gestate only one offspring at a time.
It is therefore notable that, among lesbians, there is little evidence of the sort of rampant promiscuity common among gay men. Instead, lesbian relationships seem to be characterized by much the same features as heterosexual coupling (e.g. long-term mate bonds).
The similarity of heterosexual coupling to that of lesbians, and the striking contrast with that of male homosexuals, suggests that it is women who exert decisive influence in dictating the terms of heterosexual coupling.
Thus, Symons reports, "there is enormous cross-cultural variation in sexual customs and laws... yet nowhere in the world do heterosexual relations begin to approximate those typical of homosexual men"; leading him to conclude, "heterosexual relations are structured to a substantial degree by the nature and interests of the human female" (p300).
This conclusion, of course, is contrary to the feminist assumption that it is men who dictate the terms of heterosexual coupling and for whose exclusive benefit such relationships are structured.
It also suggests that most men are ultimately frustrated in achieving their sexual ambitions to a far greater extent than are most women. "The desire for sexual variety," he writes, "dooms most human males to a lifetime of unfulfilled longing" (p228).
One is reminded of Camille Camille Paglia's description of men as "sexual exiles who wander the earth seeking satisfaction, craving and despising, never content" and in whose anguished longing "there is nothing... for women to envy" (
Sexual Personae
: p19).
Criticisms of Symons's Use of Homosexuality as a Test-Case
There is, however, a potential problem with Symons's use of homosexual behaviour as a window onto the nature of male and female sexuality. The whole analysis rests on a questionable premise – namely that homosexuals are, their preference for same-sex partners aside, otherwise similar, if not identical, to heterosexuals of their own sex in their psychology and sexuality.
Symons defends this assumption, arguing "there is no reason to suppose that homosexuals differ systematically from heterosexuals in any way other than their sexual object choice" (p292).
Indeed, in some respects, Symons sees even "sexual object choice" as analogous among homosexuals and heterosexuals of the same sex.
For example, he observes that, unlike women, both homosexual and heterosexual men tend to evaluate prospective mates primarily on the basis their physical appearance and youthfulness (p295). Thus, in contrast to the failure of periodicals featuring male nudes to attract a substantial female audience (see below), Symons notes the existence of a market for gay pornography parallel in most respects to heterosexual porn – i.e. featuring young, physically attractive models in various states of undress (p301).
This, of course, contradicts the feminist notion that men are led to 'objectify' women only due to the portrayal of the latter in the media.
Instead, Symons contends, "pictures of attractive women are used to sell products because these pictures appeal to men (and perhaps women as well), not the other way around", concluding "that homosexual men are at least as likely as heterosexual men to be interested in pornography, cosmetic qualities and youth seems to me to imply that these interests are no more the result of advertising than adultery and alcohol consumption are the result of country and western music" (p304).
However, this assumption of the fundamental similarity of heterosexual and homosexual male psychology has been challenged by David Buller in his book,
Adapting Minds
.
Buller cites evidence that male homosexuals are in some respects 'feminized' in aspects of their behaviour. Indeed, one of the few consistent early correlates of homosexuality is gender non-conformity in childhood and some evidence (e.g. digit ratios, the 'fraternal birth order effect') suggests that the level of prenatal exposure to masculinizing androgens (e.g. testosterone) in utero affects sexual orientation.
As Buller notes, although gay men seem, like heterosexual men, to prefer youthful sexual partners, they also appear to prefer sexual partners who are, in other respects very masculine. Thus, Buller observes, "the males featured in gay men's magazines embody very masculine, muscular physiques, not pseudo-feminine physiques" (Adapting Minds: p227) and indeed seem in most respects similar in physical appearance to the male models, pop stars, actors and other 'sex symbols' and celebrities fantasised about by heterosexual women and girls.
How then are we to resolve this apparent paradox?
One possible explanation that some aspects of the psychology of male homosexuals are feminized but not others – perhaps because different parts of the brain are formed at different stages of prenatal development, at which stages the levels of masculinizing androgens in the womb may vary.
Indeed, there is even evidence that homosexual males may be hyper-masculinized in some aspects of their physiology. For example, it has been found that homosexual males report larger penis-sizes than heterosexual men (Bogaert & Hershberger 1999).
This then may be because, "if it is supposed that the barriers against androgens with respect to certain brain structures (notably those concerned with homosexuality) lead to increased secretion in an effort to break through, or some sort of accumulation elsewhere… then there may be excess testosterone left in other departments" (
Born Gay: The Psychobiology of Sex Orientation
: p80).
Another possibility is that male homosexuals actually lie midway between heterosexual men and women in their level of masculinization. They therefore appear relatively feminine only when compared with other (i.e. heterosexual) men.
Compared to women, on the other hand, they may be relatively masculine, as reflected in the male typical aspects of their sexuality focussed upon by Symons.
Interestingly, this latter interpretation suggests the rather disturbing possibility that, freed from the restraints imposed by women, heterosexual men would be even more indiscriminately promiscuous than their homosexual counterparts!
Pornography as a "Natural Experiment"
For Symons, fantasy represents another window onto sexual and romantic desires. Like homosexuality, fantasy is, by its very nature, unconstrained by the conflicting desires of the opposite sex (or indeed by anything other than the imagination of the fantasist).
Symons later collaborated in an investigation into sexual fantasy by means of a questionnaire (Ellis and Symons 1990). However, in the present work, he investigates fantasy indirectly by focusing on "the natural experiment of commercial periodical publishing" – i.e. pornographic magazines (p182).
In many respects, this approach is preferable to a survey because, even in an anonymous questionnaire, individuals may be less than honest when dealing with a sensitive topic such as their sexual fantasies. On the other hand, they are unlikely to regularly spend money on a magazine unless they are genuinely attracted by its contents.
Before the internet age, such magazines, largely featuring female nudes, commanded sizeable circulations. However, their readership (if indeed 'readership' is the right words, since there was typically little reading involved) was almost exclusively male.
In contrast, there was little or no female audience for magazines containing pictures of naked males. Instead, magazines marketed towards women (e.g. fashion magazines) contain, mostly, pictures of other women.
Indeed, when, in the 1970s, attempts was made, in the name of feminism, to market magazines featuring male nudes to a female readership, one such title, 'Viva', abandoned publishing male nudes after just a few years then went bust a few years later, while the other, 'Playgirl', although it did not entirely abandon male nudes, was notorious, as a consequence, for attracting a readership composed in large part of homosexual men.
Symons concludes, "the notion must be abandoned that women are simply repressed men waiting to be liberated" (p183).
Romance Literature
Unfortunately, however, there is notable omission in Symons's discussion of pornography as a window into male sexuality – namely, he omits to consider whether there exists any parallel genre that offers equivalent insight into the female psyche.
Later writers on the topic have argued that romance novels (e.g. Mills and Boon, Jane Austin), whose audience is as female as pornography's is male, represent the female equivalent of pornography, and that analysis of the the content of such works provides insights into female mate preferences parallel to those provided into male psychology by pornography (e.g. Kruger et al 2003; Salmon 2004; see also
Warrior Lovers: Erotic Fiction, Evolution and Female Sexuality
).
Female Orgasm as Non-Adaptive
An entire chapter, Chapter Three ("The Female Orgasm: Adaptation or Artefact"), is devoted to rejecting the claim that the female orgasm represents a biological adaptation.
This is perhaps excessive but at least conveniently contradicts the claim of some critics of sociobiology such as Stephen Jay Gould that the field is 'ultra-Darwinian' or 'hyper-adaptionist' and committed to the misguided notion that all traits are necessarily adaptive.
[Incidentally, Symons also rejects the theory that the menopause is adaptive (p13), a theory which has subsequently become known as the 'grandmother hypothesis'. His discussion of rape also implicitly favours the theory that rape is a by-product of the greater male desire for commitment free promiscuous sex, rather than the product of a specific rape adaptation in males (see Palmer 1991; and
A Natural History of Rape
).]
In contrast, Symons champions the thesis that the female capacity for orgasm is a simply non-adaptive by-product of the male capacity to orgasm, the latter of which is of course adaptive.
On this view, the female orgasm (and clitoris) is, in effect, the female equivalent of male nipples (only more fun!).
Certainly, Symons convincingly critiques the notion, popularised by Desmond Morris among others, that the female orgasm functions as a mechanism to enhance 'pair-bonding' between couples.
However, subsequent generations of evolutionary psychologists have developed less naïve models of the adaptive function of female orgasm.
For example, Geoffrey Miller argues that the female orgasm functions as an adaptation for 'mate choice' (
The Mating Mind
: p239-241).
Of course, at first glance, experiencing orgasm during coitus may appear to be a bit late for 'mate choice', since, by the time coitus has occurred, the choice in question has already been made. However, given that, among humans, most sexual intercourse is non-reproductive and does not result in conception, the theory is not altogether implausible.
On this view, the very factors which Symons views as suggesting female orgasm is non-adaptive – such as the relative difficultly of stimulating female orgasm during vaginal sex – are positive evidence of its adaptive function in carefully discriminating between suitors/lovers to determine their desirability as father for a woman's offspring.
Nevertheless, at least according to the stringent criteria set out by George C Williams in his classic 'Adaptation and Natural Selection', as well as the more general principle of parsimony (or 'Occam's Razor'), the case for female orgasm as an adaptation remains unproven (see also Sherman 1989;
Case Of The Female Orgasm
).
Out-of-Date?
Much of Symons' work is dedicated to challenging the views of Sixties ethologists such as Desmond Morris. Although scientifically now somewhat obsolete, Morris's work still retains popular resonance and therefore this aspect of Symons's work is not entirely devoid of contemporary relevance.
In place of Morris's rather idyllic notion that humans are a naturally monogamous 'pair-bonding' species with a mating system analogous to that of gibbons, Symons advocates an approach rooted in the individual-level (or gene-level) selection championed by figures such as George C Williams and Richard Dawkins.
This leads to some decidedly cynical conclusions regarding the true nature of sexual and romantic relations among humans.
For example, Symons argues that it is adaptive for men to be less sexually attracted to their wives than they are to other women – because they are themselves liable to bear the cost of raising offspring born to their wives but not those born to other women with whom they mate (e.g. those mated to other males).
Another cynical conclusion is that the primary emotion underlying the institution of marriage, both cross-culturally and in our society, is neither love nor lust, but rather male sexual jealousy and proprietariness (p123). Marriage, then, is an institution borne not of love, but of male sexual jealousy and the behaviour known to biologists as 'mate-guarding'.
Meanwhile, in his excellent chapter on 'Copulation as a Female Service', Symons suggests that most heterosexual romantic relationships may be in some respects analogous to prostitution.
'The Evolution of Human Sexuality' is also out-of-date in a more serious respect. Namely, it fails to incorporate the vast amount of empirical research on human sexuality which has been conducted since the first publication of his work.
For a book first published thirty years ago, this is inevitable – not least because much of this empirical research was inspired by Symons' own ideas and specifically designed to test theories formulated in this very work.
In addition, potentially important new factors in human reproductive behaviour that even Symons did not foresee have been identified, for example role of levels of fluctuating asymmetry functioning as a criterion for, or at least correlate of, physical attractiveness.
For an updated discussion of the evolutionary psychology of human sexual behaviour, complete with the latest empirical data, readers should consult the latest edition of David Buss's
The Evolution Of Desire
.
In contrast, in support of his theories Symons relies largely on classical literary insight, anecdote and, most importantly, a review of the ethnographic record.
However, this focus in some respects ensures that the work remains of more than merely of historical interest.
After all, one of the more legitimate criticisms recent research in evolutionary psychology is that it is insufficiently cross-cultural and, with several notable exceptions (e.g. Buss 1989), relies excessively on research conducted among undergraduate samples at Western universities.
Given costs and practicalities, this is inevitable. Undergraduates in the very institution where one is employed to research and teach represent, of course, a 'convenience sample'. However, for a field that aspires to understand a human nature presumed to be universal, such a method of sampling is highly problematic.
'The Evolution of Human Sexuality' therefore retains its importance for two reasons. Not only is it the founding work of modern evolutionary psychological research into sexual behaviour, and hence of importance in the history of the field and of science, but it also remains of value to this day for the cross-cultural and ethnographic evidence it marshals in support of its conclusions.
________
References
Bellis & Baker (1990). 'Do females promote sperm competition?: Data for humans'. Anim. Behav., 40, 997-999
Bobrow & Bailey (2001). 'Is male homosexuality maintained via kin selection?' Evolution and Human Behavior, 22, 361-368
Bogaert & Hershberger (1999) 'The relation between sexual orientation and penile size'. Archives of Sexual Behavior 1999 Jun;28(3):213-21.
Buss (1989). 'Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures'. Behavioral and Brain Sciences, 12, 1-49
Ellis & Symons (1990), 'Sex Differences in Sexual Fantasy: an Evolutionary Psychological Approach', The Journal of Sex Research 27 (4): 527-555
Havlíček et al (2016) 'Men's preferences for women's breast size and shape in four cultures' Evolution and Human Behavior 38(2): 217–226
Kenrick & Keefe (1992). 'Age preferences in mates reflect sex differences in human reproductive strategies'. Behavioral and Brain Sciences, 15, 75-133.
Kruger et al (2003) 'Proper and Dark Heroes as Dads and Cads' Human Nature 14(3) 305-317
Manning et al (1997) 'Breast asymmetry and phenotypic quality in women' Ethology and Sociobiology 18(4): 223–236
Miller (1998). 'How mate choice shaped human nature: A review of sexual selection and human evolution'. In C. Crawford & D. Krebs (Eds.), Handbook of evolutionary psychology: Ideas, issues, and applications (pp. 87-129). Mahwah, NJ: Lawrence Erlbaum
Møller et al (1995) 'Breast asymmetry, sexual selection, and human reproductive success' Ethology and Sociobiology 16(3): 207-219
Palmer (1991) 'Human Rape: Adaptation or By-Product?' Journal of Sex Research 28(3):365-386
Penton-Voak et al (1999) 'Menstrual cycle alters face preferences', Nature 399 741-2.
Puts (2010) 'Beauty and the Beast: Mechanisms of Sexual Selection in Humans', Evolution and Human Behavior 31 157-175
Salmon (2004) 'The Pornography Debate: What Sex Differences in Erotica Can Tell Us About Human Sexuality' In Evolutionary Psychology, Public Policy and Personal Decisions (London: Lawrence Erlbaum Associates, 2004)
Sherman (1989) 'The clitoris debate and levels of analysis', Animal Behaviour, 37, 697-8.
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