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 guides most research in the field. This does it result from the prurience of researchers. Rather, given that reproductive success is the ultimate currency of natural selection, reproductive behaviour is perhaps the form of behaviour most directly subject to selective pressures (see Miller 2000).
Almost all of this research ultimately traces its ancestry to the work which forms the subject of the current review. Indeed, much of it was explicitly designed to test his claims and predictions. For example, Symons discusses the age at which women are most attractive to men (p189). If human evolutionary history were characterised by fleeting one-off sexual encounters, this would be the age of greatest fertility. However, if men evaluate women for the purposes of more lasting unions, then men should be maximally attracted to women of the highest 'reproductive value' - in other words, those at the beginning of their reproductive careers such that a male is able to monopolise their entire reproductive output. A decade later, Kenrick and Keefe (1992) gathered the relevant data, confirming Symons 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" 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. Data gathered by Bellis and Baker (1990) found that women indeed appear to time extra-pair copulations to coincide with ovulation. Similarly, other 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 indicating 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).
Interestingly, Symons also anticipated many of the mistakes evolutionary psychologists would be led into. He warns that researchers in modern western societies (where arranged marriages are unlawful) may be prone to overestimate the importance of female choice as a factor in human evolution, a warning almost entirely ignored by a subsequent generation of researchers and before being forcefully reiterated by Puts (2010).
Homosexuality as a "Test-Case"
An idea of the importance of Symons work can be ascertained by comparing it with contemporaneous works addressing the same subject-matter. Edward Wilson's On Human Nature was first published only a year before Symons'. Yet Wilson's chapter on sex bears little resemblance to the subject matter of modern evolutionary psychology. 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.
In contrast, Symons' 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 claiming all behaviours are adaptive even under current environments, Symons instead focuses on instead admittedly non-adaptive (or even maladaptive) behaviours as a window onto the 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 opposite sex. Hence the relative promiscuity of homosexual men is seen as reflecting the universal male desire for sexual variety when freed from the constraints imposed the conflicting desires of women. (In contrast lesbian relationships are more similar to those among heterosexuals, suggesting, contrary feminist assumptions, that women exert decisive influence dictating the terms of heterosexual relations.)
This analysis rests on the contention that "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 homosexual and heterosexual men - both evaluating prospective mates primarily on the basis physical appearance and youthfulness (p295). Contrary to the feminist claim that men are led to objectify women by the portrayal of the latter in the media, he notes the existence of a market for gay pornography parallel in most respects to heterosexual porn (p301).
However, this assumption of the fundamental similarity of heterosexual and homosexual male psychology has been challenged by David Buller (Adapting Minds: Evolutionary Psychology and the Persistent Quest for Human Nature (Bradford Books)). Buller cites evidence that male homosexuals are in some respects feminised in aspects of their behaviour. One of the few consistent correlates of homosexuality is gender non-conformity (which manifests in childhood before sexual orientation itself becomes apparent and therefore cannot be a case of gay men merely conforming to stereotype) and some evidence suggests level of exposure to prenatal masculinizing androgens in utero affects sexual orientation (Born Gay: The Psychobiology of Sex Orientation). As Buller notes, while male homosexuals may, like heterosexual men, prefer youthful sexual partners, they also appear to prefer sexual partners who are, in some respects (e.g. muscularity) highly masculine.
The resolution of this apparent paradox may lie in the possibility that some aspects of the psychology of male homosexuals are feminised but not others (perhaps because different parts of the brain are formed at different stages of development when levels of masculinising androgens may vary). Alternatively, male homosexuals lie midway between heterosexual men and women in their psychology, appearing relatively feminine only when compared to heterosexual men. Compared to women, they may be relatively masculine, as reflected in the male typical aspects of their sexuality focussed on by Symons. (This interpretation suggests the 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 focussing on "the natural experiment of commercial periodical publishing" (p182), namely pornographic magazines, for which there exists a sizable male, but no equivalent female, audience. In many respects, this approach is preferable because, even in an anonymous questionnaire, individuals may be less than honest when dealing with a sensitive topic such as their sexual fantasies.
Unfortunately, when discussing pornography extensively, Symons' omits any discussion of romance literature, which can be viewed as the female equivalent of pornography. Subsequent research has suggested that romance novels provide insights into female psychology 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
Perhaps excessively, an entire chapter is devoted to rejecting the claim that the female orgasm represents an adaptation. This conveniently contradicts the claim of critics of sociobiology such as Stephen Jay Gould that the field is 'ultra-Darwinian' and claims that all characteristics are necessarily adaptive. (Symons also rejects the claim that the menopause is adaptive (p13), a theory which has subsequently become known as the 'grandmother hypothesis'.) In contrast, Symons claims that the female capacity for orgasm is a by-product of the adaptive male capacity to orgasm - roughly the female equivalent of male nipples (only more fun).
Although Symons convincingly critiques the notion, popularised by Desmond Morris among others, that the female orgasm functions to enhance pair-bonding, subsequent generations of evolutionary psychologists have developed less naïve models of the adaptive function of female orgasm. Geoffrey Miller (2000) argues that the female orgasm functions as an adaptation for mate-selection. 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 evidence of its adaptive function in carefully discriminating between potential suitors.
Although experiencing orgasm during coitus may appear to be a bit late for mate-selection, the claim is not altogether implausible given that, among humans, most sexual intercourse is non-reproductive. Nevertheless, at least according to the stringent criteria set out by George Williams (Adaptation and Natural Selection) the case for female orgasm as an adaptation remains unproven. (See also Sherman 1989; The Case of the Female Orgasm: Bias in the Science of Evolution)
Concluding Thoughts
Challenging the naïve ethological view of writers like Morris that humans are a monogamous 'pair-bonding' species analogous to the Gibbons is the focus of a large part of Symons discussion. Symons replaces this with a more cynical view, rooted in the individual-level (or gene-level) selection advocated by Richard Dawkins and George C Williams. On this view, the primary emotion underlying marriage cross-culturally, according to Symons, is neither love nor lust, but rather male jealousy (p123). Similarly, he argues that it is adaptive for men to actually be sexually attracted less to their wives than to other women, since they are themselves liable to bear part of the cost of rearing children born to their wives but not those born to other women with whom they mate. Meanwhile, his excellent chapter on 'Copulation as a female service' suggests that most heterosexual romantic relationships may be in some respects analogous to prostitution.
To some extent, Symons work is outdated in that it fails to incorporate the vast amount of empirical research on human sexuality which has been conducted since the first publication of his work, much of it inspired by Symons analysis and most of it providing further support for his conclusions. In addition, potentially important new aspects of mating behaviour have been identified in the subsequent years, 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 David Buss's The Evolution Of Desire - Revised Edition 4.
In contrast, in support of his theories Symons relies largely on classical literary insight, anecdote and, more 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 since one criticism of much of the more recent research in evolutionary psychology is that it is insufficiently cross-cultural and, with several notably exceptions (e.g. Buss 1989), relies overly on research conducted on undergraduate samples at Western universities. Given costs and practicalities, this is inevitable. But for a field that aspires to understand a human nature presumed to be universal, it is problematic. The Evolution of Human Sexuality is therefore not only the founding work of modern evolutionary psychological research into sexual behaviour, but remains of value for the anthropological evidence it marshals in support of its conclusions.
Bellis, M.A. & Baker, R.R. (1990). Do females promote sperm competition?: Data for humans. Anim. Behav., 40, 997-999
Buss, D. M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences, 12, 1-49
Ellis, BJ & Symons, D (1990), "Sex Differences in Sexual Fantasy: an Evolutionary Psychological Approach", The Journal of Sex Research 27 (4): 527-555
Kenrick, D. T. & Keefe, R. C. (1992). Age preferences in mates reflect sex differences in human reproductive strategies. Behavioral and Brain Sciences, 15, 75-133.
Kruger, DJ, Fisher, M & Jobling, I 2003 'Proper and Dark Heroes as Dads and Cads' Human Nature 14(3) 305-317
Miller, G. F. (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
Miller, G.F. (2000) The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature
Penton-Voak et al (1999) Mentrual cycle alters face preferences Nature 399 741-2.
Puts, D.A. (2010) Beauty and the Beast: Mechanisms of Sexual Selection in Humans. Evolution and Human Behavior 31 157-175
Salmon C 'The Pornography Debate: What Sex Differences in Erotica Can Tell Us About Human Sexuality' In Evolutionary Psychology, Public Policy and Personal Decisions
Sherman PW (1989) 'The clitoris debate and levels of analysis', Animal Behaviour, 37, 697-8.
