Chimps do it, homo erectus did it, we do it; and in a rich variety of ways!

Handbook of the Evolution of Human Sexuality. Edited by Michael Kauth. Binghamton, NY: The Haworth Press, 2006, 395 pages. Softcover, $50.00.

The Handbook of the Evolution of Human Sexuality, hereafter HEHS, published simultaneously as a special issue of Journal of Psychology & Human Sexuality, has as a principal goal the presentation of theories and research on the evolution of human sexuality. The editor, Michael Kauth, claims that, unlike any previous evolutionary-based text on human sexuality, HEHS is written for an academic audience that is relatively new to evolutionary theorizing. Accordingly, the Introduction and the first chapter, both written by Kauth, provide outlines of issues with which evolutionary psychologists are likely to be very familiar. These include such basic evolutionary concepts as ultimate and proximate causes and natural and sexual selection, as well as brief histories and critiques of eugenics, sociobiology, and evolutionary psychology.

The content of many other chapters, however, is likely to be unfamiliar to evolutionary psychologists. This is because, whereas most of the research conducted by evolutionary psychologists focuses on the sexual behavior of heterosexuals and does not consider the evolutionary origin of heterosexuality, a large part of HEHS is concerned with the sexual behavior of individuals other than heterosexuals and the origin of human sexual orientations. Of the eight chapters not written by Kauth, five deal exclusively with sexual behavior other than that practiced by heterosexuals: one has as a main focus the history of societies' perceptions of various sexual orientations and another focuses on the origin of different sexual orientations. Accordingly, much of HEHS may be new to evolutionary psychologists. It is also likely to be new ground for any reader who, although well versed in evolutionary theory, is not familiar with the literature on the origin of same-sex sexual attraction and the behaviors associated with it. Consequently, the text might be better titled Handbook of the Evolution of Human Sexual Orientation.

In the first chapter not written by Kauth, Timothy Taylor invokes archaeological evidence to argue that in preagricultural societies sexual behaviors were viewed as being fluid. With the emergence of farming, however, the various sexual behaviors began to be viewed as natural categories. Sexual orientation, therefore, became a means of social control by which individuals could be classified and their place in the social hierarchy determined.

Next, Felicia De labor Garza-Mercer takes an evolutionary approach to the question of why sexual behavior feels good. She argues that sexual pleasure does not serve solely to promote reproduction because it can be secured through a variety of means other than vaginal penetration and is pursued by men and women when it cannot lead to reproduction. She concludes that "sexual behavior and reproduction are conceptually divergent" and that "reproduction can be viewed as a consequence of sexual pleasure" (p. 122).

The general theoretical background of the next two chapters is likely to be familiar to evolutionary psychologists, although the findings presented may not be. Jon Sefcek, Barbara Brumbach, Geneva Vasquez, and Geoffrey Miller note that, although research within evolutionary psychology is dominated by a consideration of sex differences in heterosexual mating behavior, there is much overlap between the sexes and individual differences within each sex. Accordingly, they discuss the role played in determining adaptive heterosexual mating behavior by ecological factors, such as sex ratio, the availability of resources, and the prevalence of pathogens. Also considered is how nonadaptive sensory biases can lead to the evolution of adaptive preferences for sexual ornaments.

The theme of mating strategies is continued by evolutionary psychologist David Schmitt. He reports the results secured from investigations into the sexual strategies of men and women from four major world regions across personality traits, sociosexual orientations, and the three sexual orientations of heterosexual, homosexual, and bisexual. Among the findings is that, across all four world regions and regardless of sexual orientation, men more than women are sociosexually unrestricted, being relatively open to multiple mating opportunities. Another finding is that, across world regions, gay and bisexual men reported engaging in more unrestricted sociosexual behaviors than did heterosexual men. Contrary to expectations, however, bisexual women reported engaging in more unrestricted sociosexual behavior than did either heterosexual women or lesbians.

As same-sex sexual behaviors cannot lead to reproduction, the bottom line in evolutionary accounting, there is much controversy about how such behaviors could be a significant part of sexual behavior today. This debate centers on whether same-sex sexual behavior is an adaptation or a by-product of an adaptation. The authors of the next three chapters each take a different approach to answering this question. The primatologist Paul Vasey argues for an approach combining a functional perspective that considers the adaptive value of behaviors with a phylogenic perspective that considers the evolutionary history of behaviors. He concludes that exclusive male homosexuality among humans is a functionless by-product of selection for increased female fecundity. Next, psychologist Lisa Diamond argues for an approach in which the focus is on evolutionary history, not function. Specifically, she considers why, in comparison to men, women evolved same-sex desires that display significantly more plasticity or responsiveness to situational and interpersonal factors. Moreover, she maintains that the inherent flexibility in arousability among females serves an adaptive function.

Evolutionary psychologist Frank Muscarella offers an alliance theory for the evolution of male-male sexual behavior. He argues that male-male sexual behavior is an adaptation that was exapted from the use of the varied sociosexual behavior that served to regulate social relationships among hominines. Accordingly, male-male sexual behavior allowed men to establish alliances among themselves and, thereby, move up the male hierarchy. Consequently, male-male sexual activity enabled men to secure the social status and control of resources that promoted their reproductive success by facilitating sexual access to women.

Finally, biochemist James Kohl offers a proximal model for exclusive male-male sexual attraction by which olfactory/pheromone inputs produce neroendocrine responses. This process, posits Kohl, leads to an unconscious conditioning that modulates the development of sexual preferences.

In addition to presenting theory and research, the other principal goal of HEHS is to highlight the implicit assumptions that frequently underpin writings about sexuality. Accordingly, in the Introduction Kauth distinguishes between "sex" and "gender," "opposite sex" and "other sex." In addition, he notes that "sexual orientation" can be viewed as binary, bipolar, or multidimensional and can be seen as representing a natural kind or a social construction. Kauth comments, "Unexamined, implicit assumptions contribute to confused, incoherent conceptions; non-standard definitions; misused terms; methodological biases; and flawed conclusions" (p. 14). Kauth wrote that he asked contributors to take care to explicate these and other concepts they might discuss; most complied.

In the Epilogue, however, Kauth admits that, contrary to his expectations, there emerged neither agreement among contributors on how to define sexual orientation nor a clearer, more precise definition of it. He notes that, after providing a definition of sexual orientation, most contributors avoided it, preferring instead to describe sexual behaviors and relationships. Kauth therefore concluded,

   It makes no sense for evolutionists to talk about sexual
   orientation--a social construct specific to this culture and
   time--when referring to sexuality in the ancient past. Categories
   of sexual orientation gay, lesbian, bisexual, heterosexual--are not
   evolved traits. That is not to say that some sexual attractions are
   not biologically based, not inherited, or not functional. While a
   human sexual nature and sexual attractions are universal, the
   concept of sexual orientation is not. (p. 381)

Kauth also urges researchers to avoid considering sex of partner as the most important variable of attraction.

Kauth's assertions, however, may be unsatisfying for evolutionary psychologists, for they suggest that sexual attraction to opposite-sex individuals does not have a privileged place in evolution. Because reproductive success is the bottom line in evolution, and because the only form of sex that can promote this is that between individuals of the opposite sex, evolutionary psychologists are likely to continue to consider the desire for sex in that form as a "sexual orientation" that has been selected for. Consequently, evolutionary psychologists are likely to continue to be concerned primarily with investigating "heterosexuality" and recruiting "heterosexual" participants for their research.

Aside from theoretical and terminological controversies, HEHS is a testament to how weird and wonderful human sexuality is. The book reminds the reader that, even without contraception, most of the sex in which humans engage cannot produce children, highlighting the many implicit assumptions that underlie writings about human sexuality. Accordingly, HEHS may serve to alert researchers to the fact that many of the sexual attractions and behaviors of "heterosexual" participants could never directly promote their reproductive success and may involve individuals other than those of the opposite sex.

Reviewed by Alastair P. C. Davies, M.A., and Todd K. Shackelford, Ph.D., Florida Atlantic University, Department of Psychology, 2912 College Avenue, Davie, FL 33314. E-mail: [e-mail].