Male, Female: The Evolution of Human Sex Differences.

Male, Female: The Evolution of Human Sex Differences. By David C. Geary. Washington, DC: American Psychological Association, 1998, 410 pages. Cloth, $49.95. Why Sex Matters. By Bobbi Low. Princeton, NJ: Princeton University Press, 1999, 328 pages. Cloth, $29.95. Sex Differences: Developmental and Evolutionary Strategies. By Linda Mealey. San Diego, CA: Academic Press, 2000, 492 pages. Cloth, $79.95.

The books under review here exemplify a simple principle: If you're going to do something, do it right! All three of these authors--Geary, Low, and Mealey--have chosen to enter the contentious territory of sex differences and have done so with the best possible armor: exhaustive knowledge of their subject matter and a determination to communicate this knowledge. These books represent a fine harvest of the maturing movement toward "evolution mindedness" in psychology and the sexual sciences.

Indeed, one of the books, Male, Female: The Evolution of Human Sex Differences by Geary is undoubtedly the finest scholarly work to date reviewing theory and research findings on sex differences. However, to a large degree, comparisons are not fair because each of the books under review fills a fairly specific niche. For example, unlike Male, Female, which is written primarily for a professional audience, Sex Differences is written as an upper-level undergraduate or graduate-level textbook, and Mealey has not aspired to do what Geary has done. (It is clear from her text, however, that she could have had that been her goal). In Why Sex Matters, Low presents her case for taking an ecological evolutionary perspective on sex and sex differences to an educated lay audience. Thus, each of the books under review addresses a very different group of readers.

Male, Female

Perhaps the most important accomplishment of Male, Female is not its massively comprehensive look at evidence of sex differences in human and nonhuman animals but its serious attempt to integrate analyses of phylogeny (ultimate evolutionary history) with ontogeny (developmental mechanisms and proximate mediating variables). Thus, Geary pays more than lip service to the notion that evolutionary mechanisms interact with social-ecological, developmental, and neurohormonal contingencies. He attempts to explore these contingencies at some depth. Male, Female, therefore, reads quite a bit differently from some of the more well-known works of evolutionary psychology which stress ultimate mechanisms, sometimes to the virtual exclusion of serious considerations of proximate mechanisms--including emotion and motivation, hormonal fluctuation, cultural differences, sex roles and ideology, and historical/demographic factors such as the operational sex ratio.

In Chapter 1, Geary starts "at the beginning" and describes theories of the origin of sexual reproduction. Because sexual reproduction results in variability among the individuals of a sexually-reproducing species, there is also variability in the "mate value" of individuals. This in turn results in (a) competition for the most desirable mates, and (b) the exercise of choice among each sex for mates with the most desirable characteristics. The name Darwin gave to this process is sexual selection and it is the theoretical basis for Geary's exploration of sex differences to follow. Geary defines and elaborates on this theory in Chapter 2.

Chapter 3 is devoted to exploring the work of sexual selection among nonhuman primates and early hominids. For those (such as creationists and many social scientists) who appear to view humans as somehow "special" and only marginally related in behavior to nonhuman primates and other animals, this material should be sobering. Contrary to the exclusive emphasis on female choice and male competition found in some evolutionary accounts, Geary presents ample evidence among primates for choice and competition among both sexes.

Chapter 4 begins coverage of sex differences among modern humans with a discussion of maternal and paternal investment in reproduction. Of particular interest is the discussion of paternal investment. Because Trivers' influential theory of parental investment and sexual selection focuses on minimal parental investment as the determining factor in the evolution of sex differences, it is sometimes assumed that, in fact, men typically invest relatively little in offspring. Geary disabuses us of this idea.

Chapter 5 discusses the dynamics of choice and competition among the sexes. Geary brings in copious cross-cultural and historical data as exemplars.

Chapter 6 presents Geary's original theoretical contribution to Male, Female, and, for me, the meat and potatoes of the book. Here Geary attempts to weave analyses of ultimate and proximate mechanisms into a theory of motivation, moored in sexual selection. The basic idea is that childhood is the formative period for the adaptation of human emotional, motivational, and cognitive-brain mechanisms to local ecological conditions. Selection has shaped these mechanisms so that they will assist the individual in his or her attempts to control the available resources--social and environmental--that support survival and reproduction. Thus, the central motivation of the behavior of human beings, according to Geary, is control over resources.

Because the focus on childhood renders Geary's theory primarily one of development, Chapter 7 presents evidence to support the view that sex differences appear during infancy and childhood, and are also expressed in parenting practices. Chapter 8 appears to some degree to be "tacked on" without logical flow from Chapter 7. However, it is fascinating and deals with sex differences in brain and cognition. The book ends with a brief consideration of the nature of sex differences in the modern world. All in all, a most impressive accomplishment. I cannot imagine teaching a class in sex differences or writing an article on the topic without referring to this work.

Sex Differences

Mealey is President of the International Society for Human Ethology, and her perspective is thus somewhat less psychological and even more closely linked to findings among nonhuman animals than is Geary's. On the other hand, she is considerably more sensitive to the political and ideological side of the study of sex differences than is Geary, and her opening comments address certain controversies within the field left alone in Male, Female.

Mealey continues displaying this awareness through Chapter 2 wherein she takes a valiant (but ultimately unsatisfying) stab at discriminating the terms sex and gender. To her credit, Mealey details a number of different ways to consider and define sex and gender, so that you may take your pick. This also alerts the untutored reader to the complexities involved in establishing sex to everyone's satisfaction. However, in the end, we are still left with the dubious implication that sex is "biological" and gender "psychosocial." As the rest of Sex Differences makes abundantly clear, psychosocial and biological are not so easily differentiated. Indeed, it may be counterproductive even to attempt to do so. Mealey herself chooses to use the term sex virtually exclusively throughout the text, apparently because she believes that gender is subsumed within sex.

As previously mentioned, Sex Differences is a survey textbook that may be used in upper-level undergraduate or graduate courses. It contains 12 chapters broken into three parts: Theory, Nonhuman Systems, and Human Systems. The layout is very reader-friendly, and Mealey's prose is exceptionally engaging, suggesting a fine sense of irony, a strong intellect, and a very broad knowledge of her subject matter. The book is a pleasure to read throughout. From the student's perspective, it is the perfect textbook. It never condescends, does not use ten words when two will do, and, while in no way jovial in prose, shows a playful sense of humor in the selection of high-quality cartoons sprinkled throughout (Larsen's "Far Side," Lyn Johnston's "For Better or Worse," etc.). Unfortunately, the book is in black and white. One wishes that Academic Press had given this a high-budget color treatment, because it surely deserves it.

Part One, Theory, begins by differentiating the concept of ultimate and proximate explanations (phylogeny and ontogeny), and detailing life-history theory, Mealey's vehicle for applying general evolutionary theory to human behavior. Life history theory focuses on "the life story of the functionally integrated individual as expressed within its species-defined boundaries, from conception to death, and its genetic and other contributions to the population" (Morbeck et al. cited in Mealey, 2000, p. 8). To give you a feeling for Mealey's perspective, this is what she has to say in this section about the notorious EEA--the Era of Evolutionary Adaptedness--which has been so misunderstood and abused as a concept:

   In theory, our EEA is the multidimensional hyperspace described by all the
   events and conditions that our ancestors experienced throughout our
   phylogenetic history. Unfortunately, this concept is often overly
   simplified as "the Pleistocene" (as if there was only one environment
   throughout the entire Pleistocene period) or, even worse, assumed to be
   reconstructed by compiling the life histories of modern African tribes. (p.
   9)

Thus, Mealey avoids reifying the heuristic of the EEA, which, after all, is more about generating hypotheses for testing than describing past environments with confidence.

After fully exploring the implications of life-history theory and its relation to sexual selection and sex differences in mating strategies, Mealey moves on to Part II, Nonhuman Systems, which applies the life-history approach and sexual selection theory to the world of nonhuman animals. Mealey does an excellent job of making the importance of comparative studies apparent to students who might not intuitively see the relevance of the mating habits of dunnocks to their own sex lives.

Part III, Human Systems, which represents the bulk of the book, opens with an important discussion of the rocky relationship between the fields of evolutionary study, particularly evolutionary psychology (which focuses on species-wide similarities), and behavioral genetics (which focuses on individual differences). Mealey uses game theory in an attempt to show how humans can be "all the same" and "each one different" at the same time.

Sex Differences moves on to a discussion of women's, and then men's, strategies and tactics in mating. The discussion of women's strategies is particularly comprehensive. The book ends with a chapter on sexual politics, which, unlike some other evolutionary theorists, Mealey takes seriously. In summary, Sex Differences surpasses all earlier attempts at reaching the textbook market in the field of evolutionary behavioral studies. (However, it should be stressed that the book is probably too technical for lower-level undergraduate classes.)

Why Sex Matters

Low was trained as an evolutionary biologist (systematics and ecology), but for the past 20 years she has been studying humans in the behavioral ecology tradition. Interestingly, she turned to the study of humans for the following reason:

   When my son was a few months old, I was teaching and working on digger
   wasps at the UM's Biological Station. I was a single parent, and he hated
   being schlepped out to the site. I found myself thinking I needed something
   I could do on the computer after he was in bed ... and that started my
   first human-focused paper, on sexual selection and ornamentation. (personal
   communication, date unkown)

Since then, she has been a consistent voice emphasizing the belief that theorizing must be accompanied by the collection of data in ecologically-relevant settings.

Why Sex Matters is another entry in the scholarly-but-accessible genre of books such as How the Mind Works (Pinker, 1997). While to some degree it rearticulates now familiar basic evolutionary theory of human behavior, emphasizing sexual selection, reciprocity, and kin selection, Low's viewpoint is that of the behavioral ecologist rather than the evolutionary psychologist. Behavioral ecology is a naturalistic perspective somewhat similar to sociobiology (perhaps the most unfairly--and ignorantly--maligned scientific discipline in recent history; Segerstrale, 2000). Therefore, the book takes very seriously the assumption that human beings are animals who evolved in the same manner as other animals and whose core nature can be best understand using the same principles one applies to the study of animal life in general. In accord with this tradition, there are plenty of cross-cultural data and attempts at hypothesis generation matching specific mechanisms to specific environments. Thus Why Sex Matters, like the other two books under review, offers sophisticated, holistic perspectives on phylogeny and ontogeny, and attempts to unite innate mechanism and environment.

The book is broken into 15 chapters. Following introductory remarks, Low explains the importance of genes as a unit of selection and the nature of kin selection and sexual selection. She also tackles the thorny issue of the importance of understanding how evolutionarily novel environments modify predictions of behavior, and she sets down five important assumptions involved in predicting human behavior:

1) Organisms are generally well-suited to the environments where they live.

2) Only heritable variation should be considered when predicting effects of natural selection (changes in gene frequencies).

3) Organisms more efficient in getting resources in any environment will survive and reproduce better than others.

4) No organism has evolved to perceive or assess directly the spread of genes, rather they behave as though these proximate correlates were their goal (e.g., people do not have sex "to spread their genes" but rather, because sex feels good).

5) Humans are like other organisms in that they have evolved in the same manner.

Chapter 3 explores the nature of sexual reproduction itself, its probable evolutionary history, and the logic of sex-differential mating strategies that flow from differences in mating costs and benefits accruing to each sex. Chapter 4 looks at mating systems among nonhuman primates, and Chapters 5-8 discuss in detail various aspects of sex-differential behavior in parental effort (including fascinating material on maternal conflicts of interest resulting in infanticide, abortion, and child neglect); division of labor; life history "paths" (including important data collected among a variety of pre-literate and early agricultural societies); and a provocative discussion of fertility transitions in later life.

Chapter 9 tackles the issue of reciprocity, a central problem of evolutionary theory. Low makes sense of the (illusory) contradiction between a "selfish gene" approach and the many examples of apparent altruism we see every day. Chapter 10 provides the framework for her consideration of cultural transmission, including morality, religion, and intelligence. Low's discussion of the social nature of a highly biological event--lactose tolerance/intolerance--in this chapter provides an excellent palliative to simplistic "is it nature-or-nurture" perspectives.

Chapter 11 presents a consideration of sex differences in the formation of coalitions, including a nice discussion of play differences among boys and girls. Chapter 12 is a unique presentation of cross-cultural data on politics and reproductive competition. This is Low at her strongest. Of all the evolutionary theorists currently writing she is one of the principle voices arguing for empirical verification through detailed cross-cultural data sets. Chapters 13 and 14 analyze the nature of warfare and attempt to understand it in terms of sexual selection and ecology. The book ends with a thoughtful analysis of the nature of current novel environments and what they portend for the future of fertility. In sum, Why Sex Matters is an important and extremely well-written book, one of the very finest of the new works on evolution directed to a general, educated readership.

Symons has claimed that there "is no such thing as evolutionary psychology." That is, there is only "psychology"--which must be evolutionary by definition (personal communication). The three authors under consideration here all assume that there is no such thing as evolutionary behavioral science--there is only behavioral science which must be evolutionary by definition. With any luck, this will be the wave of the future.

REFERENCES

Pinker, S. (1997). How the mind works. New York: Norton. Segerstrale, U. (2000). Defenders of the truth: The battle for science in the sociobiology debate and beyond. New York: Oxford University Press.

Reviewed by Paul Okami, Ph.D., Department of Psychology, University of California-Los Angeles, Los Angeles, CA 90095; e-mail: [e-mail].