In her book, Testosterone Rex, Cordelia Fine considers the extent to which the behavior of men and women is a manifestation of ancient evolutionary urges. In popular evolutionary psychology, men, the embodiment of Testosterone rex (the testosterone tyrant), are programed to fight for dominance, women to choose the winner. The problem, Fine argues, is not the application of evolutionary theory to our daily lives, but that the theory itself is outdated, biased, and wrong. Nevertheless, the idea of Testosterone rex remains prominent in marketing and psychology as well as in every day conversations. Our understanding of behavioral evolution has changed in recent decades, but the ways in which we talk about evolution, men, and women in daily life have not.

Evolutionary biologists have long been interested in mate competition and child-rearing, and the ways in which they are conditioned by the general rules of evolution. Charles Darwin is best known for his work on natural selection. The idea is simple enough. Some organisms are more likely to pass on their genes than others. Natural selection explained much about the biological world, but not everything. It did not explain sex and the differences in behavior between the sexes. Darwin attempted to explain these differences in his later book, The Descent of Man, and Selection in Relation to Sex. But, after Darwin, the attention of biologists tended to focus on the general case of natural selection rather than the specific case of sexual selection, until the 1970s, and work by researchers such as Robert Trivers. In 1972, Trivers stumbled upon what seemed to be a central law of sex and evolution, an idea both simple and sweeping.

In On the Origin of Species, Darwin spent just three paragraphs discussing the differences between the sexes in birds, mammals, and insects. Those paragraphs said little about behavior. It was not until his later book, The Descent of Man, that Darwin began to discuss male and female behavior. To Darwin, there seemed to be two ways in which behavior influenced, and was in turn influenced by, natural selection. Males competed with each other for access to females. Natural selection favored traits useful in competition, such as antlers, tusks, and horns. For their part, females chose traits that might not otherwise be favored by natural selection, such as the color of male cardinals, the tails of male peacocks, and even the bowers of male bower birds. Darwin described these patterns, or at least what was known of them at the time, but provided no simple rule that could account for the differences in sexual behavior.

Enter Robert Trivers.

In 1972, Trivers published a paper entitled “Parental Investment and Sexual Selection,” in a volume that aimed explicitly to consider what had been learned about sexual selection since The Descent of Man.1 His paper presented what he called “a general framework within which to consider sexual selection.”2 Building on the work of Angus Bateman, Trivers argued that males and females incur quite different costs in reproduction and child rearing. Different costs should favor different behaviors. Males should be expected to invest less in parental care, and more in competition with other males. His theory seemed to render intelligible a bewildering behavioral diversity. It did something else. It offered a scientific basis for thinking about our own behavior.

Trivers’s theory became a framework for considering human sexuality, sexual behavior, investment in child rearing, and much more. Evolutionary psychology, which is almost a separate field, builds upon this work, and it has been evolutionary psychologists who have disseminated it to the public. Trivers’s theory reinforced a cultural narrative of male power and promiscuity, and, as Fine notes, has influenced everything from the marketing of children’s toys to the discussion of domestic violence. But for all its influence, Fine argues, Trivers’s theory is wrong. Fine revisits the story of sex, choice, and parenting in the light of the forty years of research since Trivers published his essay in 1972. What this research reveals are some simple patterns and a great many more idiosyncratic cases.

Evolutionary biologists have long noted that Trivers’s work contained flaws, but when the scientific literature updates itself, society at large does not always follow. Fine cites the excellent work by Patricia Gowaty, in particular her paper, “Sexual Natures: How Feminism Changed Evolutionary Biology.”3 What truths we see in nature are influenced by the questions that we ask, and this, as both Gowaty and Fine note, is especially true when the questions refer to sex and power.

Cultural norms often creep into science in the form of facts that are thought to be true in the absence of data. Evidence of high mating frequencies in males was often taken as evidence of low mating frequencies in females. This changed for two reasons. New genetic techniques made it easier for scientists to trace the parents of particular offspring: in many species females were mating with more and different males than their partners. Some species were studied in detail, and in many of those cases the females were far more promiscuous than the work of Trivers might have suggested.

The behavior of animals in the wild often seemed to reveal messy exceptions to Trivers’s general rules. Echoing Gowaty, Fine emphasizes the work of Sarah Hrdy on langurs. In India, Hrdy found that to some extent langur behavior confirmed the predictions of evolutionary theory. Male langurs sometimes killed babies in order to gain mating opportunities with their mothers. But the langurs were also behaving in ways that did not conform to Trivers’s evolutionary theory. Female langurs were encouraging sex, even when not ovulating, and mating many times with many different males. Hrdy did not, at first, trust her observations. Nor did reviewers of her papers. As it would turn out, she was right. Female langurs are very promiscuous. Later work would also show female chimpanzees to be promiscuous. So, too, bonobos. The most conspicuous exceptions to Trivers’s general rule may be found among the cleverest animals.

Hrdy and others also showed that males are sometimes uninterested in mating. Males, too, can be choosy. Yet, Gowaty notes, despite studies like Hrdy’s that suggest more complexity in the behavior of males and females than simple rules might suggest, “passive females and eager males remain the normative expectation. [emphasis original]”4

In Testosterone Rex, Fine explores many other examples in which theory and the real world part company. These examples, nearly all of them from studies in the last ten or fifteen years, have much in common. They are all based on very careful empirical studies of animals that have long figured in the literature. Ethologists have had to update their general theory in the light of these specific cases. Fine concludes her book by considering what new insights in evolutionary biology mean for how we think about gender, sex, and sexuality in human society. She draws from recent studies in psychology that show both the ways in which cultural norms influence the behavior of men and women, and also how flexible those norms can be.

In biology in general there is always a tradeoff between general rules, such as those embodied in Trivers’s theory, and the importance of specific and well-studied cases. We frame our general rules based on specific well-studied cases, then use our general rules to extrapolate to cases we haven’t studied well. But our general rules are, as a consequence, also subject to how we have chosen those cases in the first place. This can easily become problematic. Those original well-studied cases might not be as well studied as we thought, or they might not be representative.

Fine’s critique is, in some measure, grounded in Fine’s own perspective as a psychologist on the differences between the behavior of men and women. Industrialized, modern cultures, those on which psychologists have focused, are in essence her case studies. She notes that the behavior of men and women is flexible and depends upon circumstance and culture. But she misses the chance to point out just how important the influence of culture can be. In this she does as the fields of psychology and neurobiology themselves tend to do. Most data in psychology come from relatively affluent subjects in Western cultures. The broader array of human cultures reveals far more by way of gender variation.

  1. Robert Trivers, “Parental Investment and Sexual Selection,” in Sexual Selection and the Descent of Man, ed. Bernard Campbell (Chicago: Aldine, 1972), 136–79. 
  2. Ibid., 137. 
  3. Patricia Gowaty, “Sexual Natures: How Feminism Changed Evolutionary Biology,” Signs 28, no. 3 (2003), 901–21. 
  4. Ibid., 908.