Testosterone Rex: Myths of Sex, Science and Society. By Cordelia Fine.

BOYS like sticks and girls prefer dolls, or so the tidy evolutionary story goes. Because stone-age men hunted game and competed for mates, boys want to play rough, take risks and assert dominance. Because women mainly cared for babies, girls still hope to nurture. Given these hard-wired differences, it is only natural that it can sometimes seem that men are from Mars and women from Venus. But some recent research disputes this.

In “Testosterone Rex” Cordelia Fine of the University of Melbourne takes aim at those who suggest that evolutionarily determined sex differences—and the power of testosterone—can explain why most CEOs are men and few physicists are women. She argues that essentialist presumptions that rationalize an unequal status quo are “particularly harmful to women”.

Evolutionary determinists suggest that females are a resource that males fight over. A female’s reproductive output is limited by her physiology no matter how many mates she has. A male’s is limited by the number of females he can inseminate. Because of this, males are more likely to seek status, take risks and fight rivals in order to woo as many fertile partners as possible. Females either choose the winners, or the winners choose them, depending on the species. But tell this to the wildly promiscuous female Savanna baboon or the fiercely competitive female bush cricket. Ms. Fine uses studies of behavior from across the animal kingdom to argue that neither sex has a monopoly on competitiveness, promiscuity, choosiness or parental care. Females who sample widely tend to be more reproductively successful (which is why a lioness may mate up to 100 times a day with different lions during estrus), and those who jockey for dominance are often rewarded with more food.

Among humans, the conventional view is that men are programmed to act like Casanova. After all, a man can ejaculate 100 times in the time it takes a woman to complete a menstrual cycle. But Ms. Fine argues that relentless male promiscuity has limited benefits. Because randomly timed sex will impregnate a healthy woman only around 3% of the time, she finds that a man would have to have sex with more than 130 women just to have a 90% chance of beating the fertility rate of a monogamous couple. This, she notes, may be one of the reasons why a majority of men—like women—say they would prefer to be in a sexually exclusive relationship.

Ms. Fine does not dispute that sexual selection has shaped brains and bodies, or that genes and hormones influence how animals think and behave. An ever-changing “mosaic” of features—some more common in females, others more common in males, some common in both—guides both men and women. But, she argues, people tend to overestimate these differences and underestimate the value of environmental factors, such as rearing conditions, ecological resources and social conditions: that is, the nurture side of the nature-nurture debate. She points to a recent study of young Chinese men and women playing a risk-taking game, which found that the women were every bit as bold as the men when they played privately, but they took fewer risks—and the men took more—when their games were observed by an attractive member of the opposite sex. Ms. Fine suggests that a desire to appeal to the observer nudged the players to heed gender norms.

Some neuroscientists speculate that sexual traits are vulnerable to environmental forces to ensure that animals can adapt to different habitats. This seems to be especially necessary for humans, who must learn how to cope in groups as diverse as matrilineal Arctic foragers and patrilineal tropical horticulturalists.

In her zeal to challenge evolutionary determinists, however, Ms. Fine takes a swipe at some straw men. Few serious theorists argue that male and female brains are categorically different, or that individuals are not influenced by environmental pressures. Parents who have both boys and girls may cock an eyebrow at the way she largely ignores studies of actual sex differences, preferring to blame much of gendered behavior on socialization. As for testosterone, only the most reductive observer would claim that absolute levels of the hormone “cause” behavior, so it is not surprising when Ms. Fine explains that its effects on brains and bodies is more nuanced. She also offers evidence that seems to undermine her point that testosterone does not necessarily make men more risky or competitive: apparently the testosterone levels of Wall Street traders go up as they make more money (a phenomenon known as the “winner effect”), which seems to spur them to take more risks.

Despite this, Ms. Fine’s is a provocative and often fascinating book. Armed with an array of studies on everything from rats to humans, she shows that adaptive traits can take different forms depending on the circumstances, and nothing is fixed.

This article appeared in the Books and arts section of the print edition of The Economist under the headline “Gender fluidity”.

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