I teach a course at the National University of Singapore on evolutionary psychology. I am critical of the field, but I did accept a central tenet of evolutionary psychology, which is that the greater biological investment of women into reproduction impacts the psychology and behaviour of men and women. The logic is compelling. In order to reproduce, women are obliged to become pregnant and give birth. Pregnancy is demanding. Often pregnant women suffer increased blood pressure as their bodies are forced to deliver more blood and nutrition to the fetus, and many suffer morning sickness. All pregnancies involve weight gain, stretching, difficulty moving and discomfort, which renders the woman less able to protect and feed herself. Then, at the end of pregnancy, there is the painful, exhausting and dangerous birth. Assuming the woman survives pregnancy and childbirth, there is breastfeeding, which can extend the period of discomfort and exhaustion for several more years.

In contrast, men merely provide a sperm to fertilise the egg. After that, they can be free. No pregnancy. No birth. No breastfeeding. Consequently, argue the evolutionary psychologists, women evolved adaptations to mitigate their greater investment and risk following sex. Those adaptations include a preference for a mate who is healthy, resourceful and dependable, so that the woman is not abandoned and is provided for. In short, when it comes to sex, women will be choosy and faithful while men will be casual and promiscuous. In the first chapters of Testosterone Rex: Myths of Sex, Science and Society, Cordelia Fine takes that theory of parental investment apart. First, there are the many female animals that are not at all choosy. During estrous, a lioness might mate as many as a hundred times a day with multiple lions, and savannah baboons also actively seek numerous, brief pairings. Then there are the very many male animals who provide far more than just sperm, such as the St Andrew’s Cross spider. The cost of mating for the St Andrew’s Cross spider is his life – during mating, he breaks his copulatory apparatus and the female then eats him. ‘No wonder he is so cross’, quips Fine.

Animals, however, occupy many different ecological niches, and it might not be surprising that a single evolutionary pattern fails to explain the sexual activity of every species. The important issue is whether it explains human sexual behaviour. Fine makes a convincing case that differential parental investment fails to explain human sexuality today or historically. The standard evolutionary psychology argument is that promiscuous men are more likely to have children, creating evolutionary pressure for male promiscuity. One source of evidence is the historical reproductive success of some men, such as Ghengis Khan who is believed to have fathered over a thousand children (and now has 16million living descendants). The Inca Kings kept houses stocked with as many as 1,500 virgins, most of whom did not remain virgins for long. But Fine rightly points out that these cases are extreme, rare and prone to failure. A promiscuous man would need to have sex with 130 women in a year to have a 90 per cent chance of fathering more than the one child a monogamous man could achieve in that time. Historically, most men never had any realistic possibility of sex with more than a small number of women, and their best reproductive efforts were never likely to yield more than a dozen children, close to the peak reproductive potential of women.

Despite all the talk of male promiscuity, the evidence that males are more promiscuous than women is slim. The most striking modern evidence is the Clark and Hatfield study, where a man and a woman asked total strangers, of the opposite sex, if they would have sex with them. While 75 per cent of the men agreed to the offer of sex, not a single woman consented. That difference is massive, but Fine punches several holes in it. First, nobody actually had sex, so it is unknown if the men were saying ‘yes, let’s go!’ or ‘yes, sure, let’s have sex, LOL’. Second, there are many practical reasons why women might automatically say no to such an offer, including concerns about allowing a creepy stalker into their lives, fears about being robbed and the simple concern that sex with a random guy will be all about him and so completely unfulfilling. In a version of the Clark and Hatfield study where it was not a complete stranger immediately proposing sex, which is fairly strange, but a known person or celebrity, the men and women consented to sex in equal numbers. The discussion of sex motivated by evolutionary thinking is also quite strange because it is all about the copulatory act, which makes it about mating rather than sex. Twenty-first century human beings almost never mate. Whereas women a few thousand years ago might have spent almost their entire adult life pregnant or breastfeeding, modern women spend at most about five per cent of their adult life pregnant or breastfeeding. Almost all sex is now entirely removed from its reproductive function and is engaged in for reasons associated with fun, companionship, expectations, desire and so on. The biological function of sex, to make babies, is not the basis of human sexuality, which for both men and women is about relationships and bound up with many layers of culture, belief, habit and so on.

The female emphasis on child rearing is also held to be responsible for women being more empathic and caring, and less prone to risky behaviour. Fine takes those ideas apart as well. Large numbers of studies have shown that the differences in such behaviours are so small that if you select a male and female at random, and test them for empathy, about 40 per cent of the time you will end up with the woman scoring lower (and therefore being more ‘masculine’) than the man. Moreover, Fine notes that nothing is set in stone: ‘The Pew Research Center in the United States, for instance, recently reported that young women have now overtaken men in the importance they place on success in a high-paying career, and the sexes are equally likely to count being a good parent and having a successful marriage as more important than lucrative workplace success.’ In conclusion, Fine argues that the myriad different conditions that humans are likely to be born into shows that, despite obvious differences in reproductive capacity, men and women remain essentially similar: ‘our genes don’t know in advance what that cultural group’s consensus will be on the appropriate roles for men and women. A baby girl could potentially be born into a society that expects her to play piano and embroider, study at university, walk dozens of miles a day to fetch water, plant crops, tend animals, prepare animal skins, or hunt animals – and to grow up to live a life of chaste wedded monogamy, or to have two or three husbands simultaneously… Regardless of our biological sex, life will likely demand we all, at some point, cherish and care for others; take risks; and compete for status, resources, and lovers.’

It’s here that I have some argument with Fine. In her closing chapter, she laments the lack of equality between men and women, especially when it comes to positions of power and authority (senior police officers, political office, tenured professors and so on). She argues that the lack of equality is a consequence of still treating boys and girls as though they are essentially very different when the evidence shows that they are essentially very similar. Fine argues for an authoritarian control of images and toy manufacture so that women will be equal, but that argument infantilises women, and invalidates the choices women are making. Women have never been better placed, historically, to pursue life on their own terms. In the industrialised West, women are making the choice to put their children and home life ahead of their careers. Fine may not like it, but it is legitimate for women to choose a traditional domestic role, and it is not the result of toy manufacturers forcing women into a stereotype. In Asia, where sex stereotyping is at least equal to that in the West, many women are choosing to reject the traditional domestic role and are putting career before family, resulting in fertility levels plummeting. Fine rightly dispenses with the myth of essential sex differences and argues that men and women do not need to be defined and constrained by fashionable views of what makes a man a man and a woman a woman. But by conceiving of the self as purely a product of cultural conditioning at the hands of evil toy makers, she also shrinks the self and our rational capacity for autonomy. Fine lands in a place where she begins to deny any legitimacy to a definition of men and women based on differences in appearance, or sexual desire, or reproductive ability. If women want to define themselves with respect to their ability to be pregnant and give birth, they can do that, and men cannot, because women really can be pregnant and give birth and men really cannot. Women might feel differently about being caregivers, not because a stereotype-obsessed society is telling them to think that way, but because they choose to endorse a view that helps them make sense of, and feel comfortable with, their different shape and reproductive ability. It is an unfortunate end to an otherwise excellent book. The possibility of gendered toys creating sexism motivates Fine to shrink the scope of individual and free exploration of gender roles.