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.
During mating, the St Andrew’s Cross spider breaks his copulatory apparatus and the female then eats him. ‘No wonder he is so cross’, quips Fine
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.