Nowadays, most people who regard themselves as intelligent, sophisticated, and well-educated despise prudery about sex. They regard the Victorians as ludicrously inhibited about it, and think that they could have done with a bit of modern sex education. But this is only in relation to the physical aspects of sex. Where the psychological and social aspects are concerned, the situation is the exact opposite, and the Victorians were less inhibited than we are.

What I have in mind here are innate psychological, cognitive, and behavioral differences between the sexes. Where the Victorians would have been quick to accept that men and women could be strikingly different in these respects, modern people—especially those who regard themselves as right-minded—are likely to be as horrified by the prospect of such differences as the Victorians were where physical sex is concerned. Indeed, we have even invented a euphemism for sex, which many now call “gender.”

But there are some problems with this. First, gender strictly speaking refers to language, where nouns can be masculine, feminine, or neuter. As language teachers correctly point out, sex and gender are two different things, and equating one with the other abuses language, to say the least. The second problem is that in modern biology sex can be defined objectively and quantitatively—unlike gender, which looks subjective and arbitrary by comparison. But worst of all, gender tends to hide the real difference between the sexes in ways which could be construed to favour male self-interest. By insisting on equality—or even near identity—users of gender mask the reality of sex differences in its most important respect: the costs and benefits to the individual.

Any cost to the parent which benefits offspring in any way is called parental investment. This covers such diverse things as feeding, transportation, protection, instruction, warming, cooling, or whatever. In modern biology, the sex with maximum parental investment and the largest sex cell is always female and that with the least and smallest, male. (In the very rare cases where there is no difference, they are arbitrarily labeled plus and minus.)

In the case of mammals, the imbalance in parental investment between the sexes has reached bizarre proportions thanks to internal gestation and lactation, which are exclusive to females. Look at it this way: in the most tangible terms of weight added to a new-born, the mother’s contribution is billions of times greater than the father’s, which amounts to a single sperm! And in our evolutionary past—not to mention much more recent times—any further weight gains prior to weaning would be the result of breast-feeding, an exhausting and time-consuming task which can only be undertaken by a woman, never by a man.

The same is true of intangible costs. The risk of death in childbirth in Western countries today is about 1-in-10,000, but in sub-Saharan Africa—which probably reliably represents the greater part of human evolutionary history—the figure is about 1-in-100, and the lifetime risk of death from all factors associated with pregnancy for a woman is a staggering 1-in-21! But throughout history, the corresponding risk to males has been exactly zero. And similar asymmetries are seen where benefit is concerned: 37% of women are treated for psychiatric problems related to infertility, but only 1% of men. However, almost the exact opposite is the case with sexual dysfunction: many more men than women seek medical help, and the diagram suggests why. Assuming an average of 10 children per woman, a man mated to 10 women has 10 times as many children as a woman mated to 10 men! And as far as evolution is concerned, this is all that matters: reproductive success is the bottom line, but the male and female accounts work out very differently.

In short, as every woman knows, sex does not mean the same thing for a woman that it does for a man in objective terms of costs and benefits—even when the term refers merely to the act, rather than differences between being male and being female.

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