The science of women and sex: Is Stephen Fry right after all?

Evolutionary theory says yes – but that's not the whole story.

Can science shed some light on Stephen Fry's comments about female sexuality? Do women really find sex disgusting and only partake of the gruesome act in order to get their man to "commit", as he suggested in an interview with Attitude magazine?

The deep frying of Fry for his ill-chosen words, which he insists were made in jest and taken out of context, is perhaps unjustified. The science suggests that he may have a point, but only if the long view of human sexuality is taken into account – in other words, the reason why sex has evolved in the first place.

Biologically, sex is a way of mixing the genes between two individuals in order to produce a genetic variety in the offspring that would not exist with asexual reproduction, such as cloning. Many animals and plants engage in sexual reproduction because it confers an advantage, and the fact that sex has been practised for many hundreds of millions of years by a vast plethora of lifeforms attests to its biological importance.

But explaining the reasons for sexual reproduction does not explain why we have just two sexes, and why males and females are so different to one another. To understand that we need to understand the two competing and mutually exclusive "strategies" employed by each sex in order to reproduce.

Females produce egg cells, which are relatively large structures representing a sizeable investment in the future compared to sperm cells. This investment gets magnified substantially in female mammals, including humans, who get pregnant, lactate and are involved in years of strenuous childcare.

For a female mammal, therefore, reproduction is not something to be undertaken lightly. Choosing the right mate, especially in a monogamous species involving shared childcare, is therefore crucial. Females of many species, including humans, should therefore be "choosy" – that may mean looking for commitment from a male if shared offspring-rearing is important in that species.

Males have taken an opposing strategy. They invest very little in each sperm cell, which is no more than a package of tightly wrapped DNA attached to a propeller. Instead of quality, males have gone for quantity, because their strategy is to inseminate as many females as possible. This explains why one man can inseminate many hundreds of women in a lifetime, but a woman could only bear children from no more than a dozen or so men.

The problem for the male, however, is that even in an utterly promiscuous society where females are available to everyone, there is a high risk of not finding a female who hasn't already been inseminated, because each male can inseminate so many.

This leads to intense competition between males for mates and the rise of elaborate ways for them to beat their fellow sperm-producers to the precious female eggs. At its most complex, this results in males trying to exclude their females from other males by force – leading to social organisations based on harems that are defended by the size of your antlers (or your wallet).

Although some human societies are polygamous, most are based on some variation of monogamy, either serial or lifetime. But monogamy does not preclude promiscuity, as exemplified by married footballers.

As a species, human males fall somewhere between gorillas and chimpanzees in terms of their propensity to promiscuity. We can say this from looking at the relative size of a man's testicles compared to those of the gorilla (slightly promiscuous, small testes) and chimps (highly promiscuous, very large testes).

So what has this got to do with whether women enjoy sex as much as men? The answer is that it provides the long evolutionary backdrop to human sexual behaviour, which may explain differences between the two sexes.

That there is a difference between the sexes is self-evident. It manifests itself in terms of physical and psychological distinctions that are, in general, fairly easy to categorise as either "male" or "female", although there are always going to be some individuals who fall between the two statistical "norms".

Charles Darwin took a keen interest in why such differences between the sexes have evolved and came up with the idea of sexual selection, which is a special type of natural selection, the driving force of evolution. In essence, sexual selection explained how the peacock got his tail. Sexual selection says it was because the peacock is the victim of female choice – choosy peahens are attracted to the males with the longest, most elaborate tails.

Without getting into the details of the evolutionary reasons for sexual selection, many of the physical distinctions between men and women, such as beards and breasts, are also thought to have resulted from sexual selection. It is a reminder, if one is needed, that we are at heart sexual animals who evolved over many thousands of millennia.

Evolutionary theory, however, can only go so far in explaining human sexuality, which is after all controlled by that most mysterious of human features, consciousness. It is often said that the human brain is the sexiest organ, and there is abundant evidence to suggest that is especially true for women. Men's sexuality appears to be less cerebral, and more deep-rooted in the far more ancient "reptilian" part of the brain.

One other mystery of human sexuality is the female orgasm, which serves no obvious biological function. Humans may be one of the few species where females actually enjoy sex physically as much as males when it comes to orgasmic pleasure.

So although biology can go a long way to explaining sexuality in humans, it cannot explain everything – including why some people are attracted to the same sex.

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