The female, once pregnant, is out of circulation for some time. This includes the entire period of pregnancy and lactation, perhaps even longer if we count the early care of the infant, and this amounts to a very significant proportion of her breeding life. She has little to gain from promiscuous or multiple matings and a great deal to lose, especially when lethal sexually transmitted diseases are epidemic.

By maintaining a degree of reserve and control over her sexual arousability a female can give herself time to select males on certain important criteria. One major basis of choice will be the degree of genetic fitness shown by the male – in terms of health, physical skills, mental ingenuity and other attributes that increase the chances that her offspring sired by him will survive. Another criterion might be the willingness of the male to commit himself to staying around to help care for her during pregnancy and to support the offspring. Elaborate courtship rituals that extend the male’s investment in the mating sequence will effectively ensure that he is not committed elsewhere and that he is ‘sincere’ enough in his affection not to abandon his mate immediately after sexual consummation. The female distaste for ‘wham, bam thank you, ma’am’ sex has deep biological significance.

An interesting demonstration of female control over the occurrence of intercourse has been provided by Peplau, Rubin and Hill (1977). Studying a large number of dating couples on American campuses they found that the length of time they would go out together before sexual ‘intimacy’ first occurred depended on various characteristics of the female partner, such as her religion and previous experience. The characteristics of the men were irrelevant. It thus appears that the willingness of men to have intercourse may be taken for granted; it is women that decide whether and when it will take place.

A similar conclusion can be reached on the strength of studies in Californian singles bars of optimal pick-up lines. For a man, what he says is crucial to whether or not he will be successful in getting off with a potential partner. Success rates vary from zero with openers like ‘Let’s go back to my place right now’ and ‘Bet I can drink you under the table’ to around 50 per cent with more respectful lines like ‘Excuse me, my name is Robert. I’ve been watching you for some time and can’t help finding you extremely attractive. Would you allow me to buy you a drink?’ In the case of a woman chatting up a man in this situation, it makes little difference what she says; almost any opening line is 100 per cent successful.

Virginity was once held to be so important that girls were afraid their boyfriend would lose respect for them if they allowed him to ‘go all the way.’ Since he would be bound to boast of his conquest to all his friends, this would probably also diminish her marriage prospects. Today, pre-marital sex has become so commonplace that a young woman often fears that she will lose her boyfriend if she does not permit intercourse within a reasonable period of time. Christensen and Gregg (1970) found that nearly a quarter of their sample of college women had ‘given in’ to their first sexual experience without feeling desire on their own part. Rather, they had yielded either to force or some sense of obligation to their boyfriend. Less that 3 per cent of men questioned gave answers that could be classified the same way. Similarly, when Bardwick (1971) asked college women why they had first engaged in intercourse, the answers she obtained were typically along the following lines: ‘Well, a great strain not to. Fairly reluctant for a while, but then I realized it had become a great big thing in the relationship and it would disintegrate the relationship.... I wanted to also.’ ‘He’d leave me if I didn’t sleep with him.’ ‘Mostly to see my boyfriend’s enjoyment.’ ‘I gave in to Sidney because I was lonely.’

Very few of the girls interviewed said they had started having sex because they wanted to. Usually it was regarded as a price that had to be paid for continuation of a relationship, or it was an attempt to prove to their boyfriend that they loved him. Despite the great increase in permissiveness in our society over the last few decades, the motives and concerns of the two sexes remain quite different.

The characteristically female trait of coyness makes its appearance very early on in life. At around the age of one or two, little girls are observed to hide their eyes from strangers in a manner that seems to express something half-way between embarrassment and flirtation. This gesture is more typical when the stranger is an adult man other than the father and is seldom observed in little boys. That it is an instinctual behaviour pattern rather than one learned by imitation is suggested by the fact that it also occurs in girls who are born blind and who would therefore have no chance to learn it off other girls or adult women. The seductive element in this gesture has led some ethologists to interpret it as a ritual invitation to chase (Eibl-Eibesfeldt, 1971). They note that a very similar sequence of making eye contact with a man and then modestly diverting the gaze is shown by adult women at parties and other sexually charged situations. This sequence is perceived by men as appropriate and appealing, whereas brazen staring and overly explicit invitations are considered ‘unfeminine’ and apt to be counterproductive.

There are a great many other deductions that may be made from the ‘parental investment theory,’ as it is often called (Trivers, 1972). For example, we would expect the motives for extra-marital sex to be different for men and women, with men committing adultery primarily for the sake of novelty and women inclining towards adultery when they perceive the other man as superior or more attentive than their husband. Similarly, we would expect jealousy to centre on the act of sexual penetration in the case of men and fear of losing the attention of the mate (the relationship) on the part of women. We would expect women to be less easily aroused by visual stimuli and slower to warm up sexually. We would expect competition between males to be more severe than that between females, with the result that successful high-ranking males would monopolize more than their fair share of females, and the others miss out to some extent. This would lead to a situation in which sexual ‘perversions’ would be more common in men than women and prostitution would be aimed almost entirely at male clients. Surveys and observational evidence discussed by Symons (1979) and Wilson (1981a) generally support these expectations.