Even so, the idea that young men fantasise about sex every seven seconds is untrue. Their sexual responses, however, are easily aroused. Fantasies may be compared to the playing of someone's own private video in his mind, in which he has the starring role in his preferred sexual activity. These activities may be as a couple, a threesome, a group or even solitary.

It is said that women tend to fantasise about sexual experiences they have had, although these will become distorted to give greater pleasure. There is a much stronger romantic element to female fantasies than is found in those of a man. The female fantasies aren’t concentrated so much, if at all, on the visualisation of the male genital region, or even on coupling, but more on the situation and the presence of the desired person.

Great play is made of the idea that all women have a rape fantasy, even though they wouldn’t want it to happen. This is nonsense. But it is likely that most women want to be enticed into sexual situations that they know are unwise, but even in their fantasies want to be able to blame their man for being the instigator.

Men’s fantasies are said to include either a desire to possess and control the woman of their fantasies, or to be controlled by them, and are therefore based on domination or submission. Both desires represent a return to childhood, when parental power was absolute. In reality, most men’s fantasies, although they include sex, are more daydreams and are more likely to be about loving and being loved than involving whips.

The sexual side of male imagery is more explicit and anatomical. Most will fantasise about run-of-the-mill sex with women they find attractive, but those who wish to be submissive may be attracted to someone who has the characteristics of a powerful female figure of their childhood; others may want to be in control and play the fatherly role, looking after a dependent woman.

Both men and women most commonly fantasise about sex with a previous partner. Most wouldn’t actually want to act out this fantasy, even if they had the chance, but it is an oft-used device by people to alleviate sexual boredom.

In the male the role of fantasy in sex is said to increase as sexual prowess diminishes. And later in life, as the late Lord Hailsham of St Marylebone observed, the end of the everyday obsession with sex is the one advantage of old age.