Academic psychologists have studiously avoided sex. Very little is known about why you might prefer the missionary position to being on top, or different sexual activities with different partners or in different moods.
It's an astonishing oversight, perhaps a reaction against Freud (with whom, until very recently, psychos wanted no association); or it could be strong evidence for his theory that sex is a subject which makes us jumpy, as well as excited; or both.
When it comes to explaining why we fancy blondes or muscles or shapely hands, evidence suggests we are indeed, as Freud suggested, generally attracted by characteristics of our opposite-sexed parents. If mum was a bit standoffish and dad a real poppet, then son will tend to plump for standoffish lovers, daughter for poppets.
The psychologist John Money argues that our childhood experiences draw a unique lovemap. We develop an ideal partner, linking lust and love, with whom we enact fantasy scenarios and for whom we search in reality. According to him, the topography of our lovemap is drawn by the end of childhood, and he asserts that, 'Once a lovemap has been formed, it is, like native language, extremely resistant to change.'
There is evidence that children of fiercely religious parents tend to be inhibited, that those with little or no parental input are liable to shag everyone and everything, and that men who were sexually abused are more at risk of becoming abusers. Very specific perversions evolve out of very specific childhood maltreatments.
Among conventional heterosexuals, there is scientific evidence that the one half of people with an insecure pattern in relationships, fearing abandonment or rejection or both, have predictable sexual habits. Among those who are avoidant - emotionally distant, remote people - the women do not tend to have many partners, whereas the men are more active, with a penchant for one-night stands, sex without love and with partners who are already spoken-for.
Avoidants of both sexes report disliking the lovey-dovey side of sex, like caressing, embracing, kissing or gazing into a partner's eyes. They favour oral or masturbatory practices, ones that involve less emotional contact.
The other main insecure pattern is the Clinger - people who are desperate for love and constantly demanding more of it. They tend to use sex to get love. They strongly prefer embraces, caresses and displays of affection to genital action. Clinger women may get turned on by exhibitionism, voyeurism or bondage, whereas the men tend to be sexually reticent. Both sexes are liable not to have many different partners or to be very highly sexed.
If I was a psychologist starting out today, I would make a beeline for researching why people have different sexual practices. Get out there, trainee psychology lads and lasses, it's a vast unmapped territory.
· Next week: Being gay