I can still vividly recall my first sexual experience, aged four. At nursery school a small coterie of us had taken to having occasional orgiastic breaks from the rest of the class in an adjacent spare room.
My general memory is of four of us, two boys and two girls, getting uncommonly excitable for no very obvious reason. My specific memory is of groping a girl's genitals and her rather vacant look of dreamy pleasure, a bit like the expression that used to appear on the face of that woman in the Cadbury's Flake advert before she took her first bite.
It's interesting to reflect that even today, after decades of sexual liberation, some of you will still be a bit uncomfortable with that reminiscence.
But before you consign me to the file marked 'Pervert', and turn to the more edifying Comment pages, I would ask that you listen to the evidence that this memory is in fact a sign of my normality.
However much you may not like to admit it, genitally focused sex play among three- to six-year-olds is the norm. Be it Doctors and Nurses, Mummies and Daddies or 'I'll show you mine, if you show me yours', most of us can remember something of the sort.
Even in the far more sexually inhibited era (1948) when Alfred Kinsey published his famous study of sexual behaviour, more than half of his male respondents could recall mutual masturbation with other boys in childhood, and one-third remembered touching girls' genitals. More recent surveys, based on mothers' accounts of their children, show similar results.
Two-thirds of young children are reported by their mothers to touch their own sex parts. They are very liable to be voyeuristic and exhibitionistic, sometimes engaging in outrageous flirtation, adopting a seductive manner, often impersonating their parents, older siblings or television characters.
Boys favour public masturbation more than girls, who are partial to exhibitionism. According to their mothers, two-thirds of girls like to wander about in the nude (versus half of boys) or sit with their crotch exposed (versus one-third of boys).
Around the age of five, like Adam and Eve, they begin to feel uncomfortable if others see their naked bodies. They demand bathroom privacy and become self-conscious when changing at the public swimming pool.
Hence eight-year-olds are four times less likely than four-year-olds to be reported by their mothers as touching their sex parts or showing them to other children or adults. Female modesty replaces exhibition, so that older girls are five times less likely than younger ones to walk around in the nude or their underwear.
This repression happened to us all. But precisely how our parents reacted to our infantile gropings - punitive horror, tolerance or inappropriate interest - has been crucial in determining our later attitude to sex. If reading this has been a bit unsettling, your early experience might explain why.
· Next week: how early repression affects teen sex in boys.