In a study of 1,500 London teenagers, published recently by Dr Richard Burack of St Bartholomew’s Hospital, London, 87 per cent of 18-year-olds said they’d had penetrative sex. Shockingly, 20 per cent of 13-year-olds said they’d already had full sexual intercourse or oral sex.

The issue of early sex goes to the heart of anti-poverty policies: the younger you are when you have sex, the more likely you are to have an unwanted pregnancy with a partner who won’t stay to help rear your child.

Not only do most families produced by teenage pregnancies become the backbone of the underclass, but as soon as teenagers start having sex, their academic performance at school deteriorates dramatically; those who stay at school beyond 16 earn 60 per cent more than those who leave.

The evidence is that family environment determines when teenagers first have sex, and how promiscuous they become. For example, higher levels of parental monitoring predict later sexual initiation (it is well established that girls from single-parent families are more likely to become sexually active at an earlier age, and be more promiscuous, than those who grow up in two-parent families). In particular, it seems that the presence of a father limits girls’ sexual precocity; he might, for instance, discourage boys from being sexually aggressive with his daughters. But the absence of a father is linked with increased negative attitudes to men which could make girls dissatisfied with the opposite sex and therefore change sexual partners frequently.

The latest theories from evolutionary psychology emphasise that the absence of a father and a chaotic family background may operate at an even more basic biological level to produce early onset of puberty, sexual experimentation and promiscuity in girls. This might explain the current epidemic of teenage pregnancy and early sex (the UK has the highest level of teen pregnancy in Europe), for the declining age of first sexual encounter parallels increasingly early menstruation. Girls from disturbed families have suffered prolonged psychological stress, which can stimulate the nervous system, so accelerating maturation.

Also, in evolutionary terms, the onset of puberty marks the beginning of an adolescent’s separation from the family, a process that will begin earlier if staying in the family environment is perceived as risky or harmful. In a high-risk environment, evolutionary psychologists now argue, the most adaptive response for a female is to reproduce early. In contrast, the most adaptive strategy for a female in a stable home environment is to defer sexual activity and reproduction, produce fewer offspring, and invest more time and resources in each child, thus increasing their chances of survival. This is a new way of explaining why impoverished, chaotic families typically have more children than their stable counterparts.

Several recent studies support these theories. For example, a survey of 1,200 Canadian women found that girls whose fathers were absent before the age of 10 began menstruation almost half a year earlier, on average, than girls from two-parent homes. An even younger start to puberty was identified in an American study among girls from divorced families, compared with those whose parents were still married. Other studies have also found that the earlier the experience of father absence, the earlier the onset of puberty, and that parental divorce before the age of seven can produce particularly premature menstruation.

Professor Raj Persaud is a consultant psychiatrist at the Maudsley hospital and senior lecturer at the Institute of Psychiatry in London. He is this year’s visiting Gresham professor for public understanding of psychiatry and will give three free public lectures at Barnard’s Inn Hall, London EC1 on September 29, October 20 and November 29. See www.gresham.ac.uk for more details. Email: rajpersaud@tes.co.uk