The key, though, is surely the behaviour of girls, always the pivotal partners in sexual relationships. Clearly, some teenagers are ignorant of the facts of life; and accidents do happen even when they use contraception. Nevertheless, illegitimacy rates were far lower at the turn of the century, when both sex education and contraception were much more scarce and unreliable.
It is clear from areas with very high rates of teenage pregnancy that there are two particularly significant factors: a change in the behaviour of girls, and the influence on both boys and girls of catastrophic levels of family breakdown among their own parents. Teenage pregnancy is umbilically linked to a culture which promotes sexual freedom, indifference to marriage and the disposability of fathers.
In researching the background for my book, I talked to teachers, health visitors and vicars on Merseyside. They painted a grim picture of social breakdown which feeds on itself with every new generation. Lone mothers, they said, often give their children few boundaries of behaviour. The children are furious with the mother because the father isn't there; the mother is resentful at the children for being a burden on her. Because the mother has herself been so badly parented, she in turn is emotionally little more than a child. So she won't bother to collect the children from school, or will leave them alone while she goes to the pub. Such children come to view parenthood more as a biological fact than a role. They themselves are extremely immature. "These teenagers may think they are in love; two weeks later they feel completely differently," said one headteacher I spoke to.
The crucial factor, say the teachers, is that girls in these areas become sexually available very early. Girls of 11 are taunted by other girls for being virgins. "The main factor is the changing nature of girls and their relationships," said the head. "They are much more willing to have sex." Another teacher said: "They have the attitude towards sex that previously only the boys had; they want the sex rather than the relationship. They interpret equality as doing exactly the same as the boys. They think it means initiating, not just the sex, but also the smoking and the drugs." The boys, however, including those who have sex with them, are contemptuous of sexually available girls, calling them "slags" or "slappers". "They also treat their own mothers with contempt for having a different boyfriend every month," said a teacher.
Such a girl tends to think of her opportunities in terms of having a baby. But whereas once she saw herself as a mother within marriage, now she knows she will be housed and receive benefit as a single mother, so if the boy who fathered her baby doesn't measure up, she knows she can manage without him.
According to one health visitor, the boys want to be around their babies - but they also want to come and go and see other girls. The young mothers, not surprisingly, resent this.
Many girls become very preoccupied with their babies while the boys, who are profoundly needy because of their own disastrous family backgrounds, feel pushed out. "A lot of the women here feel they are better off without a man," said the health visitor. "After the boy has gone the friction stops. A lot of these boys go back to their mothers."
Boys from areas with shatteringly high unemployment, with no prospects for the future, suffer from cripplingly low self-esteem.
Doing badly at school, and with no job to aim for, they may feel that the only way they can notch up any achievements at all is to father a number of children. Why should these lads be expected to stick around when the girls reject them and say they want to go it alone? For a while the baby fulfils their needs; most disastrously, the young mothers think the baby will be someone to love them.
The saddest thing, say teachers, is to see baby buggies aimlessly pushed around the local shopping precinct by young girls who the previous year were sitting in their classrooms. They are the teenage victims of the devastating message that girls can match boys in opportunistic sex and that mothers can go it alone.
The Sex-Change Society: Feminised Britain and the Neutered Male, by Melanie Phillips; Social Market Foundation, pound;12.99.