A particularly startling example of the impact of our relationship with our children is the age at which girls enter puberty. Girls are entering puberty younger and the main reason seems to be the absence of dads.

The average age at which a British girl had her first period was 13 and a half in the 1950s and 1960s. Today, the average age for a girl to have her first period is 12 years 11 months, and it happens at primary school for one in eight girls. Breast and pubic hair growth are now under way in the average 10-year-old.

This is not an insignificant fact, because entering puberty young (before 11) correlates with a host of problems, from teenage pregnancy to depression. Only 2% of those who do so go on to enter higher education, regardless of their parents' IQ and educational level.

Changing genes could not possibly explain the trend (they take millennia to alter), although there is some evidence that it could be partly due to the greater quantity of body fat in our increasingly obese child population. But by far the most important factor seems to be a girl's relationship with her dad.

On average, a girl whose father divorces or separates from her mother and leaves the family home before she is 10 comes into puberty five months earlier than a girl from an intact family. But the impact of fathers is not limited to whether they are physically present.

In intact families, girls reach puberty later if they have a positive rather than a negative relationship with their father; the more he is involved in her upbringing, the later she will have her first period.

If the father is absent through illness or work rather than as a result of divorce or separation, the girl's pubertal age is unaffected. Interestingly, too, an absent mother or a girl's quality of relationship with her, does not affect the point at which she comes into puberty.

Early pubescence threatens depression, although not all such suffer. If the girl has an advanced ego - the capacity to integrate internal and external changes, and to deal with complex ambiguities - she is not more likely to be depressed by it.

Overall, the enormous increase in the divorce rate and in single-parent households since 1960 seems very likely to have played a major role in the decreasing age of puberty. However, it is not clear precisely why an absent or emotionally unengaged father should trigger earlier puberty.

The strongest clue comes from the fact that if the father leaves the family home before the girl is six, she is twice as likely to have early first periods and four times more likely to start sex early. It suggests that the disruption to the mother, a lack of cash and all the other problems that go with single parenthood, probably make the girl more likely to be emotionally needy and to be eager to be able to use sexual allure as soon as possible to make people love her.

The more times a girl's family environment changes (with the mother taking new partners) in childhood, the greater the risk of early puberty. If there are three or more new partners, a girl is five times more likely to have a teenage pregnancy.

Staying together for the sake of the children is not the only way to avoid all these problems for your daughter, although it would appear to help.

The other crucial implication is that fathers - whether or not they are home-based - need to start seriously bonding with their daughters if they do not want them to turn into women at the age of 11.

• Evidence relating to decreased puberty age: Lee, JM et al, 2007, Pediatrics, Volume 119, No 3, 624-30. For more Oliver James, see selfishcapitalist.com