Another environmental factor brings me to the moral reason that stands in the way of equalising the educational success of those in the professional classes and those towards the bottom - moral, at least, for those of us who think that whether a baby has a father as well as a mother is a moral issue. One more empirical finding that is no longer in dispute in the United States among scholars of child development is that children do best when they are raised by married biological parents.

Children raised by a divorced mother do next best (whether she remarries doesn't make much difference). Children raised by an unmarried mother do worst of all. Let me emphasise that these results are found after controlling for a host of socio-economic background variables. Poor and rich children alike benefit from growing up with both biological parents and suffer from being born to a lone mother. In the United States, even the scholars who used to believe otherwise have changed their minds on this one.

The advantages of growing up with married biological parents do not include higher academic ability. But they do include all sorts of other advantages that affect success in school. Children raised by both biological parents are more likely to grow up psychologically healthy, accustomed to a regular routine and self-disciplined than children who grow up with unmarried mothers (again, even after controlling for socio-economic variables). They are more likely to have someone watching over their homework and noticing if there are problems at school.

If the incidence of marriage and lone parenthood were evenly distributed throughout the social classes, the advantages of marriage would not benefit the children of the professional classes. But lone parenthood is concentrated in the lowest socio-economic levels of English society. Consider the examples of Wokingham and Southwark, local authorities that epitomise the socio-economic top and bottom. As of the 2001 census, 13 per cent of Wokingham households with dependent children had a lone parent. In Southwark that figure was 45 per cent.