If the publishers of Freakonomics needed to give the weird title a push - they don't, because it is already a best-seller - they would probably find a quote from Tony Blair a handy sales pitch. When the British prime minister floated the idea of 'pre-birth' intervention to stop teenage mothers bringing up delinquent children, he sounded as if he was totally absorbed by one of the book's most profound revelations - that allowing mothers to terminate their unwanted pregnancies had led to fewer crimes in the United States. Economist Steven Levitt and journalist Stephen Dubner, who co-authored Freakonomics, came to that view after ploughing through mountains of data. They found a high correlation between the US Supreme Court's 1973 decision to legalise abortion and a big plunge in the country's crime rates in the late 1980s and early 1990s. Their conclusion: as legalised abortion led to fewer unwanted babies being born - and unwanted babies are more likely to become troublemakers - legalised abortion had led to less crime. It is not known whether Mr Blair has read the book, which has been hailed as pioneering a new way of looking at the world. While the British prime minister is not advocating teenage mothers from at-risk families be forced to have abortions, he would still be making a grave mistake if he were to make 'pre-birth' intervention by the state a condition for helping young parents. Perhaps nine years as prime minister has made Mr Blair feel he can sort out all society's ills. He has never hesitated to voice his opinions and float sometimes controversial solutions to a range of problems. But to make predictions about the future behaviour of children as yet unborn, and to use that as a starting point for radical changes in government policy towards families in trouble, is going too far. That would be a daring bid at social engineering and an insult to many gifted people who have overcome an unpromising start in life. Mainstream economists have since found holes in Levitt's analysis and moralists have faulted him for demeaning the value of life. His contribution lies in drawing interesting links between seemingly unrelated phenomenons and opening up a new way of viewing everyday life. Dubner has helped make his message intelligible. But the two have made no attempt to use the findings of their retrospective study as a basis for planning for the future. Social workers may instinctively agree with Mr Blair's assertion that the children of teenage mothers have a higher chance of growing up to become a menace to society. But the solution should lie in helping the mothers foster an environment conducive to the healthy growth of their children. The state would be crossing a dangerous line if it were to influence young people's parenting decisions because of judgments about their suitability to raise children.