the costs.

One of the them will be obvious to anyone faced with this supposedly generous offer. The father, if he takes his leave, will lose five and half months pay. That is a major financial shortfall for the family. It will probably affect his pension, too. It will reduce his lifetime earnings. It makes the whole idea of paternity leave into a luxury for the rich that will be useless for most people. It is odd. One day the government is warning us that we should work until we are older because otherwise we will never pay for our pensions. The next day it is encouraging us to take half a year off.

The second burden will be carried by companies. Look at it from their point of view. They are being told, "You must let your men leave your company for up to six months whenever their wives give birth. You will probably have to get someone to replace them. And then, if it suits them, the men will have the absolute right to come back to their old jobs." That makes life very difficult. It means recruiting and training up new people and then suddenly dispensing with them. It means living with uncertainty. People usually say it would be difficult 'for small businesses' but it could be a real nuisance to larger ones too.

The sudden absence of personnel could interfere with good public services too. Imagine that the surgeon who is due to operate on your heart decides to take six months off because baby has arrived. The NHS is short of heart surgeons and cannot conjure up extra ones out of nowhere.

But before teasing out all the problems that could be caused, is this law really necessary in the first place?

We don't need to speculate. We can read the book. Other countries, especially in Scandinavia, have already tried this sort of thing.

Just as here in Britain, Sweden established maternity leave. But then, as here, it was thought that this was 'discriminatory' and 'sexist' and that men should take a bigger part in raising children. So a law providing for paternity leave was created to enable and encourage men to share bringing up baby. The trouble was, even in strongly egalitarian Sweden, hardly any men used it. The take-up rate remained under five per cent compared to virtually 100 per cent of maternity leave take up by mothers.

Did this set-back deter the amateur social-engineers who call themselves politicians? Not at all. The Swedish government proceeded to make one month's paternity leave almost compulsory. If men did not want to change nappies, they would be virtually forced to get out the Pampers and get wiping. But that did not work either. The men have generally used the leave for leisure activities, to go fishing or take courses. Recently a Swedish government official visited London and admitted that paternity leave days were being added to August and Christmas holidays. They were making no difference whatever to patterns of childcare. So much for Alan Johnson's vain idea that he is going to give children 'the best possible start in life' by his measures.

Catherine Hakim, a senior fellow at the London School of Economics, who has studied these matters refers to "the remarkable failure of Swedish policy on paternity leave". That is the kind of failure which the government now wishes to immitate.

It largely comes down to human nature. The way we are brought up makes a big difference to our behaviour. But there is increasing evidence that there are underlying psychological differences between men and women. Men appear generally to be more aggressive, for example. They commit 80 per cent of murders across all sorts of different societies.

In a similar way, it is probably the case that men are fundamentally much less likely than women to be interested in looking after babies for long periods of time. In which case, no amount of politically correct legislation is going to make any difference.

There is no strait-jacket here. Men do vary in their interest in babies, as do women. But the majority of men - perhaps 60 per cent, according to Catherine Hakim - are 'work-centred'. Some women are also work-centred but they are a minority - 20 per cent. The majority of women - 60 per cent - are what she calls 'adaptive', meaning they want some combination of work and family.

Ironically women are the ones who are most against men giving up work and sharing equally in child care. There is plenty of evidence that they generally want their men employed, not at home. The unhappiest marriages with the most discontented women, tend to be ones where the man is not working.

By all means if Mr Johnson can change the nature of men and women he should get on with his new law. If he can't he should stop meddling.

(This is the unedited version of an article which appeared in the Daily Express today.)