For those of you who don't read the sports pages, there has recently been a row about David Beckham, the England football captain. In his latest match for England he deliberately fouled someone.

Why? Not because his team would gain any advantage.

He had sustained an injury to his ribs, so he knew he would be unable to play in the next England game. By fouling someone in the way he did, he knew he would get a yellow card. This would be added to the yellow card he had previously received in another match and the two together would cause him automatically to be banned from his next game. But that was fine, because he knew he could not play in the next game anyway. By getting himself banned for one game, he would wipe away the yellow card already on his record. He would then be able to play in the rest of the tournament with less risk of getting banned for a game.

David Beckham was being, as he put it, "clever". Of course Beckham is well known, despite his footballing talent, for not being an outstanding intellectual. But he is obviously bright enough to work the system (if not quite bright enough not to brag about it).

When soccer's governing body drew up the rules on fouls, did it intend to incentivise the England captain deliberately to foul someone?

No, of course not. The people involved surely thought they were creating rules which were fair. They probably felt they were writing rules which were the footballing equivalent of 'social justice' - a fair amount of punishment for a particular amount of crime. They assumed that behaviour on the football pitch was a given and that the rules should merely reward and punish in a "fair" way. They did not think that changing the rules would change behaviour. But, as Beckham has shown, the rules certainly did change behaviour.

This very same mistake is at the core of the failure of the welfare state. People - including all the famous politicians and many clever civil servants - have thought they could change the rules - even whole structures - without changing behaviour.

In 1948, Bevan nationalised the hospitals. He assumed that the standard of hospitals was a given. He assumed that the behaviour of doctors, nurses and administrators was fixed. They would not be affected by a change in the structure - in the rules of the game under which all those people were operating. They would perform just as efficiently, just as charitably.

Their time would be deployed without any change in the amount of time spent in bureaucratic work. The hospitals would be just as clean. The authority of the consultants would be just as great. Innovation would be just as notable as before. There would be just as many beds and just as many small, local hospitals. All would be just as good, regardless of a wholeseale change in the rules of play which resulted from changing from an independent, part commercial, part charitable structure to a state-owned structure.

They were wrong. In this case, as in many, many others, the creators of the welfare state fell foul of the Beckham rule. It can be defined thus:

If you change the rules, players will change their behavour.