Nothing is more certain, in an uncertain world, than the Iron Law of Unintended Consequences. When the Tories decided that town halls' dreary registry offices needed a competitive kick up the backside, and made it much easier for couples to get married in a hotel, a country house or even, in due course, a rotating pod of the London Eye, they didn't realise it would also undercut church weddings... It did, though. Many couples, it turned out, only married in church because this was the prettiest backdrop for the photos. Since the rules changed in 1994, Anglican ceremonies in England and Wales have fallen by 40 per cent, to about 60,000; civil ceremonies, presided over by a registrar in 'approved premises', have risen to 50,000. (The chilly breeze of competition did also make town halls jazz up their act; local registry offices still marry as many as both these rivals put together.) The worried church has decided to go in for marketing. Its wedding shows at Birmingham's NEC and London's Earls Court, this month and next, trumpet the thought that "ancient tradition and modern experience are held together in the skilful hands of the clergy, who seek to have a personal relationship with the couple."

Roll over Bede, Cranmer and Lancelot Andrewes. The Iron Law of Unintended Consequences is more powerful than the Thirty–Nine Articles or the Book of Common Prayer.

The Iron Law pervades all areas of social policy. All governments have an unstoppable itch to try to manipulate behaviour (and the present government more than most). But people are remarkably capable of derailing almost any such Pavlovian projects. Left is as vulnerable as right to having its lovely plans go awry.

How many of those who advocated abortion law reform, as the woman's right to choose, expected abortion to be used as a belated form of contraception (which the statistics seem to show it regularly is)? This was something you associated with the Soviet Union, where contraceptives could often not be got for love or money, and didn't work when you did get them.

How many of those who argued that it was unfair of building societies not to allow the wife's (or, later, partner's) earnings to be included in mortgage applications realised that this would simply pump more money into the housing market? Prices would rise. Young mothers, who might have preferred to stay at home with infant children, can be obliged to keep working just to service the mortgage.

How many of those who criticised council housing allocations for not helping the needs of the poorest families, and campaigned for what became the 1977 Housing Act (giving absolute priority to neediness and 'homelessness'), had reckoned that this would destroy the coherence of the estates, handing them over increasingly to the socially marginal. One dilemma was resolved, in a way; but it gave rise, within a decade, to another dilemma, just as hard to resolve. I should put my own mea–culpa into this equation. The magazine I then edited, New Society, was one of the fervent advocates of change.

And so it goes on. Whatever the rights and wrongs of Ken Livingstone's congestion charge, for example, it has made exemption cards for the disabled so valuable — £300 to £400 each on the black market — that card theft has rocketed upwards, especially in boroughs like Camden and Westminster which straddle the congestion zone boundary. Nor is the Iron Law restricted to the United Kingdom in its operations. When, out of a reasonable sentiment of guilt, international agencies pumped money into Vietnam, to help it found a coffee–growing industry, did they foresee that the Vietnamese would be so efficient at this that their cheap beans would destroy the economies of many small African or Central American states which depended on high coffee prices?

Now, of course, the Iron Law is beloved by conservatives, because you can twist it into an argument for never carrying out any reform. That's not my own conclusion. I remember Cromwell's celebrated letter to the assembled Scots : "I beseech you, in the bowels of Christ, think it possible you may be mistaken." Neither he nor the Scots heeded this wise advice. But the rest of us should. Reform should pay close heed to what people want, not what others think they ought to want. And then give time for the consequences to play out, before plunging into the next reform. Otherwise, the Iron Law will play its all–too–practical jokes on you yet again.