Have you ever attended Weight Watchers or Slimming World? Or bet on a horse? I’m sorry. I don’t mean to be personal. It’s just that if you have, I have good news. Even if you put all that weight back on when you stopped going, or lost all your money when Dobbin refused at the first fence, your experience can still be put to use. It can help you to understand how the Tories can solve their dilemma over marriage.

Let me set the dilemma out. There is strong evidence that living in a house with a stable family and two parents is good for children – it makes them more likely to do well at school, less likely to commit crime and so on. There is also good reason to believe that marriage helps to produce longer, more stable relationships than does cohabitation.

Naturally, therefore, Conservatives want to encourage marriage. David Cameron promised to recognise marriage in the tax and benefit system and it is commonly thought that his policy will be some sort of tax allowance paid only to married people. Retreating from this promise wouldn’t just make Mr Cameron look a fool – it would also be a blow to marriage itself. It would signal that marriage as a norm is no longer politically defensible.

But sticking with the promise brings no end of problems. There is, for instance, a technical difficulty – many of the people the Tories wish to influence don’t earn enough to pay tax. That, however, is just the start. The policy discriminates against single parents, divorced people and widows. The Tories can’t get round this – discrimination is how the thing works. It also appears to judge the relationships of nonmarried parents and denigrate their ability to care for their children, angering millions of potential Tory voters.

And much of this anger will be justified. Marriage may be better for children, but only on average and once all other social factors have been adjusted for. There are, therefore, no end of examples of nonmarried parents raising their children in a far superior way to married contemporaries. Britney Spears was married when she took to driving with her son on her lap.

For a party trying to persuade people that it is modern and “for all”, it’s a bit of a nightmare. David Cameron was happy to drop grammar schools because he had decided supporting selection was the wrong policy. This is different – he thinks recognising marriage is right, it is central to his idea of healing a broken society and, anyway, he’s stuck with his words. But that doesn’t make the politics any better.

So what should he do? Begin by thinking how a policy to encourage long and stable relationships might actually work.

This is where betting on horses and attending slimming clubs come in. Numerous social experiments show that making public commitments has a very strong effect on those signing up to them. And you can see the application of this research all around you.

Organisations seeking compliance from their staff often get them to set out their goals for the coming year in writing. When you enter a competition on the back of a cereal pack, you usually have to send in a postcard, answering a tie breaker question “I like this cereal because . . .”. The purpose is not to break ties, but simply to get consumers to write down that they like the product. Slimming clubs use the power of commitment to get you to move towards a publicly announced target weight.

One of the most interesting features of the academic work on commitment is this – once you have made a choice, you instantly become more confident, more convinced that it was the right decision than you were before you made it. This is called postdecision dissonance, since you ask. A study of gamblers, for instance, showed that immediately after placing a bet they became far more confident that their horse would win.

Marriage works in exactly this way. The public commitment is a powerful force that keeps the couple together. And the feeling that so many people have immediately after their wedding? The feeling that things are different? Why, that’s postdecision dissonance.

So a sensible public policy would do two things to encourage stable relationships. The first is to provide just enough of an incentive to tip cohabiting couples willing to consider marriage into actually getting married. The second is to make marriage a social norm, showing confidence and respect in the institution so that those having children feel that people like them generally marry. Once the couple is married, social psychology – the power of commitment – takes the strain.

How would you do this? Well, paying money makes sense. It might provide that incentive, that little extra push, at the margin and help to establish the norm. But once the money has done the trick, once the couple has got married, why would you keep paying them?

There is no evidence, or theory, that suggests that paying people continual instalments of small sums of money (or taking it off the tax bills of those who pay tax) would increase the longevity of their relationship. In fact, I am surprised so many pro-marriage advocates believe in it. After all, if simply paying people worked to extend relationships, you wouldn’t need to bother with marriage at all. You could just pay couples to stay together.

So the right policy is to pay people one lump sum – a dowry – when they get married. A dowry of, say, £5,000 would be a real incentive to formalise a relationship and a public political statement of faith in the institution of marriage. There are, of course, technical details to be worked out – timing of the payment, the eligibility of divorcees and so forth. But the Government might be able to pay out even more: the money only gets paid to newlyweds (and civil partnerships), so you can afford an amount big enough to make a difference. And it reaches those who don’t pay tax, too.

Just as attractive are the political advantages. The one-off dowry doesn’t discriminate against anybody who is already a single parent, say, or widowed. It doesn’t judge their relationships or even appear to do so. It keeps Mr Cameron’s promise, encouraging people to get married, but doesn’t divide society into taxpayers who conform and those who don’t. It’s liberal and Conservative at the same time.

The dowry. Dilemma solved.

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