HERE’S an idea. Children are, the data overwhelmingly proves, better off in households who own ponies than in those who don’t. They do better at school, grow up to earn higher incomes, live longer lives and generally suffer less*. When they grow up, they are more likely to own ponies themselves, and so perpetuate the virtuous cycle. Obviously then the state ought to subsidise ponies, to extend the benefits of the institution of pony-owning to as wide a section of society as possible.

A joke. Most people would not accept that ponies have such wonderful effects on children (though your correspondent can name at least one who fervently believes it). The point is that pony ownership is one of those things that is highly correlated with wealth, which in turn is correlated with children doing well at school. That doesn’t mean that ponies are the cause. Wealth is the cause, together with a thousand other smaller things that go with it—possibly including, in a tiny way, pony-ownership. Similarly, it is extremely clear that on average, children do better when their parents are married. What is not so clear is how much the institution of marriage actually has to do with it.

This debate matters because government—or at least its Conservative main part—is extremely keen to promote marriage—and it plans to introduce a tax break worth around £200 to some married couples to do so, as a total cost of some £600m. Today, Ed Balls, the shadow chancellor, has pledged that he will scrap the tax break if Labour wins the election, prompting a furious reactionfrom the right. They argue that their tax break will help encourage stable families, reducing benefit dependency and helping children. Mr Balls says that the tax break is “perverse and unfair”. He is absolutely right.

For all that Conservatives promote the role of their tax break in helping children, according to the 2011 census, just 37% of married couples have dependent children. There are a bunch of other practical problems—for example, dual income earners are unlikely to benefit, ruling out a load more children. But let’s put that aside. The key question is whether marriage itself actually helps anything—that is, as opposed to cohabitation.

The evidence seems to suggest not. For example, only one major study tried to account for the selection effects of marriage, and this showed that “once observable and unobservable differences between married and cohabiting parents in Sweden are taken into account, marriages (at least those by parents responding to financial incentives) provide no educational advantage to children”.

What really matters is first, how—or indeed whether—we should try to encourage two-parent families and second, whether we should try to improve the circumstances of single parenthood. Last year, 36% of lone parent households were workless, against just 5% of two parent households. To make up for the income costs of that, we rely heavily on the welfare system, which costs a lot of money and creates a lot of resentment. If more couples stayed together, that would be an improvement.

But how much can we realistically do? The reality is, we live in a world where people divorce and relationships don’t always last. Relatively few people plan to raise children alone; it just happens. If they are middle-class, they can usually rely on some sort of support from the absent parent, but even then it is tough. If money is tight, it is tougher still. How would those people feel about tax breaks which in effect penalise them in favour of people who are doing quite well already?

Perhaps the liberal thing to do would be to stop trying to influence people’s private lives and start trying to ensure that people can do the best thing for their children whatever the circumstances they end up in. Since 1997, the proportion of lone parents living in workless households has fallen from 51% to 36%: that is a significant victory, far outweighing any of the likely benefits of the marriage tax break. That is at best an enormous sideshow—moralising through public money. A tax break for ponies would be about as justified.

*NB: For the purposes of this blog post, your correspondent has not actually checked that any of this is true, but he is pretty sure that it is.