THE story of Schrödinger's cat illustrates a nice finding of particle physics: you cannot measure something without changing its outcome. Curiously enough, that is true of a lot of social policy too. Take poverty. Until the late Victorian period, poverty in Britain was well documented–read almost any Dickens novel. But it wasn't well measured. That meant that it tended to be seen in individual terms: this person is poor for that reason. It was only in the 1890s, when social researchers such as Charles Booth and Seebohm Rowntree started knocking on doors and actually counting poor people, that Victorian Britain got a sense of the sheer amount of poverty in their midst. That made the rise of the welfare state, which really began with old age pensions in 1909, slightly more inevitable.

All of which is a long way of saying that measuring poverty matters. This is something that Iain Duncan Smith, the welfare secretary, is well aware of. Yesterday, he gave a speech arguing that poverty "simply cannot be captured by assessing whether their household has more or less than 60% of the average income." He wants the measure to be broader–and not primarily about income. We've published a story on the proposed change here. Mr Duncan Smith's argument that the current poverty line isn't an effective measure. But his real motivation for wanting to change it is surely how it affects government policy.

A dedicated, income-based poverty measure committed the last government to supporting household incomes, with tax credits and other benefits. That, after all, was the only effective way to reduce child poverty, as the government measured it. The Children's Society, a charity, has created a helpful online calculator here which shows quite how this worked: input characteristics for any family (how many children, etc) and you get a poverty line based on those characteristics. You can then see quite how the generosity of benefits have changed over time for that particular family. Essentially, they've become a lot more generous for pensioners and families with children, and a fair amount less generous for childless people.

And so it should be no surprise that pensioner and child poverty fell under the last government, while poverty rates for childless people increased. The measure dictates the policy. But all benefit rates are likely to become less generous now, thanks to the government's shortage of cash. That might be why Iain Duncan Smith wants a new measure: one that includes variables like family stability and educational attainment, rather than just how much money a family has. Quite apart from whether that is a good idea (I happen to think not), it raises an interesting question: will it be able to influence government policy in quite the same way?