Imagine that your daily earnings were less than the price of this newspaper. Would you consider buying private education and private healthcare?

Before you make up your mind, here are a few considerations: government healthcare and primary education are free; the private-sector doctors are ignorant quacks and the teachers are poorly qualified; the private schools are cramped and often illegal. It doesn't sound like a tough decision. Yet millions of very poor people around the world are taking the private-sector option. And, when you look a little closer at the choice, it's not so hard to see why.

Take the doctors of Delhi, who were studied carefully by two World Bank researchers, Jishnu Das and Jeffrey Hammer. These doctors are busy people – the average household visits a doctor every two weeks, and the poor are particularly likely to visit. And, surprisingly, three-quarters of those visits are to private practitioners – despite the fact that public-sector doctors are better qualified. Why?