“Life’s not fair,” my parents always used to say. Bill Gates and the Mexican business magnate Carlos Slim each have fortunes of about $60bn, according to the rich-list boffins at Forbes. A 10 per cent return on that lot would produce a $6bn income, or about $200 a second. That is, very roughly, about what an American makes in a day or an Ethiopian makes in nine months. Small wonder that income inequality is a hot topic.

But reliable numbers on inequality are hard to find. Even in the US, there is no agreement over what is going on. Look across the globe, and the data problems are far more acute. The most commonly reported scare statistic compares the richest country with the poorest. But this method overlooks the fact that about 2.5 billion poor people live in rapidly growing India and China. A better, but more demanding approach summarises inequality both across countries and within countries. Such efforts suggest no strong recent trends in global income inequality, either up or down.