IT IS the economics book taking the world by storm. "Capital in the Twenty-First Century", written by the French economist Thomas Piketty, was published in French last year and in English in March of this year. The English version quickly became an unlikely bestseller, and it has prompted a broad and energetic debate on the book’s subject: the outlook for global inequality. Some reckon it heralds or may itself cause a pronounced shift in the focus of economic policy, toward distributional questions. This newspaper has hailed Mr Piketty as "the modern Marx" (Karl, that is). But what’s it all about?

"Capital" is built on more than a decade of research by Mr Piketty and a handful of other economists, detailing historical changes in the concentration of income and wealth. This pile of data allows Mr Piketty to sketch out the evolution of inequality since the beginning of the industrial revolution. In the 18th and 19th centuries western European society was highly unequal. Private wealth dwarfed national income and was concentrated in the hands of the rich families who sat atop a relatively rigid class structure. This system persisted even as industrialisation slowly contributed to rising wages for workers. Only the chaos of the first and second world wars and the Depression disrupted this pattern. High taxes, inflation, bankruptcies, and the growth of sprawling welfare states caused wealth to shrink dramatically, and ushered in a period in which both income and wealth were distributed in relatively egalitarian fashion. But the shocks of the early 20th century have faded and wealth is now reasserting itself. On many measures, Mr Piketty reckons, the importance of wealth in modern economies is approaching levels last seen before the first world war.