The end of Western convergence, then, isn’t an indicator of some kind of failure. Meanwhile, the coming of rapid convergence by emerging markets is a huge success story.

When I was in grad school in the 1970s, I thought I should do development economics — because it was clearly the most important subject — but didn’t, because it was too depressing. At that point it was mostly non-development economics, the study of why Third World countries seemed to fall ever further behind the West. True, we were already seeing a growth takeoff in smaller East Asian economies, but few saw this as a trend that would spread to China and India.

Then something happened; we still don’t know exactly what. It’s a good guess that it has something to do with hyperglobalization, the unprecedented surge in world trade made possible by breaking up value chains and moving pieces of production to lower-wage countries. But we don’t really know even that.

One thing is clear: at any given time, not all countries have that mysterious “it” that lets them make effective use of the backlog of advanced technology developed since the Industrial Revolution. Over time, however, the set of countries that have It seems to be widening.

Once a country acquires It, growth can be rapid, precisely because best practice is so far ahead of where the country starts. And because the frontier keeps moving out, countries that get It keep growing faster. Japan’s postwar growth was vastly faster than that of the countries catching up to Britain in the late 19th century; Korea’s growth from the mid-60s even faster than Japan’s had been; China’s growth faster still.

The It theory also, I’d argue, explains the U-shaped relationship Subramanian et al find between GDP per capita and growth, in which middle-income countries grow faster than either poor or rich countries. Countries that are still very poor are countries that haven’t got It; countries that are already rich are already at the technological frontier, limiting the space for rapid growth. In between are countries that acquired It not too long ago, which has vaulted them into middle-income status, but are able to grow very fast by moving toward the frontier.