For two hundred years, the middle class has enjoyed legendary status in Western economic thought. First the British and then the American middle classes, Weber, Marx and many others said, served as vaunted engines of economic growth and political stability throughout the 19th and 20th centuries.

When large segments of the population moved beyond subsistence living, they invested their excess capital in savings, education and the purchase of increasingly high-quality consumer goods. A focus on thrift, education and hard work produced entrepreneurial small companies that drove economic growth.

For a remarkable forty-year stretch after World War II, that model proved largely accurate in many parts of the world. The American, European and Japanese economies drove global growth. And a middle class the likes of which the world had never seen emerged in the United States. American incomes, educational opportunity and home size seemed destined to grow inexorably generation after generation.

In an astonishingly short period, that economic model has disintegrated. In practice – just ask the American worker whose real wages have declined for decades – and even in theory.

Last month, the British economist Charles Kenny declared the middle class’ importance and purportedly magical role in economic growth a “myth.” A 2010 paper by the Brookings Institution’s Homi Kharas argued that the best hope for the world economy was the emergence of western-like, consumer middle classes in China and India. And my colleague Chrystia Freeland wrote a recent column describing how companies are increasingly trying to sell to the rich at home and the emerging middle class abroad instead of the cash-strapped American middle class.