Classical economics suggests that market economies are self-correcting in times of recession or depression, and tend toward full employment and output. But English economist John Maynard Keynes disagrees.

In his groundbreaking 1936 book The General Theory of Employment, Interest, and Money, Keynes argues that traditional economics has misunderstood the causes of unemployment. Employment is not determined by the price of labor; it is directly linked to demand in the economy. Keynes believes market economies are by nature unstable, and so require government intervention. Spurred on by the social catastrophe of the Great Depression of the 1930s, Keynes sets out to revolutionize the way the world thinks about and understands economics—and in this he succeeds.

In the latter half of the twentieth century, Keynesian economics became mainstream policy for most Western governments. Although his ideas fell out of fashion, the global market turmoil in the opening decade of the twenty-first century once again saw interventionist government fiscal and monetary policy based on Keynesian thinking.