Causes of the Great Depression
Second, there are the monetarists, who believe that the Great Depression started as an ordinary recession, but that significant policy mistakes by monetary authorities (especially the Federal Reserve) caused a shrinking of the money supply which greatly exacerbated the economic situation, causing a recession to descend into the Great Depression. Related to this explanation are those who point to debt deflation causing those who borrow to owe ever more in real terms.
Great Depression
The two classical competing theories of the Great Depression are the Keynesian (demand-driven) and the monetarist explanation. There are also various heterodox theories that downplay or reject the explanations of the Keynesians and monetarists. The consensus among demand-driven theories is that a large-scale loss of confidence led to a sudden reduction in consumption and investment spending. Once panic and deflation set in, many people believed they could avoid further losses by keeping clear of the markets. Holding money became profitable as prices dropped lower and a given amount of money bought ever more goods, exacerbating the drop in demand. Monetarists believe that the Great Depression started as an ordinary recession, but the shrinking of the money supply greatly exacerbated the economic situation, causing a recession to descend into the Great Depression.