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
Economists and economic historians are almost evenly split as to whether the traditional monetary explanation that monetary forces were the primary cause of the Great Depression is right, or the traditional Keynesian explanation that a fall in autonomous spending, particularly investment, is the primary explanation for the onset of the Great Depression. Today the controversy is of lesser importance since there is mainstream support for the debt deflation theory and the expectations hypothesis that building on the monetary explanation of Milton Friedman and Anna Schwartz add non-monetary explanations.