One episode, several accounts

The most studied episode in economic history, and the place where a curriculum either teaches honestly or becomes partisan.

Seven lessons. The first is the record in published figures, with the publisher and the series named each time, and with the measurement problems stated rather than hidden. Everything after it is an argument about why.

Then four accounts, each given in the words of the people who made it. The Austrian account, which locates the cause in the credit expansion of the 1920s. The monetarist account of Friedman and Schwartz, which holds that a severe recession was turned into a depression by a collapse in the stock of money the Federal Reserve had the power to prevent. The account in Keynes's General Theory of 1936, which runs through a collapse in the marginal efficiency of capital and the limits of what a lower interest rate can do about it. And Bernanke's 1983 addition, which asks what the destruction of banking relationships cost quite apart from the money supply.

The sixth lesson is the one people skip. The four accounts agree about a great deal more than the argument between their supporters suggests, and setting out the agreed facts first is what makes the remaining disagreement legible.

The last asks what evidence would favour one account over another and whether that evidence can exist. It does not declare a winner, and a reader who wants one should notice that the economists themselves did not produce one.

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