For the world's bankers, it is not April that is the cruellest month, but September. It is when most financial crises hit and the world's stock markets have their biggest tumbles. During the 19th and 20th century it was thought that the pattern was caused by the crop seasons. But even as agriculture has declined dramatically in significance, the seasonal spike continues. Perhaps it has to do with the end of the vacation season: all those well-heeled bankers, returning to work after the long hazy days of summer, are jolted back into reality.