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Robert Shiller asserts that investors already had a "crash mentality" before October 19, associated with views about borrowing, government debt, and the perception that portfolio insurance was affecting markets. The proximate cause of the crash, he believes, was a response to price declines: the assumption by investors that the crash had arrived. In Shiller's opinion, the crash was as much a sociological or psychological phenomenon as an economic one.
Robert J. Shiller (Fri,) studied this question.