Crash-Testing the Efficient Market Hypothesis

Kenneth French believes that neither investor panic nor a breakdown of market mechanisms on October 19 drove prices to an irrationally low level. Rather, the crash may have been the response of an efficient market to news about expected future cash flows or returns. French theorizes that prices were irrationally high before the crash, that investors were unaware of this, and that events on or about October 19 brought prices back to rational levels. His conclusion is that no new regulation of the stock market is necessary.