The 1987 Crash was a worldwide phenomenon. The FTSE 100 Index lost 10.8% on that Monday and a further 12.2% the following day. In the month of October, all major world markets declined substantially. The least affected was Austria (a fall of 11.4%) while the most affected was Hong Kong with a drop of 45.8%. Out of 23 major industrial countries, 19 had a decline greater than 20%.
From October 6–10 the Dow Jones Industrial Average (DJIA) closed lower in all five sessions. Volume levels were record-breaking. The DJIA fell over 1,874 points, or 18%, in its worst weekly decline ever on both a points and percentage basis. The S&P 500 fell more than 20%. The week also set 3 top ten NYSE Group Volume Records with October 8 at #5, October 9 at #10, and October 10 at #1.
Thirty-two percent of Americans who were invested in the stock market during at least one of the last five financial downturns pulled some or all of their money out of the market. That’s according to a NerdWallet-commissioned survey, which was conducted online by The Harris Poll of more than 2,000 U.S. adults, among whom over 700 were invested in the stock market during at least one of the past five financial downturns, in June 2018. The survey also found that 28% of Americans would not keep their money in the stock market if there were a crash today.
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The crash followed an asset bubble. Since 1922, the stock market had gone up by almost 20 percent a year. Everyone invested, thanks to a financial invention called buying "on margin." It allowed people to borrow money from their broker to buy stocks. They only needed to put down 10-20 percent. Investing this way contributed to the irrational exuberance of the Roaring Twenties.
Research at the Massachusetts Institute of Technology suggests that there is evidence the frequency of stock market crashes follows an inverse cubic power law. This and other studies such as Prof. Didier Sornette's work suggest that stock market crashes are a sign of self-organized criticality in financial markets. In 1963, Mandelbrot proposed that instead of following a strict random walk, stock price variations executed a Lévy flight. A Lévy flight is a random walk that is occasionally disrupted by large movements. In 1995, Rosario Mantegna and Gene Stanley analyzed a million records of the S&P 500 market index, calculating the returns over a five-year period. Researchers continue to study this theory, particularly using computer simulation of crowd behaviour, and the applicability of models to reproduce crash-like phenomena.