In finance, Black Monday refers to Monday, October 19, 1987, when stock markets around the world crashed, shedding a huge value in a very short time. The crash began in Hong Kong and spread west to Europe, hitting the United States after other markets had already declined by a significant margin. The Dow Jones Industrial Average (DJIA) fell exactly 508 points to 1,738.74 (22.61%). In Australia and New Zealand, the 1987 crash is also referred to as "Black Tuesday" because of the time zone difference.

A popular explanation for the 1987 crash was selling by program traders, most notably as a reaction to the computerized selling required by portfolio insurance hedges. In program trading, computers perform rapid stock executions based on external inputs, such as the price of related securities. Common strategies implemented by program trading involve an attempt to engage in arbitrage and portfolio insurance strategies. After the crash, many blamed program trading strategies for blindly selling stocks as markets fell, exacerbating the decline.

Some economists theorized the speculative boom leading up to October was caused by program trading, and that the crash was merely a return to normalcy. Either way, program trading ended up taking the majority of the blame in the public eye for the 1987 stock market crash. U.S. Congressman Edward J. Markey, who had been warning about the possibility of a crash, stated that "Program trading was the principal cause."

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