On Monday, Oct. 19, 1987, the Dow Jones Industrial Average plunged by nearly 22%. Black Monday, as the day is now known, marks the biggest single-day decline in stock market history. The remainder of the month wasn’t much better; by the start of November, 1987, most of the major stock market indexes had lost more than 20% of their value.
No single event caused the stock market to crash in 1987. Instead, a series of factors drove the sell-off, including a widening U.S. trade deficit, computerized trading, and tensions in the Middle East. The rise of program trading, which occurs when computers make automated trades, likely played the biggest role in this crash. The computers tended to produce more buy orders when prices were rising and more sell orders when prices fell. As those sell orders flooded the market on Oct. 19, it caused other investors to sell in a panic.
Because the Black Monday crash was caused primarily by programmatic trading rather than an economic problem, the stock market recovered relatively quickly. The Dow started rebounding in November, 1987, and recouped all its losses by September of 1989.