Which U.S. stock index fell 20.47% during the Crash of 1987?

The S&P 500 fell 20.47% during the Crash of 1987. That decline occurred on October 19, 1987, the day widely known as Black Monday.

The S&P 500 tracks a broad group of large U.S. companies and is different from the narrower Dow Jones Industrial Average. Although the Dow’s 22.6% fall is the figure most often quoted for Black Monday, the S&P 500 also suffered an extraordinary one-day percentage loss.

Analysts have linked the crash to elevated valuations, interest-rate concerns, trade tensions, and automated trading strategies. Portfolio insurance programs were designed to limit losses but could generate additional selling as prices dropped, contributing to the speed of the decline.

The index eventually recovered, and the crash did not produce a depression comparable to the 1930s. Regulators responded with trading halts and other safeguards intended to give markets time to absorb extreme order imbalances.