Black Monday: The Day the Market Fell 22% in One Session
The Dow Jones Industrial Average fell 22.6% in a single trading day — still the largest one-day percentage decline in its history — with no single piece of news to explain it.
- One-day decline
- −22.6%
- Index
- Dow Jones Industrial Average
- Time to recover
- About 2 years
What happened
On October 19, 1987, the Dow Jones Industrial Average fell 508 points — 22.6% of its value — in a single session. Markets around the world followed within hours, with Hong Kong, Australia, the UK and other major exchanges posting double-digit losses of their own.
There was no single headline behind it. Investigators later pointed to a combination of overvaluation after a strong multi-year rally, rising interest rates, and — critically — a new practice called portfolio insurance, where large institutions used computer-driven programs to automatically sell stock index futures as prices fell, intended to protect their portfolios.
The problem was that thousands of institutions were running similar programs at the same time. As prices dropped, the programs sold, which pushed prices down further, which triggered more automated selling. The mechanism designed to protect individual portfolios amplified the crash for the market as a whole.
Why it still matters
Black Monday is the reason exchanges now have circuit breakers — automatic trading halts triggered when an index falls a set percentage in a short window, giving the market a pause to absorb information instead of free-falling on automated momentum.
The deeper lesson is about correlated, automated behavior. A strategy that looks safe when you are the only one using it can become dangerous when thousands of participants run the same logic simultaneously — the market has no mechanism to know that everyone is trying to exit through the same door at once.