The 1987 Black Monday Crash

How the largest single-day percentage drop in stock market history unfolded on October 19, 1987, and the safeguards it inspired.

On October 19, 1987, a date now known as Black Monday, the Dow Jones Industrial Average fell by roughly 22.6 percent in a single trading day - still, decades later, the largest one-day percentage drop in that index’s history. It happened with no obvious single trigger, no major company collapse, and no clear news event that morning that explained a fall of that size, which is part of what has made it such a widely studied stock market crash.

What actually happened that day

Stock markets around the world had already been sliding somewhat in the days before October 19, and worries about the US trade deficit and rising interest rates had been building. But nothing in the news that Monday morning obviously justified a drop of nearly a quarter of the market’s value within hours. Trading volume was enormous, and in some cases exchanges struggled simply to process the number of orders coming in, which made the actual chaos worse: investors placing sell orders sometimes didn’t know at what price their trade would eventually execute.

Program trading and portfolio insurance

Much of the debate over what caused the crash to be so severe centers on program trading: the use of computer systems to automatically execute large batches of trades based on preset rules, without a human deciding on each individual trade in real time. A related strategy called portfolio insurance used computer-driven selling to try to limit losses as prices fell. The trouble was that many large institutional investors were using similar strategies at the same time, so as prices dropped, computer systems across many firms triggered more automatic selling, which pushed prices down further, which triggered still more automatic selling.

Why a strategy that protects one investor can hurt everyone together

Imagine one investor sets up a rule: "if prices fall by 5 percent, automatically sell to limit my losses." On its own, this seems sensible. But imagine thousands of large investors have programmed nearly identical rules into their own trading systems. Once prices dip slightly, all those systems can start selling at roughly the same time, and that combined selling pressure itself pushes prices down further - triggering the next round of automatic sales. A strategy that looks protective for one investor can end up amplifying the very crash everyone was trying to protect themselves from.

Was the market simply overvalued?

Assuming economists agree on a single cause

Some economists emphasize program trading and portfolio insurance as the main accelerants. Others argue stock valuations had simply climbed too high relative to underlying company earnings in the months before, and a correction of some kind was likely regardless. Both explanations have supporting evidence, and more than three decades later, economists still don't fully agree on how much weight each factor deserves - a genuinely open question rather than a settled one.

The aftermath: circuit breakers

One of the crash’s clearest legacies is the introduction of circuit breakers: rules that automatically pause trading on an exchange for a set period if prices fall by a large enough amount in a short time. The idea is straightforward - give human judgment, and the flow of accurate information, a chance to catch up before automated selling can spiral further, rather than letting a rapid free-fall run entirely unchecked. Circuit breakers have since been triggered during other periods of market stress, including parts of the 2020 COVID economic shock covered elsewhere in this module, and are now a standard feature of major stock exchanges worldwide.

Key takeaways

Black Monday, October 19, 1987, remains the largest one-day percentage drop in Dow Jones history.
No single clear news event explained the size of the crash that day.
Program trading and portfolio insurance strategies likely amplified the selling as many firms reacted similarly at once.
Economists still debate how much overvaluation versus automated trading contributed to the crash's severity.
The crash led directly to circuit breaker rules that pause trading during extreme price swings.