On October 19, 1987, the Dow Jones Industrial Average fell 508 points in a single session, a 22.6 percent drop nearly double the worst day of 1929; within two years, the market had erased the loss entirely.

By the closing bell on October 16, 1987, investors had already watched the Dow Jones Industrial Average fall for five straight trading days, its worst week since 1940. What nobody at the New York Stock Exchange expected was for it to get very much worse, very much faster, in a single session three days later.

What actually happened on October 19, 1987?

The Dow opened lower and never recovered. By 4 p.m., the index had lost 508.32 points, a 22.61 percent decline: the worst single-day percentage drop in its history, before or since. More than $500 billion in stock value disappeared from the New York Stock Exchange in one session, a loss the exchange had not suffered in a single day since the outbreak of World War I in 1914.

Wall Street had a name for the day within hours.

Traders and reporters started calling it Black Monday before the market even closed, borrowing the label from October 28, 1929, when the Dow fell 12.8 percent in the worst single day of that earlier crash. Monday's decline nearly doubled it.

The trading floor could not keep pace with itself. A record 604.33 million shares changed hands, roughly three times an ordinary session's volume for 1987, and the New York Stock Exchange's ticker tape (the running strip that reported executed trades to the rest of the world) fell more than two hours behind the actual selling. Investors who called their brokers that afternoon were often quoted prices that no longer existed.

What caused Black Monday?

Economists have spent nearly four decades arguing about that question, and they still do not fully agree.

The leading suspect is portfolio insurance, a hedging strategy a large share of institutional money had adopted over the preceding several years. The idea was simple: as stock prices fell, a computer-driven formula would automatically sell stock-index futures to offset the loss, in theory protecting a fund's portfolio without anyone having to sell the underlying stocks. In practice, when many funds ran the same formula at once on a day prices were already falling, the automatic selling pushed futures prices down, which pushed index arbitrageurs to sell the actual stocks those futures tracked, which pushed prices down further and triggered still more automatic selling.

It was a feedback loop with no one at the wheel.

A federal task force led by former Treasury Secretary Nicholas Brady studied the crash for the White House and concluded, in early 1988, that portfolio insurance and related program trading had substantially worsened the decline. Later research has been less certain: some economists point instead to a broader loss of confidence after a run of bad economic news that week, including a wider-than-expected US trade deficit and rising interest rates, arguing the automated selling was more a symptom of the sell-off than its cause. Nobody disputes that the strategies existed and sold heavily that day; how much of the crash they actually caused remains a genuinely open question.

How did the crash spread around the world?

Black Monday didn't start in New York, and by the time American markets opened, the rest of the world had already been selling for most of a day.

Hong Kong's exchange fell so hard that officials there closed it for the rest of the week rather than let the selling continue; when it reopened, prices fell again. London's FTSE 100 dropped nearly 11 percent that same Monday and another 12 percent on Tuesday, a decline compounded by a windstorm that had already shut British markets the previous Friday. By the time trading opened on Wall Street that morning, sell orders were already arriving from Tokyo, Hong Kong, London, and Frankfurt.

By the end of October, almost nowhere had escaped.

Hong Kong finished the month down 45.8 percent, Australia 41.8 percent, Spain 31 percent, and Britain 26.4 percent; the US, at 22.68 percent, was far from the worst hit. New Zealand's market fell about 60 percent from its 1987 peak and took years to recover.

How did Washington respond?

Alan Greenspan had been chairman of the Federal Reserve for barely two months, sworn in that August after Paul Volcker's eight-year term. Before trading opened on Tuesday, October 20, the Fed released a single sentence: it stood ready, the statement said, "to serve as a source of liquidity to support the economic and financial system."

It was a deliberately narrow promise, and it worked. The Fed pumped reserves into the banking system through its open-market desk and leaned on major banks privately to keep extending credit to securities firms whose own cash positions the day's trading had shredded, a move that helped keep a stock-market crash from turning into a banking crisis.

The Dow, for its part, recovered fast.

The index clawed back roughly 60 percent of Monday's loss within two trading sessions and finished 1987 with a small gain for the year. It reached a new all-time high by the late summer of 1989, about two years after the crash, and the recession many analysts had warned about never arrived.

What Black Monday changed

Exchanges did not wait for the next crash to make changes. Within a few years, the New York Stock Exchange adopted circuit breakers, automatic trading halts triggered once the market falls by a set percentage in a single day, meant to give investors time to think rather than sell on reflex. Those breakers have been recalibrated several times since (most recently in March 2020, when they tripped four separate times in two weeks) and remain the market's basic defense against a repeat of 1987's runaway feedback loop.

Automated trading itself never went away; it grew. High-frequency and algorithmic strategies now account for a majority of daily US equity trading volume, executing orders in fractions of a second rather than the minutes portfolio insurance needed. The mechanism that amplified Black Monday was primitive by comparison to what runs markets today, and regulators have spent the years since trying to build safeguards against exactly the kind of self-reinforcing sell-off that program trading proved was possible.

What Black Monday left behind was not a recession or a wave of bank failures, the way its 1929 predecessor had. It left a standing question that circuit breakers only partly answer: what happens the next time thousands of automated systems decide, within the same few minutes, to sell.

Sources

• Wikipedia, "Black Monday (1987)"
• Britannica, "Black Monday (1987)"
• Federal Reserve History, "Stock Market Crash of 1987"
• Goldman Sachs, "Global Financial Markets Crash on Black Monday"