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Black Monday 1987

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Black Monday 1987

On October 19, 1987, the Dow Jones Industrial Average fell 508 points—22.6 percent in a single trading session. It remains the largest single-day percentage decline in the Dow's history. What made Black Monday different from all previous crashes was its cause: not a banking panic, not an economic shock, but a feedback loop between computer-driven trading strategies that had been designed to protect portfolios and instead amplified the very collapse they were meant to prevent.

Portfolio insurance and its fatal flaw​

Portfolio insurance was a strategy, developed by academic finance professors Hayne Leland and Mark Rubinstein, designed to give institutional investors downside protection without the cost of purchasing options. The strategy involved dynamically adjusting a portfolio's equity exposure as markets moved: selling futures as prices fell (to reduce exposure) and buying as prices rose. In theory, this created a synthetic put option. In practice, it created a selling program that responded to falling prices with more selling.

By October 1987, an estimated $60–90 billion in U.S. equity portfolios were subject to portfolio insurance programs. The strategy worked fine in markets with adequate liquidity—small price moves triggered modest adjustments. But when a sharp initial decline in the week before October 19 triggered large-scale selling by portfolio insurers, the selling drove prices lower, which triggered more selling by other portfolio insurers, which drove prices lower still. The cascade was self-reinforcing and self-accelerating.

A day unlike any other​

Markets opened October 19 with a significant gap down from Friday's close, reflecting weekend selling pressure. The New York Stock Exchange's specialists—the firms responsible for maintaining orderly markets in specific stocks—were overwhelmed. Some simply stopped answering phones. The futures markets in Chicago were declining faster than the underlying stocks, creating arbitrage opportunities that were too large to exploit because neither market had adequate liquidity.

The Dow's 508-point decline happened in a market where the infrastructure for handling such a day simply did not exist. There were no circuit breakers—those would come later, recommended by the Brady Commission. Computer systems were overwhelmed. Communication between the futures markets in Chicago and the equity markets in New York broke down.

The immediate response​

The Federal Reserve, under recently appointed Chairman Alan Greenspan, acted swiftly. On October 20, before markets opened, the Fed issued a brief statement affirming its readiness to serve as a source of liquidity to support the economic and financial system. This commitment—later called the Greenspan Put—helped stabilize markets. The Dow recovered much of its losses over the following months.

The Brady Commission report in January 1988 identified portfolio insurance and program trading as key contributors, and recommended circuit breakers—automatic trading halts triggered by large price moves—to allow markets to regroup during extreme stress.

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