Which 1987 market-crash mechanism automatically sold futures or stocks to limit portfolio losses?

Portfolio insurance was the 1987 market-crash mechanism that automatically sold futures or stocks to limit portfolio losses.

Portfolio insurance used computer-based rules to reduce equity exposure as markets fell. A common approach involved selling stock-index futures when prices declined, with the aim of protecting a portfolio against further losses. The strategy was designed for gradual market movements, but many investors following similar rules could sell at the same time during a rapid fall.

On October 19, 1987, heavy selling in index futures and underlying stocks interacted with already fragile investor confidence. The feedback loop increased downward pressure and contributed to the extraordinary one-day decline. Researchers and regulators have debated how much of the crash portfolio insurance caused versus amplified.

Portfolio insurance did not mean that every investor held an identical product, and it was not the only explanation for Black Monday. Other factors included valuation concerns, interest rates, trade disputes, and market-structure weaknesses. The event encouraged reforms such as circuit breakers and coordinated exchange procedures.