What automated trading program helped amplify selling during the 1987 Black Monday crash?

Portfolio insurance helped amplify selling during the 1987 Black Monday crash.

Portfolio insurance was a computerized risk-management strategy designed to limit losses in an investment portfolio. As markets declined, the strategy directed managers or computer systems to sell stock-index futures and other assets. The intended protection could therefore create more selling during a fast fall.

The program was not the only cause of Black Monday. Market valuations, interest-rate concerns, trade tensions, and investor anxiety were already placing pressure on prices. However, the interaction of automated strategies, futures trading, and cash markets made the decline move unusually quickly.

The crash helped reshape market regulation and trading infrastructure. In the United States, exchanges later introduced circuit breakers that temporarily halt trading after especially large price movements. The event is a key example of how a strategy intended to reduce individual portfolio risk can contribute to broader market instability when many participants use similar rules at the same time.