Which technology accelerated automated selling during the 1987 Black Monday stock-market crash?

The technology and strategy that accelerated automated selling during the 1987 Black Monday stock-market crash was portfolio insurance.

Portfolio insurance used computer models to determine when to sell stock-index futures as markets declined. Its designers intended the strategy to limit losses by reducing equity exposure, but many similar programs could generate sell orders at the same time.

On 19 October 1987, selling pressure moved rapidly through futures and cash markets. The Dow Jones Industrial Average lost 22.6 percent, the largest one-day percentage decline in its history. The crash also exposed weaknesses in market coordination and settlement procedures.

Researchers continue to debate how much portfolio insurance contributed compared with other factors, including high valuations, investor psychology, liquidity problems, and international market links. It was therefore a major mechanism examined after the crash, not a single universally accepted cause.