Black Monday: Portfolio Insurance Algorithms Cascade and Wipe $500 Billion in One Day
What happened
On 19 October 1987, global stock markets crashed simultaneously. The Dow Jones Industrial Average fell 22.6% in a single session — the largest single-day percentage drop in its history — wiping approximately $500 billion from US equities in one day. The primary accelerant was "portfolio insurance," an algorithmic trading strategy that automatically sold futures contracts as markets fell. With hundreds of billions of dollars executing identical instructions simultaneously, automatic selling triggered lower prices which triggered more automatic selling. Human traders stepped back; the machines kept going.[1]
What went wrong
Portfolio insurance was individually rational: it allowed institutional investors to hedge against downturns by systematically selling index futures as a portfolio's value fell, locking in a floor. By 1987, an estimated $60–90 billion in assets were managed using portfolio insurance strategies that were mathematically similar — effectively cloned across many institutions. The strategy assumed that liquid futures markets would absorb the selling. When all the algorithms triggered simultaneously on the morning of 19 October, there were no buyers large enough to absorb the sell side. Lower prices triggered more selling; more selling drove prices lower. Liquidity evaporated faster than any human could respond.[1]
Lesson learned
Trading algorithms that are individually rational become collectively catastrophic when deployed at scale across the same market with the same strategy simultaneously. Black Monday directly created market circuit breakers — mandatory trading pauses that give humans time to intervene before algorithmic feedback loops complete their work. The same failure mode — identical algorithms, simultaneous trigger, insufficient liquidity on the other side — has since recurred in multiple "flash crash" events.
Sources
- [1] Brady Commission Report / SEC Black Monday: Portfolio Insurance Algorithms Cascade and Wipe $500 Billion in One Day