Black Monday 1987
Black Monday (19 October 1987) was a one-day ~22% crash in the DJIA, amplified by portfolio insurance — a mechanical selling program that turned a decline into a gap.
Definition
Portfolio insurance tried to synthesize puts by selling futures as the market fell. The same signal hit many books at once; futures and cash disconnected; liquidity vanished. It was not a banking solvency crisis. The Brady report named the feedback.
Why it matters
It is the cleanest modern example of identical hedges as a crowded mechanical seller. 2018 vol targeting and some 2020 CTAs are cousins.
Case
S&P futures cheap to cash, NYSE order-book withdrawal, and a Fed statement the next day that liquidity would be supplied. Circuit breakers were the later rule.
How to read it
Ask what the mechanical rule is and how much AUM shares it. If the rule sells into weakness, you have 1987 in the limit.
Key papers
Landmark references with real publisher or arXiv links — not token-overlap noise.
- Report of the Presidential Task Force on Market MechanismsBrady Commission · 1988 · US GovernmentThe official 1987 post-mortem.