On 19 October 1987 — Black Monday — the Dow Jones Industrial Average fell 22.6% in a single trading session. It remains the largest single-day percentage fall in the index's history. The UK's FTSE 100 fell 26% over two days. Markets around the world fell 20-45% within days. The crash had no single identifiable cause: a confluence of overvalued markets, rising interest rates, trade deficit concerns, and — critically — a technological mechanism that made the crash worse than it needed to be.
Portfolio insurance and the doom loop
Portfolio insurance was a popular institutional strategy in the mid-1980s. The theory was elegant: hedge a portfolio of equities against falls by dynamically selling stock index futures as the market falls. The more the market falls, the more futures you sell. In theory, this creates a synthetic put option, protecting the portfolio.
The problem: when many large institutional investors are all running the same strategy, their simultaneous selling creates its own doom loop. As the market falls, portfolio insurance triggers selling. The selling drives the market lower. Lower prices trigger more selling. Liquidity evaporates. The futures market falls faster than the underlying equity market, creating unprecedented basis between futures and spot prices.
On Black Monday, the systems were overwhelmed. Exchanges could not process orders fast enough. Some market makers withdrew from the market entirely. The financial system nearly broke down.
The regulatory and market response
The 1987 crash directly prompted several structural changes to financial markets:
*Circuit breakers*: NYSE and other exchanges introduced circuit breakers — automatic trading halts triggered by large market falls — to pause markets and allow orderly processing of orders. Today, circuit breakers are standard on virtually every major exchange globally.
*Margin reform*: The Brady Commission investigated the crash and recommended coordinated margin requirements across futures and equity markets. The crash exposed that margin requirements in futures markets were far lower than in equity markets, creating dangerous leverage differentials.
*Intermarket coordination*: The crash revealed how equity, futures, and options markets interacted in stressed conditions. The Federal Reserve, under Alan Greenspan — in only his second month as Chair — issued a statement guaranteeing liquidity support for the financial system. This early intervention is credited with preventing a deeper recession.
*Risk management frameworks*: Portfolio insurance as a market-wide strategy was discredited. The crash drove banks and asset managers to develop far more sophisticated risk management frameworks, including scenario analysis and stress testing — asking 'what happens to our portfolio if markets fall 20% today?'
*Options pricing*: The crash permanently changed implied volatility surfaces. Before 1987, the Black-Scholes model was used with a roughly flat volatility surface. After the crash, a persistent 'volatility smile' and 'skew' emerged — out-of-the-money puts became permanently more expensive, reflecting market participants' willingness to pay for tail risk protection.