Black Monday 1987

On Monday 19 October 1987 the S&P 500 fell roughly 20% in a single trading session — the largest one-day percentage decline in its history. The plunge was global: equity markets from Hong Kong to London to New York collapsed in cascade. There was no single news catalyst; portfolio-insurance programs that mechanically sold futures as prices fell, combined with thinly-staffed market-makers and primitive electronic order routing, are widely cited as accelerants. Despite the violence of the move, the economy did not enter recession and the S&P 500 finished the calendar year roughly flat in price terms.

• Cash and short-duration Treasury bills, which simply did not participate in the equity drawdown.
• Long-duration Treasury bonds rallied during the panic as investors moved into safe-haven duration.
• Staying invested: by the end of 1987 the S&P 500 was approximately flat for the year, and it recovered its pre-crash high within roughly two years.

• Portfolio insurance — the strategy of dynamically hedging equity exposure with short futures — pro-cyclically amplified the sell-off.
• Stop-loss orders executed at gap-down prices well below their trigger levels because liquidity vanished.
• Margin-financed equity positions faced forced liquidation at the worst prices of the session.

• Mechanical, correlated selling strategies can become the market — when too many participants run the same playbook, liquidity evaporates exactly when it is needed.
• A 20% one-day shock is not necessarily a recession. The 1987 crash was largely a microstructure event; macro fundamentals did not justify the move and prices recovered.
• Liquidity at the moment of stress is qualitatively different from liquidity in normal markets — bid-ask spreads, slippage and partial fills make pre-trade risk estimates optimistic.