A stock market crash is a sudden, dramatic drop in stock prices driven by panic selling and underlying economic factors, often following speculation and economic bubbles. These crashes are social phenomena where external events combine with crowd psychology, creating a feedback loop of selling, typically occurring after prolonged bull markets, excessive optimism, high price-earnings ratios, and heavy use of margin debt and leverage. While there is no strict numerical definition, a decline of over 10% in a market index over several days is commonly considered a crash, distinguishing them from longer-term bear markets—though they don't always coincide, as seen with Black Monday (1987) which didn't lead to a bear market, and the Japanese asset bubble which burst over years without a single crash. Historical examples include Tulip Mania (1634–1637), the first recorded economic bubble, and the Panic of 1907, which led to the creation of the Federal Reserve in 1913. The Wall Street Crash of 1929 saw the Dow Jones Industrial Average rise from 63.9 in 1921 to 381.2 by September 1929, only to crash and not regain that level for 25 years, illustrating how crashes are generally unexpected, as historian Niall Ferguson noted.