The Dow Jones Industrial Average fell 12.5 per cent on October 29, 1929. Many commentators at the time and later suggested that stocks had simply become overvalued; the subsequent crash a return to normalcy after an overdone equity rally.

More realistically, investors were pricing in a very negative turn in the economic landscape that would enervate economies worldwide. The change had to do with trade policy, and specifically the looming passage of the Smoot-Hawley tariff that sparked economic isolationism around the world.