The Great Depression wasn't caused by one single event; however, the 1929 stock market crash certainly started off the downward spiral. Though other factors contributed to the GDP decline, that one event (the stock market crash) began the overall erosion of confidence that individuals had in the U.S. economic system.
Compare that confidence erosion during the fall of 1929 with what happened to our recent economy in 2007-2008. Our economic downturn was also given a name --- the "worst recession since the Great Depression"; thus, contributing to an even greater confidence deflator.
