Final answer:
The stock market crash in 1929 did not contribute to American optimism in the 1920s and instead led to the Great Depression.
Step-by-step explanation:
The factor that did not contribute to American optimism in the 1920s was the stock market crash in 1929. While the 1920s were a decade of prosperity and the economic growth, the crash in 1929 led to the Great Depression and a period of widespread financial ruin. Only about 10 percent of American households held stock investments and speculated in the market, but nearly a third of Americans lost their savings and jobs in the ensuing depression.