The theory that stocks tend to decline during the month of October. The October effect is considered mainly to be a psychological expectation rather than an actual phenomenon. Most statistics go against the theory.

Some investors may be nervous during October because the dates of some large historical market crashes occurred during this month. Black Monday, Tuesday and Thursday all occurred in October 1929, after which came the Great Depression. In addition, the great crash of 1987 occurred on October 19, and saw the Dow plummet 22.6% in a single day.