In some stock markets, the October Effect also referred to as the Mark Twain effect is the phenomenon of stock returns in October being lower than in other months.[1] The reference to Mark Twain comes from a line in Mark Twain's Pudd'nhead Wilson: "October. This is one of the peculiarly dangerous months to speculate in stocks. The others are July, January, September, April, November, May, March, June, December, August, and February."[2] [3]
The quotation is a sarcastic assertion that speculation in stocks is always dangerous. Twain wrote Pudd'nhead Wilson in 1894 many years before the stock market crashes of 1929, 1987 and 2008 which roughly occurred in October.