“Samuel Langhorne Clemens (November 30, 1835 – April 21, 1910), known by his pen name Mark Twain, was an American writer, humorist, entrepreneur, publisher, and lecturer. He was lauded as the "greatest humorist the United States has produced,"[ - -Wikipedia
In some stock markets, the Mark Twain effect is the phenomenon of stock returns in October being lower than in other months. The name 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."
The quotation is a sarcastic assertion that speculation in stocks is always dangerous. The fact that Twain specifically picks out October initially is taken as a reference to an "October effect", as exemplified by the 1929, 1987 and 2008 stock market crashes which roughly occurred in October.
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