“The market can stay irrational longer than you can stay solvent” (or, “The market can remain irrational longer than you can remain solvent”) is a statement often attributed to economist John Maynard Keynes (1883-1946). There is no record that Keynes ever said it or wrote it.
 
The earliest citations for this phrase appear in the early 1990s.
 
 
Wikiquote; John Maynard Keynes
John Maynard Keynes, 1st Baron Keynes of Tilton (5 June 1883 - 21 April 1946) was a British economist whose ideas, known as Keynesian economics, had a major impact on modern economic and political theory and on many governments’ fiscal policies.
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. The market can stay irrational longer than you can stay solvent.
As quoted in The Politics of Public Fund Investing: How to Modify Wall Street to Fit Main Street (2006) by Ben Finkelstein. 
   
Google Books
1992 (month?), Business Week, pg. 100:
That’s essentially what economist John Maynard Keynes meant when he said, “The market can stay irrational longer than you can stay solvent.”
   
Google Books
January (?) 1993, Forbes magazine, pg. 236:
Above all, in 1993 remember this: Markets can remain irrational a lot longer than you and I can remain solvent.
 
January 1994, Futures, ‘Economics bottom line for Thematic” by Ginger Szala:
“And I believe fervently that markets can remain irrational a lot longer than you remain solvent.”
     
Google Groups: misc.invest.futures
Newsgroups: misc.invest.futures
From: Stewart Taylor

Date: 1995/09/17
Subject: Re: Ewing’s ideas on Corn
 
An old trading saying…. that, by the way is very true… “The market can stay irrational a lot longer than you can stay solvent!”.... Thinking about a trade in terms of what the world can and can’t afford can be a dangerous habit. 
   
investorville
diligence
posted 01-04-1999 06:55 PM
Short seller’s quote of the month: “The market can remain irrational longer than you can remain solvent.” - John Maynard Keynes.
 
New York (NY) Times
MUTUAL FUNDS REPORT; In the Stampede to Big Stocks, Some Managers Are Left Behind
By CAROLE GOULD
Published: April 4, 1999
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He remains a firm believer in asset allocation, although he said that during the quarter, a sobering market adage from John Maynard Keynes came to mind: Markets can remain irrational longer than you can remain solvent.
   
TheStreet.com
Statement Shock: 10 Questions With Tech Skeptic Robert Sanborn
10/11/01 - 07:46 AM EDT
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I remember the famous quote that the market can remain irrational longer than you can remain solvent, so the key thing is to be able to be proven right.
   
New York (NY) Times
Now You, Too, Can Enter the World of 007 Finance
By JAMES PETHOKOUKIS
Published: January 28, 2007
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He urges caution in using aggressive trading strategies. Using an aphorism sometimes attributed to John Maynard Keynes, Mr. Dillon said, “The market can stay irrational longer than you can stay solvent.”