“If it were a real $20 bill, someone would have picked it off the sidewalk already” (economics joke)

The efficient market hypothesis (EMH) assumes such efficiency that it has led to a popular economics joke. Two economists are walking down the street and one of them notices what appears to be a $20 bill (or a $100 bill—the monetary amounts vary) on the sidewalk. “It’s not a real $20 bill,” the other economist declares. “If it were a real $20 bill, someone would have picked it up off the sidewalk already.”
 
The joke has beencited in print since at least 1984, when it was printed in The Economist magazine.
 
   
Wikipedia: Efficient-market hypothesis
In finance, the efficient-market hypothesis (EMH) asserts that financial markets are “informationally efficient”. In consequence of this, one cannot consistently achieve returns in excess of average market returns on a risk-adjusted basis, given the information available at the time the investment is made.
 
Google Books
The Economist
Volume 293
1984
Pg. 172, col. 4:
This gives rise to a rational-expectations joke about an economics professor walking with a keen-eyed student across the university quad. ‘Look,’ says the student, pointing at the ground, ‘a five-pound note.’ ‘It can’t be, replies the rational professor. ‘If it were there, somebody would have picked it up by now.’
 
Google Books
The Economist Economics
By Rupert Pennant-Rea and Clive Crook
New York, NY: Penguin Books
1986
Pg. 56:
This gives rise to a rational-expectations joke about an economics professor walking with a keen-eyed student across the university quad. ‘Look,’ says the student, pointing at the ground, ‘a five-pound note.’ ‘It can;t be, replies the rational professor. ‘If it were there, somebody would have picked it up by now.’
 
Google Books
The State of Economic Science:
Views of Six Nobel Laureates
By Werner Sichel
Kalamazoo, MI: W.E. Upjohn Institute for Employment Research
1989
Pg. 18:
No doubt there is a slightly absurd aspect in assinging complete rationality and complete foresight within the limits of ineliminable uncertainty, a point illustrated by a parable widely repeated among economists: Two economists are walking down the street. One says, “Look, there’s a $20 bill lying on the street,” to which the other replies, “There can’t be; if there were, someone else would have already picked it up.”
   
Google Books
Extra Dividends Supplement to accompany Fundamentals of Investing (Fourth Edition)
By David J. McLaughlin, Lawrence J. Gitman and Michael D. Joehnk
New York, NY: Harper & Row
1990
Pg. 42
This logic is similar to the story of the finance professor and his impressionable young student. While walking down a busy corridor in the business school, the student pointed ahead and excitedly exclaimed, “Isn’t that a $10 bill on the floor?” “No,” smile the professor, “it can’t be. If it were, someone would have picked it up already!”
 
Google Books
Financial Assets, Markets, and Institutions
By Gary Smith
Lexington, MA: D.C. Heath
1993
Pg. 206:
The Mortgage Market Lawyer: Look, there’s a $100 bill on the sidewalk.
Economist: Don’t bother. It must be counterfeit, or someone would have picked it up by now.
 
Google Books
Modern Competitive Analysis
By Sharon M. Oster
New York, NY: Oxford University Press
1994
Pg. 20:
And, indeed, the spirit behind the principle of efficiency as well as its limitations are quite well reflected in the joke: Two economists, one an eminent efficient-market type and the second a bright young assistant professor, are walking down the street. Suddenly the young economist bends down and reaches, trying to pick up something from the sidewalk. “What are you doing?” asks the older professor. “There’s a $20 bill on the ground,” replies the young assistant. “Nonsense,” retorts the older man. “If there were really a $20 bill on the ground, someone else would have already picked it up.”
 
Googe Books
The Economics of Financial Markets
By Hendrik S. Houthakker and Peter J. Williamson
New York, NY: Oxford University Press
1996
Pg. 317:
A story told by James Duesenberry makes the point. Two economists are walking together. One says, “I see a quarter on the sidewalk.” The other, a beiever in the EMH, replies “Impossible. If it really were a quarter, someone would have picked it up already.”
 
Google Books
Essentials of Investments
By Zvi Bodie, Alex Kane and Alan J. Marcus
Boston, MA: Irwin/McGraw Hill
1998
Pg. 253:
There is a telling joke about two economists walkingdown the street. They spot a $20 bill on the sidewalk. One starts to pick it up, but the other one says, “Don’t bother; if the bill were real someone would have picked it up already.”
 
The lesson here is clear. An overly doctrinaire belief in efficient markets can paralyze the investor and make it appear that no research effort can be justified.
 
Google Books
Creative Capitalism:
A Conversation with Bill Gates, Warren Buffett, and Other Economic Leaders
Edited by Michael Kinsley
New York, NY: Simon & Schuster
2008
Pg. 24:
It’s sort of like the famous joke about the economist who sees the ten-dollar bill on the ground and says it can’t be there, someone would have picked it up.
   
Youram Bauman, Ph.D., The world’s first and only Stand-Up Economist
You might be an economist if…
July 25, 2009
(...)
…you see a dollar bill on the street and don’t pick it up, because if it were a real dollar bill, someone else would have already picked it up. (Moshe Kranc, with many historical antecedents)
 
Harvard Business Review
IT and the Age of $100 Bills
by Andrew McAfee |  9:00 AM January 22, 2010
(...)
During the session I was reminded of an old economics joke. In his book on stock market behavior, Andrew Lo tells it this way: “…an economist [is] strolling down the street with a companion when they come upon a $100 bill lying on the ground. As the companion reaches down to pick it up, the economist says ‘Don’t bother — if it were a real $100 bill, someone would have already picked it up.’”
 
The joke pokes fun at economists not because they’re notoriously cheap, but because many of them believe the efficient markets hypothesis or EMH — the idea that stock prices reflect all publicly available information about a company. If the EMH is in fact accurate, then public information that you can use to beat the market doesn’t really exist; it’s like the $100 bill in the joke.
 
Economists Do It With Models
An Economist Finds A $10 Bill, And A Lesson On Efficient Markets…
January 1st, 2012
One of the first things I did in 2012 was find a $10 bill on the sidewalk in Harvard Square. I decided that this was a good sign for the new year, so I tweeted about it shortly after midnight. In retrospect, I should have known better, since I pretty quickly got the following responses:
(...)
Sigh. Yes yes, there’s an economist joke for just about everything. This one goes something like the following: An economist and a normal person are walking down the street together. The normal person says “Hey, look, there’s a $20 bill on the sidewalk!” The economist replies by saying “That’s impossible- if it were really a $20 bill, it would have been picked up by now.”