“Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it” is often credited to American author Mark Twain, the pen name of Samuel Langhorne Clemons (1835-1910). However, there is no evidence that Twain ever said it.
 
Canadian educator Laurence J. Peter (1919-1990) wrote in The Peter Principle (1969):
 
“During one of my lectures a Latin-American student, Caesare Innocente, said, ‘Professor Peter, I’m afraid that what I want to know is not answered by all my studying. I don’t know if the world is run by smart men who are, how you Americans say, putting us on, or by imbeciles who really mean it.’”
 
The quotation has been falsely attributed to Twain since at least 1990.
   
   
Google Books
The Peter Principle
By Laurence J. Peter and Raymond Hull
New York, NY: William Morrow
1969
Pg. 55:
During one of my lectures a Latin-American student, Caesare Innocente, said, “Professor Peter, I’m afraid that what I want to know is not answered by all my studying. I don’t know if the world is run by smart men who are, how you Americans say, putting us on, or by imbeciles who really mean it.”
   
2 April 1969, Seattle (WA) Times, “The Peter Principle: The World: Put On or Imbecility?” by Dr. Laurence J. Peter and Raymond Hull, pg. 18, col. 2:
During one of my lectures a Latin-American student, Caesare Innocente, said, “Professor Peter, I’m afraid that what I want to know is not answered by all my studying. I don’t know if the world is run by smart men who are, how you Americans say, putting us on, or by imbeciles who really mean it.”
   
Google Books
Trend Tracking:
The System to Profit From Today’s Trends
By Gerald Celente and Tom Milton
New York, NY: Wiley
1990
Pg. 22:
“Sometimes I wonder,” said Mark Twain, “if the world is being run by smart people who are putting us on or imbeciles who really mean it.”
 
4 May 1998, The Titusville Herald (Titusville, PA), “Letter to the Editor,” pg. 4, col. 2:
To the editor:
The latest fiasco in Harrisburg has prompted me to pose the following question: Are we being governed by smart people who are putting us on or by imbeciles who really mean it?
(...)
R. L. “Les” Currens,
Titusville
     
Google Groups: 3do.bad-attitude
G’bye
Oneson
7/30/00
(...)
Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it.
—Mark Twain
 
Google News Archive
30 January 2007, Lawrence (KS) Journal-World, “Public Forum,” pg. 6B, col. 5:
Mark Twain once said, “Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it.” Either case makes me sick. If only I could afford a doctor.
David Reber,
Lawrence
 
Real Clear Politics
Putting Words in Mark Twain’s Mouth
By Carl M. Cannon - December 10, 2012
(...)
22. “Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it.” [Not Twain; source unknown.]
   
The Huffington Post
Lev Raphael Author, reviewer, and blogger
4 Fake Quotes by Famous Authors to Watch Out For
Posted: 11/19/2014 1:09 pm EST Updated: 11/20/2014 10:07 am EST
(...)
Lately I’ve been seeing this quotation from Mark Twain all over Facebook, and the first time was early in the morning, spoiling my first cup of coffee: “Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it.” My warning bells went off immediately because the whole thing sounded too contemporary, especially “putting us on.”