Source: Broca's Brain (1979), Chapter 5, “Night Walkers and Mystery Mongers: Sense and Nonsense at the End of Science” (pp. 58-59)
“Nobody ever lost a dollar by underestimating the taste of the American public.”
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P.T. Barnum 17
American showman and businessman 1810–1891Related quotes
                                        
                                         'Notes On Journalism' http://books.google.com/books?id=52L2eI9mwlcC&q="No+one+in+this+world+so+far+as+I+know+and+I+have+searched+the+record+for+years+and+employed+agents+to+help+me+has+ever+lost+money+by+underestimating+the+intelligence+of+the+great+masses+of+the+plain+people"&pg=PA28#v=onepage in the Chicago Tribune ( 19 September 1926 http://archives.chicagotribune.com/1926/09/19/page/87/article/notes-on-journalism) 
The first sentence is often paraphrased as "No one ever went broke underestimating the intelligence of the American people." (The Yale Book of Quotations, 2006, p. 512) 
1920s 
Source: Gist of Mencken
                                    
“Do not underestimate us Americans.”
                                        
                                        Post-Presidency, DNC address (2004) 
Context: I am not discouraged. I really am not. I do not despair for our country. I never do. I believe tonight, as I always have, that the essential decency and compassion and common sense of the American people will prevail.
And so I say to you and to others around the world, whether they wish us well or ill: Do not underestimate us Americans.
                                    
“You can never underestimate the stupidity of the general public”
“…the Americans must have the Almighty dollar.”
                                        
                                        Their cupidity renders them daring and indifferent to everything else. It is nothing to them to expose their lives and those of others in order to gain money. How materialistic these people are! 
To the Right Reverend J. Bouvier, Bishop of Le Mans, Saint Mary's, 1849-07-08.
                                    
                                        
                                        15 January 2005 
http://groups-beta.google.com/group/sci.crypt/msg/49c4cd60d948032d 
On testing
                                    
Creation seminars (2003-2005), Lies in the textbooks
“No one has ever gone broke underestimating the insecurities of the gay and lesbian consumer.”
Source: Skipping Towards Gomorrah (2002), p. 216
                                        
                                         Michael Scheuer interviewed by Bill Maher http://youtube.com/watch?v=ZF4_oaTIH8g, September 21, 2007. 
2000s
                                    
Source: NRA-Endorsed Christina Hagan: ‘Stop Sending Our Dollars Overseas,’ Pay Armed Veterans to Protect Schools https://www.breitbart.com/politics/2018/04/24/nra-endorsed-christina-hagan-stop-sending-our-dollars-overseas-pay-armed-veterans-to-protect-schools/ (24 April 2018)