Argumentum ad populum
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“”Eat shit. Twenty trillion flies can't be wrong.
Argumentum ad populum (literally, "an argument to the people") is the logical fallacy that just because something is popular, it is therefore true (or desirable). Undoubtedly many popular notions are true, but their truth is not a function of their popularity.
This logical fallacy is often used by children as an excuse for wanting something (everybody's got one) or getting into mischief (everybody's doing it). Despite the juvenile nature of the argument, it is often used by people who should know better, particularly by those who are trying to force other people to their way of thinking. A case in point is the push in the United States to get creationism taught in public school science classes. The argument runs along the lines of suggesting that because a majority of people in the U.S. believe in creationism, it should therefore be taught as science.
Populus (accusative populum) means people in Latin. Linguists haven't reached a consensus on the origin of the expression; one linguistic argumentum ad populum for its origins is the Etruscan language.
 Scientific consensus
So, what's the difference between most people believe X and scientific consensus which is, at the end of the day, most scientists believe X? Doesn't this make out scientists to be somehow superior to the rest of the population?
There are two significant differences:
- Scientific consensus doesn't claim to be true, it claims to be our best understanding currently held by those who study the matter. Scientific claims for truth are always tentative rather than final, even if they are often very impressive tentative claims for truth.
- Scientific consensus is built upon a foundation of logic and systematic evidence - the scientific method - rather than dogma (or that which is taught in Sunday school) or popular prejudice. The consensus comes not from blindly agreeing with those in authority, but from having their claims to be thoroughly reviewed and criticised by their peers. (Note that even long-established scientific consensus can be overthrown by better logic and better evidence typically preceded by anomalous research findings.)
 When it is not not a fallacy
When the truth-value of the proposition is really a function of the popularity. This can arguably be the case in grammar (most native speakers think a certain construction is grammatical, thus it is grammatical),and matters of convention and etiquette. Also in democratic processes.
 Argumentum ad populum... without the populum
A twist on the argumentum ad populum is claiming that some idea is more popular than it is, and then using that alleged popularity in order to justify some extreme measures against the alleged promoters of the idea. Examples include:
- Christian fundamentalists countering Islamic terrorists' farm system of suicide bombers by brainwashing children into thinking "we are enlisted in the holy military service of Christ that all our life long we should fight against the world, Satan, and our own flesh," as shown in the documentary Jesus Camp.
- The claims of Joseph McCarthy and the John Birch Society that there was a massive secret communist conspiracy to subvert the First World and capture it for the Soviet Union.
- The claims underlying homosexual recruitment conspiracy theories, as well as Bill O'Reilly's chatter about a lesbian gang epidemic: that an idea of homosexuality as superior to heterosexuality is commonplace among the gay population, thus justifying their exclusion from schools etc.
- Some exaggerated claims by such organs as Searchlight magazine about the popularity of the British National Party, positing that the party might turn Britain into a fascist dictatorship sometime next week and start curtailing civil rights. This, the magazine argues, means that the civil rights of all persons even tangentially associated with the BNP must be promptly curtailed.
The term "silent majority" is often invoked by people who decide to fall back on argumentum ad populum without any actual evidence that their view is at all popular. The flaw with the phrase is obvious: if the majority is silent, then how can one identify its views?
 Fifty million Frenchmen
"Fifty million Frenchmen can't be wrong" is used, though often sarcastically, to justify a point of view by alluding to its general acceptance. It is a demonstration of argumentum ad populum and is falsified prima facie by the French obsession with Jerry Lewis as a comic genius in the 1960s. It derives from the 1927 song "Fifty Million Frenchmen Can't Be Wrong" which compared free attitudes in 1920s Paris with censorship and prohibition in the United States.
It has also been noted that "The entire population of Germany can't be wrong twice."