Argumentum ad populum

In argumentation theory, an argumentum ad populum (Latin for "appeal to the people") is a fallacious argument that concludes that a proposition is true because many or most people believe it: "If many believe so, it is so."

This type of argument is known by several names, including appeal to the masses, appeal to belief, appeal to the majority, appeal to democracy, appeal to popularity, argument by consensus, consensus fallacy, authority of the many, bandwagon fallacy, vox populi, and in Latin as argumentum ad numerum ("appeal to the number"), fickle crowd syndrome, and consensus gentium ("agreement of the clans"). It is also the basis of a number of social phenomena, including communal reinforcement and the bandwagon effect. The Chinese proverb "three men make a tiger" concerns the same idea.

This fallacy is sometimes committed while trying to convince a person that a widely popular thought is true, based solely on the fact that it is a widely popular thought. In the Argumentum ad populum, the population's experience, expertise or authority is not taken into consideration by the author:

The argumentum ad populum is a red herring and genetic fallacy. It appeals on probabilistic terms; given that 75% of a population answers A to a question where the answer is unknown, the argument states that it is reasonable to assume that the answer is indeed A. In cases where the answer can be known but is not known by a questioned entity, the appeal to majority provides a possible answer with a relatively high probability of correctness.

There is the problem of determining just how many are needed to have a majority or consensus. Is merely greater than 50% significant enough and why? Should the percentage be larger, such as 80 or 90 percent, and how does that make a real difference? Is there real consensus if there are one or even two people who have a different claim that is proven to be true?

It is logically fallacious because the mere fact that a belief is widely held does not necessarily guarantee that the belief is correct; if the belief of any individual can be wrong, then the belief held by multiple persons can also be wrong. The argument that because 75% of people polled think the answer is A implies that the answer is A fails, because, if opinion did determine truth, then there would be no way to deal with the discrepancy between the 75% of the sample population that believe the answer is A and 25% who are of the opinion that the answer is not A. However small a percentage of those polled give an answer other than A, this discrepancy by definition disproves any guarantee of the correctness of the majority. In addition, this would be true even if the answer given by those polled were unanimous, as the sample size may be insufficient, or some fact may be unknown to those polled that, if known, would result in a different distribution of answers.

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