Argumentum ad populum
In argumentation theory, an argumentum ad populum (Latin for "appeal to the people") is a fallacious argument that concludes a proposition to be true because many or most people believe it. In other words, the basic idea of the argument is: "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, and bandwagon fallacy, and in Latin as argumentum ad numerum ("appeal to the number"), 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.
Examples
Fifty million Elvis fans can't be wrong.
Everyone's doing it.
In a court of law, the jury vote by majority; therefore they will always make the correct decision.
Google gives more hits when this spelling is applied, therefore this has to be the correct spelling.
Many people buy extended warranties, therefore it is wise to buy them.
Explanation
The argumentum ad populum is a red herring and genetic fallacy. It appeals on probabilistic terms; given that 75% of a population answer 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 is not necessarily a 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, this argument fails, because if opinion did determine truth, then there 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 the percentage of those polled is distributed among any remaining answers, 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.
This fallacy is similar in structure to certain other fallacies that involve a confusion between the justification of a belief and its widespread acceptance by a given group of people. When an argument uses the appeal to the beliefs of a group of supposed experts, it takes on the form of an appeal to authority; if the appeal is to the beliefs of a group of respected elders or the members of one's community over a long period of time, then it takes on the form of an appeal to tradition.
One who commits this fallacy may assume that individuals commonly analyze and edit their beliefs and behaviors. This is often not the case (see conformity).
Link
https://en.wikipedia.org/wiki/Argumentum_ad_populum