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."
Contents
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.
Examples
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:
Explanation
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.
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.)
The argumentum ad populum can be a valid argument in inductive logic; for example, a poll of a sizeable population may find that 100% prefer a certain brand of product over another. A cogent (strong) argument can then be made that the next person to be considered will also very likely prefer that brand (but not always 100% since they could be exceptions), and the poll is valid evidence of that claim. However, it is unsuitable as an argument for deductive reasoning as proof, for instance to say that the poll proves that the preferred brand is superior to the competition in its composition or that everyone prefers that brand to the other.
Evidence
Exceptions
Appeal to belief is valid only when the question is whether the belief exists. Appeal to popularity is therefore valid only when the questions are whether the belief is widespread and to what degree. I.e., ad populum only proves that a belief is popular, not that it is true. In some domains, however, it is popularity rather than other strengths that makes a choice the preferred one, for reasons related to network effects.
Language
Linguistic descriptivists argue that correct grammar, spelling, and expressions are defined by the language's speakers, especially in languages which do not have a central governing body. According to this viewpoint, if an incorrect expression is commonly used, it becomes correct. In contrast, linguistic prescriptivists believe that incorrect expressions are incorrect regardless of how many people use them.
Reversals
In some circumstances, a person may argue that the fact that Y people believe X to be true implies that X is false. This line of thought is closely related to the appeal to spite fallacy given that it invokes a person's contempt for the general populace or something about the general populace in order to persuade them that most are wrong about X. This ad populum reversal commits the same logical flaw as the original fallacy given that the idea "X is true" is inherently separate from the idea that "Y people believe X": "Y people believe in X as true, purely because Y people believe in it, and not because of any further considerations. Therefore X must be false." While Y people can believe X to be true for fallacious reasons, X might still be true. Their motivations for believing X do not affect whether X is true or false.
Y=most people, a given quantity of people, people of a particular demographic.
X=a statement that can be true or false.
For example, consider the arguments:
In general, the reversal usually goes: Most people believe A and B are both true. B is false. Thus, A is false. The similar fallacy of chronological snobbery is not to be confused with the ad populum reversal. Chronological snobbery is the claim that if belief in both X and Y was popularly held in the past and if Y was recently proved to be untrue then X must also be untrue. That line of argument is based on a belief in historical progress and not—like the ad populum reversal is—on whether or not X and/or Y is currently popular.