Argumentum ad populum


In argumentation theory, an argumentum ad populum is a fallacious argument that concludes that a proposition must be true because many or most people believe it, often concisely encapsulated as: "If many believe so, it is so".
Other names for the fallacy include common belief fallacy or appeal to belief, appeal to the majority, appeal to the masses, appeal to popularity, argument from consensus, authority of the many, bandwagon fallacy, consensus gentium, democratic fallacy, and mob appeal.

Description

Argumentum ad populum is a type of informal fallacy, specifically a fallacy of relevance, and is similar to an argument from authority. It uses an appeal to the beliefs, tastes, or values of a group of people, stating that because a certain opinion or attitude is held by a majority, it is therefore correct.
Appeals to popularity are common in commercial advertising that portrays products as desirable because they are used by many people or associated with popular sentiments instead of communicating the merits of the products themselves.
The inverse argument, that something that is unpopular must be flawed, is also a form of this fallacy.
The 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 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 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 based on majority opinion. This is often not the case.

Scholarship

The philosopher Irving Copi defined argumentum ad populum differently from an appeal to popular opinion itself, as an attempt to rouse the "emotions and enthusiasms of the multitude".

Valid/Strong and Invalid/Weak uses

argues that appeals to popular opinion can be logically valid in some cases, such as in political dialogue within a democracy.
The argumentum ad populum can be a cogent argument in inductive logic. For example, a poll of a sizeable population may find 100% prefer a certain brand of product over another. A cogent argument can then be made that the next person to be considered will also very likely prefer that brand, and the poll is strong evidence of that claim. An example of a weak inductive argument, however, would be saying: "Support the brand of the product. People need a practical product to ensure the fulfillment of its consumers as they say such a product from a particular brand is required according to the polls. People have suffered too long for not receiving the product from this brand." The problem stems from the conclusion likely not following from the premises. What is so special about this product? How is it certain if one product isn't used by the majority, is responsible for their suffering? How will this product lead to a better quality of life? Why is this product made? How can it be induced the polls aren't from a biased sample? These are some of the questions which concern the overall strength of the argument inductively.
For deductive reasoning as proof, for instance, to say the poll proves the preferred brand is superior to the competition in its composition or that everyone prefers that brand to the other.

Language

s 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 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.
Examples:
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.