Argumentum ad populum

In logic, an argumentum ad populum (Latin: "appeal to the people") is a fallacious argument that concludes a proposition to be true because many or all people believe it; it alleges: "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 the people, argument by consensus, authority of the many, and bandwagon fallacy, and in Latin by the names argumentum ad populum ("appeal to the people"), 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 spreading of various religious beliefs, and of the Chinese proverb "three men make a tiger".

Examples
This fallacy is sometimes committed while trying to convince a person that a widely popular thought is true.
 * Nine out of ten of my constituents oppose the bill, therefore it is a bad idea.
 * Nine out of ten of my fellow congressmen favor the bill, therefore it is a good idea.

It is sometimes committed when trying to convince a person that widely unpopular theories are false.
 * It's silly for you to claim that Hitler would not have attacked the United States if they hadn't entered World War II. Everyone knows that he planned to conquer the world.

The fallacy is commonly found in arguments over ethics:
 * Most people hold that infanticide is morally wrong. Therefore, infanticide is morally wrong.

The fallacy is also common in marketing:
 * Brand X vacuum cleaners are the country's leading brand; so buy Brand X vacuum cleaners.
 * Watch Show X - the #1 watched show on television!

Other examples:
 * Fifty million Elvis fans can't be wrong.
 * All of my friends are 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.
 * Most analysts consider Enron Corporation a well-run company with excellent management, so its common stock is a good investment.
 * Everyone jaywalks here and as long as I look carefully, nothing will happen.
 * Find me anyone who shares your preposterous notion that the Earth orbits the Sun!
 * Twelve million people play World of Warcraft, so it must be the best video game ever.

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 percent 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).

The argumentum ad populum can be a valid argument in inductive logic; for example, a poll of a sizeable population may find that 90% 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 prefer that brand, 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

 * One could claim that smoking is a healthy pastime, since millions of people do it. However, knowing the dangers of smoking, we instead say that smoking is not a healthy pastime despite the fact that millions do it.
 * One could claim Angelina Jolie is the best-looking woman in the world, because she is regularly voted as such, although the sample she is part of (celebrities) is insufficient.
 * One could claim that slavery is morally justified if the majority of people within society support it, or at least do not oppose it.

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.

Democracy
The "correctness" of electoral processes lies in the prior acceptance by the electorate that the outcome of an election shall be enacted no matter what it is.


 * "Most of the voting members at the last Rotary Club meeting thought that the Club should hold a fund-raiser in October. Therefore, the Club shall hold a fund-raiser in October."

Democracy is based on appeal to popularity. As a means of determining the truth of beliefs, it is fallacious (see consensus reality and wikiality). Democracy does not obviate this; it merely makes the fallacy irrelevant as correctness is defined by popularity in its case (possibly subject to constitutional restrictions).

Argumentum ad populum explains how some democracies (e.g. Nazi Germany, Fascist Italy) have fallen victim to the tyranny of the majority.

The following argument is an Appeal to consequences:
 * 1) Most people of the country "A" may have wrong wills.
 * 2) These wrong wills may have very bad consequences.
 * 3) Therefore governing the country "A" based on the wills of most of its people is wrong.

Similarly fallacious is the following argument: In the statement 3, free will means the ability of a human being to act based on his/her right or wrong wills.
 * 1) A human being may have wrong wills.
 * 2) These wrong wills may have very bad consequences.
 * 3) Therefore for human beings, having a free will is wrong.

Social convention
Matters of social convention, such as etiquette or polite manners, depend upon the wide acceptance of the convention. As such, argumentum ad populum is not fallacious when referring to the popular belief about what is polite or proper:
 * "Most people in Russia think that it is polite for men to kiss each other in greeting. Therefore, it is polite for men to kiss each other in greeting in Russia."

Social conventions can change, however, and sometimes very quickly. Thus, the fact that everyone in Russia this year thinks that it is polite to kiss can not be used as evidence that everyone always believed that, or that they should always believe it.

The philosophical question of moral relativism asks whether such arguments apply to statements of morals.

Safety
Whether to follow a tenet decided by popularity rather than logical design may be a matter of safety or convenience:


 * "Nearly all Americans think that you should drive on the right side of the road. Therefore, you should drive on the right side of the road in the United States."

In this case, the choice of which side to drive on is basically arbitrary. However, to avoid head-on collisions, everyone on the road must agree on it. In many cases, what is safe to do depends on what others expect one will do, and thus on the "popularity" of that choice.

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 the majority of a people believes X implies that X is false. This line of thought is closely related to the ad hominem, appeal to emotion, poisoning the well, and guilt by association fallacies given that it invokes a person's contempt for the general populace or something about the general populace in order to persuade them that the majority is wrong about X. The ad populum reversal commits exactly the same logical flaw as the original fallacy given that the idea "X is true" is inherently separate from the idea that "Most people believe X".

For example, consider the arguments:
 * "Most of your fellow Americans ignorantly think that the Earth is only 5000 years old and you're going to believe in God just like they do?"
 * "Are you going to be a mindless conformist drone drinking milk and water like everyone else, or will you wake up and drink my product?"
 * "Everyone likes The Beatles and that probably means that they didn't have nearly as much talent as &lt;Y band&gt;, which didn't sell out."
 * "The German people today consists of the Auschwitz generation, with every person in power being guilty in some way. How on earth can we buy the generally held propaganda that the Soviet Union is imperialistic and totalitarian? Clearly, it must not be."
 * "Most people still either hate gays or just barely tolerate their existence. How can you still buy their other line that claims that pederasty is wrong?"
 * "Everyone loves &lt;A actor&gt;. &lt;A actor&gt; must be nowhere near as talented as the devoted and serious method actors that aren't so popular like &lt;B actor&gt;."

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.