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...
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).
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...
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, and ideals of beauty are arguably culturally determined and thus arbitrary to a significant degree. For instance, overweight bodies have been considered more beautiful in some cultures (such as Mauritania) because only the wealthy could afford to eat enough to become overweight or obese. By contrast, contemporary high fashion generally involves women who have been criticized for eating too much.
At a time in history when most people believed the world was flat, one could have claimed the world is flat because most believed it.
Advocates of heliocentrism, such as Galileo Galilei were strongly suppressed, despite scientific evidence, now recognized as factual, that supported heliocentrism at the expense of geocentrism.
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.