The bandwagon fallacy, also called appeal to the people, appeal to popularity or argumentum ad populum, is an argument where something is 'proven' by stating that many or all people believe it. Because there is no reason to assume that the majority, however large, is right, this argument is a logical fallacy.

For example, one could claim that smoking is a reasonable pastime, since millions of people do it. However, knowing the dangers of smoking, we instead say that smoking is not a reasonable pastime despite the fact that millions do it.

While the argument may seem convincing when not examined, a simple refutation would lie in pointing out that at a time not too distant in United States history, it was common to hold other people as slaves, which few people today would consider ethical.

The bandwagon fallacy is a pillar of advertising, along with the appeal to authority fallacy.

See also: Bandwagon effect