Bandwagon

If you suggest that someone's claim is correct simply because it's what most everyone is coming to believe, then you're committing the bandwagon fallacy. Get up here with us on the wagon where the band is playing, and go where we go, and don't think too much about the reasons. The Latin term for this fallacy of appeal to novelty is Argumentum ad Novitatem.

Example:

[Advertisement] More and more people are buying sports utility vehicles. Isn't it time you bought one, too? [You commit the fallacy if you buy the vehicle solely because of this advertisement.]

Like its close cousin, the fallacy of appeal to the people, the bandwagon fallacy needs to be carefully distinguished from properly defending a claim by pointing out that many people have studied the claim and have come to a reasoned conclusion that it is correct. What most everyone believes is likely to be true, all things considered, and if one defends a claim on those grounds, this is not a fallacious inference. What is fallacious is to be swept up by the excitement of a new idea or new fad and to unquestionably give it too high a degree of your belief solely on the grounds of its new popularity, perhaps thinking simply that 'new is better.'