Bandwagon

In plain terms

The bandwagon argues that a claim is true because lots of people accept it, or that a choice is right because lots of people are making it. Popularity substitutes for evidence. "Everyone's doing it" becomes the case for doing it.

Also called appeal to popularity or argumentum ad populum.

Why it's fallacious

Truth doesn't tally votes. Plenty of things most people once believed turned out to be wrong: that heavier objects fall faster, that the sun orbits the earth, that ulcers are caused by stress. The number of people who held those views never moved the underlying physics or biology. What eventually changed minds was evidence, and evidence is what the bandwagon skips.

The move still persuades because humans are social animals. Conforming to group belief is cognitively cheap, and the conformity pressure is often invisible to the people feeling it. Recognizing the shape of the appeal is the first defense against it.

Canonical example

"Over ten million copies of this diet book have been sold. It can't be wrong."

Ten million sales tell us something: the book is well-marketed, its promise is appealing, and a lot of people tried it. They don't tell us the diet works. People buy books on many topics that turn out to be mistaken. Popularity is data about demand, not about truth.

Counter-example (not a fallacy)

"Ninety-eight percent of climate scientists who publish in the field agree that human activity is driving recent warming. That near-unanimity reflects the fact that the same conclusion emerges across different data sources, methods, and research groups, which is hard to explain if the science were weak."

This isn't a bandwagon. The argument isn't "many scientists believe it, therefore it's true." It's "many independent scientists, working with different methods, have converged on the same conclusion, which is informative about the evidence." The claim is about what produced the agreement, not about the mere count of agreeing people. A convergence of independent investigators is meaningful. A convergence of strangers on social media isn't the same thing.

How to respond when you see it

Ask what would change minds if everyone switched. "If half the believers flipped their view tomorrow, would that make the claim less true?" If the answer is yes, the claim is social, not factual. If the answer is no, popularity shouldn't have been the argument in the first place.