Appeals to Authority and to Popularity

Judge when expert opinion is good evidence, and why popularity is not.

Citing experts is often legitimate; we rely on experts constantly. An appeal to authority becomes a fallacy when the authority is outside their field, is not actually expert, has a conflict of interest, or disagrees with the consensus of other experts. Worked example: 'A famous actor says this diet cures headaches, so it must work. ' Acting fame gives no medical expertise. Compare: 'A panel of neurologists reviewing forty studies found the treatment reduced headaches for most patients. ' Here the authority is relevant and collective.

The bandwagon fallacy, or appeal to popularity, argues that something is true or good because many people believe or do it: 'Millions of people use this app, so it must be safe. ' Popularity can reflect marketing, convenience, or habit rather than truth. To evaluate either appeal, ask whether the authority is relevant and unbiased, and whether the number of believers actually bears on the question.

• 1Expert testimony is valid when relevant, credible, and unbiased.
• 2Authority outside its field is a fallacy.
• 3Popularity does not establish truth (bandwagon).