...To determine the veracity of a given statement, we often look to society’s collective assessment of it. But it is difficult to measure social consensus very precisely, and our brains rely, instead, upon a sensation of familiarity with an idea. You use a rule of thumb: if something seems familiar, you must have heard it before, and if you’ve heard it before, it must be true.

The rule obviously invites many opportunities for error. The seniors in Skurnik’s study couldn’t remember the context in which they had heard the health claims (research shows that we are quick to forget “negation tags,” like whether something is said to be false or a lie), so they relied, instead, on a vague sense of familiarity, which steered them astray. Repetition, psychologists have shown, easily tricks us. Kimberlee Weaver of Virginia Tech recently found that if one person tells you that something is true many times, you are likely to conclude that the opinion is widely held, even if no one else said a thing about it.