Oddly Wired
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truth

A rumor becomes truth if it is repeated enough.

The availability cascade is a self-reinforcing process in which an idea gains social momentum through the interaction of the availability heuristic and reputational incentives.

Individuals often adopt these ideas to appear current or avoid social sanction, regardless of their private belief, rather than simply because repetition makes them perceive the idea as fact.

Repetition is a substitute for evidence.

Where this comes from

Study
Kuran and Sunstein, 1999
What the check found
The definition conflates the availability cascade (a social-reputational process) with the illusory truth effect (a memory/cognitive bias). Kuran and Sunstein emphasize social pressure, not just repetition.
Confidence
Solid, with limits

Related

One of these every day, and every one names the study it came from. When a finding is contested, it says so.