The Representativeness Heuristic — Judging by Resemblance

When similarity hijacks probability — and how to correct the substitution

What is the representativeness heuristic and when does it lead judgment astray?

The representativeness heuristic is the mental shortcut of judging probability by how closely something resembles a prototype or stereotype. It is fast and often useful, but it reliably misfires when it overrides base rates, produces the conjunction fallacy, or treats random-looking sequences as unlikely.

Kahneman and Tversky identified the representativeness heuristic as one of the most consequential shortcuts in human judgment: we assess probability by asking "how much does this look like a member of that category?" rather than "how often does this actually occur?" The heuristic is not always wrong — resemblance often correlates with membership — but it fails systematically when base rates are ignored, when sample sizes are treated as irrelevant, or when a coherent narrative obscures the underlying frequencies. The practices below address the specific failure modes, each with the cognitive mechanism responsible.