Unpacking effect
Breaking a possibility into named pieces raises its judged probability.
What it means
The increase in estimated probability that occurs when a single, packed description of an event is decomposed into a list of its more specific components. Each explicitly named possibility brings its own examples and reasons to mind, so the unpacked version draws more cognitive 'support' than the implicit whole, even though the underlying event is unchanged. The effect produces subadditivity and is a direct prediction of support theory; it can be turned around, too, since burying possibilities inside a vague catch-all category suppresses their apparent likelihood. It has practical bite for anyone eliciting forecasts or risk estimates: the level of detail in which options are presented systematically nudges the probabilities people report, so checklists and itemized prompts can inadvertently inflate perceived risk while coarse categories deflate it.
Examples
People rate 'death from heart disease, cancer, or some other natural cause' as more probable than the single phrase 'death from a natural cause.'
Asked the chance the project slips, a manager says ten per cent. Asked separately about the vendor, the new hire, the integration, and anything else, the pieces sum to far more.
Travel insurance that itemises theft, illness, cancellation, delay, and lost luggage feels far more necessary than the same policy promising cover for 'travel mishaps'. Naming each mishap makes each one imaginable.
First described in Tversky & Koehler (1994).