Subadditivity effect

Belief that probability of the whole is lesser than the sum of probabilities of the parts.

Example

Subjects in one experiment judged the probability of death from cancer in the United States was 18%, the probability from heart attack was 22%, and the probability of death from "other natural causes" was 33%. Other participants judged the probability of death from a natural cause was 58%. Natural causes are made up of precisely cancer, heart attack, and "other natural causes," however, the sum of the latter three probabilities was 73%, and not 58%.

Cause

Subadditivity effect can be explained by a noisy conversion of objective evidence (observation) into subjective estimates (judgment).