Subadditivity Effect
The tendency to judge the probability of an overall event as less than the sum of its component parts — leading to probability estimates that add up to more than 100%.
Origin
Israeli-American psychologist Amos Tversky and Canadian psychologist Derek Koehler formalized the subadditivity effect in their 1994 paper "Support Theory: A Nonextensional Representation of Subjective Probability," published in Psychological Review. Their support theory demonstrated that breaking an event into explicit sub-categories ("unpacking") inflates its judged probability — so the parts consistently sum to more than the whole. The finding built on Tversky and Daniel Kahneman's decades of research into heuristics and biases, dating back to their landmark 1974 Science paper.