Conjunction fallacy
The error of judging that a combination of two qualities is more probable than the occurrence of one on its own.

For example, when given a behavioural thumbnail sketch about an hypothetical person†, offered two conclusions about the character described in the sketch, and asked to select which answer is the more probable most subjects choose the less-probable alternative. Such thinking permeates beyond the laboratory.

The error is not explained by research participants’ misunderstanding of what “probable” means.

(see also: Representativeness heuristic, clustering illusion, Regression towards the mean, pattern-seeking)