What is the conjunction fallacy?

The conjunction fallacy is an error in judgment that occurs when people overestimate the likelihood of two events happening at the same time. As a result, people assign higher probability to detailed descriptions. However this violates the laws of probability: the conjunction, or co-occurrence, of two events cannot be more likely than the probability of either event alone.

People are missguided because they use the representativeness heuristic to arrive at their judgment. In other words, they mistake the perceived plausibility of a scenario for probability.