In this experiment, you were given two options on every trial; one option listed a single event (e.g. Mary is a teacher) and one option listed a conjunction of events (e.g. Mary is a teacher who likes to read). In general, participants will indicate that the conjunctive option is more likely, but statistically, it never is. The probability of a single event is x% and the probability of that event and another event co-occurring is x% × y%, which is ALWAYS less than x%. Tversky and Kahneman (1983) were the first to coin the phrase conjunctive fallacy, and they suggested that the phenomenon was the result of individuals using the representativeness heuristic, rather than mathematical probability, to choose an alternative. The representativeness heuristic is a problem-solving short cut where material that is considered to be representative of a population (such as teachers liking to read) forms the basis of judgements about that population.

Question for Review