Representativeness Heuristic - Disjunction Fallacy

Disjunction Fallacy

In addition to extensionality violation, base-rate neglect, and the conjunction fallacy, the use of representativeness heuristic may lead to a disjunction fallacy. From probability theory the disjunction of two events is at least as likely as either of the events individually. For example, the probability of being either a physics or biology major is at least as likely as being a physics major, if not more likely. However, when a personality description (data) seems to be very representative of a physics major (e.g., pocket protector) over a biology major, people judge that it is more likely for this person to be a physics major than a natural sciences major (which is a superset of physics).

Further evidence that the representativeness heuristic may cause the disjunction fallacy comes from Bar-Hillel and Neter (1986). They found that people judge a person who is highly representative of being a statistics major (e.g., highly intelligent, does math competitions) as being more likely to be a statistics major than a social sciences major (superset of statistics), but they do not think that he is more likely to be a Hebrew language major than a humanities major (superset of Hebrew language). Thus, only when the person seems highly representative of a category is that category judged as more probable than its superordinate category. These incorrect appraisals remained even in the face of losing real money in bets on probabilities.