The representativeness heuristic is used when making judgments about the probability of an event being representational in character and essence of a known prototypical event. It is one of a group of heuristics (simple rules governing judgment or decision-making) proposed by psychologists Amos Tversky and Daniel Kahneman in the early 1970s as "the degree to which [an event] (i) is similar in essential characteristics to its parent population, and (ii) reflects the salient features of the process by which it is generated". Heuristics are useful because they use effort-reduction and simplification in decision-making.
When people rely on representativeness to make judgments, they are likely to judge wrongly because the fact that something is more representative does not actually make it more likely. The representativeness heuristic is simply described as assessing similarity of objects and organizing them based around the category prototype (e.g., like goes with like, and causes and effects should resemble each other). Thus, it can result in neglect of relevant base rates and other cognitive biases.
Determinants of representativeness
The representativeness heuristic is more likely to be used when the judgement or decision to be made has certain factors.
Similarity
alt=Diagram|right|thumb|243x243px|Snap judgement of whether novel object fits an existing category
When judging the representativeness of a new stimulus/event, people usually pay attention to the degree of similarity between the stimulus/event and a standard/process.
Several examples of similarity have been described in the representativeness heuristic literature. This research has focused on medical beliefs. The researcher found that clinicians use the representativeness heuristic in making diagnoses by judging how similar patients are to the stereotypical or prototypical patient with that disorder.
:<math>P(H|D) = \frac{P(D | H)\, P(H)}{P(D)}.</math>
<br />However, judgments by representativeness only look at the resemblance between the hypothesis and the data, thus inverse probabilities are equated: This was explicitly tested by Dawes, Mirels, Gold and Donahue (1993) who had people judge both the base rate of people who had a particular personality trait and the probability that a person who had a given personality trait had another one. For example, participants were asked how many people out of 100 answered true to the question "I am a conscientious person" and also, given that a person answered true to this question, how many would answer true to a different personality question. They found that participants equated inverse probabilities (e.g., <math>P(conscientious|neurotic)=P(neurotic|conscientious)</math>) even when it was obvious that they were not the same (the two questions were answered immediately after each other).
Some research has explored base rate neglect in children, as there was a lack of understanding about how these judgment heuristics develop. The authors of one such study wanted to understand the development of the heuristic, if it differs between social judgments and other judgments, and whether children use base rates when they are not using the representativeness heuristic. The authors found that the use of the representativeness heuristic as a strategy begins early on and is consistent. The authors also found that children use idiosyncratic strategies to make social judgments initially, and use base rates more as they get older, but the use of the representativeness heuristic in the social arena also increase as they get older. The authors found that, among the children surveyed, base rates were more readily used in judgments about objects than in social judgments. Base rates may be neglected more often when the information presented is not causal. Base rates are used less if there is relevant individuating information. Groups have been found to neglect base rate more than individuals do. Use of base rates differs based on context. Research on use of base rates has been inconsistent, with some authors suggesting a new model is necessary.
Conjunction fallacy
A group of undergraduates were provided with a description of Linda, modelled to be representative of an active feminist. Then participants were then asked to evaluate the probability of her being a feminist, the probability of her being a bank teller, or the probability of being both a bank teller and feminist. Probability theory dictates that the probability of being both a bank teller and feminist (the conjunction of two sets) must be less than or equal to the probability of being either a feminist or a bank teller. A conjunction cannot be more probable than one of its constituents. However, participants judged the conjunction (bank teller and feminist) as being more probable than being a bank teller alone. Some research suggests that the conjunction error may partially be due to subtle linguistic factors, such as inexplicit wording or semantic interpretation of "probability". The authors argue that both logic and language use may relate to the error, and it should be more fully investigated.
The values shown in parentheses are the number of students choosing each answer.