Availability Heuristic

Heuristics and Biases

The availability heuristic is a theoretical construct that forms part of the influential heuristics and biases framework for explaining intuitive judgment in humans, pioneered by Daniel Kahneman and Amos Tversky in the 1970s. From this perspective, because of their limited time, knowledge, and computational ability, in general, people cannot make judgments according to the often complex normative rules of logic, probability theory, and statistics. Instead, they have to resort to simpler judgment heuristics that exploit natural assessments. Natural assessments benefit from naturally existing propensities of memory and perception that are conveniently available and easily assessed. These judgmental heuristics are often useful, but sometimes they lead to systematic and serious cognitive biases.

With regard to confidence (probability) judgment, the key notion has been variable substitution, according to which the complex judgment of probability is substituted with a simpler natural assessment. Assessment of probability in the sense implied by probability theory and statistics involves consideration of all possible outcomes and their frequencies of occurrence. In the face of this complexity, it is proposed that people substitute probability with a subjective variable that is conveniently available and easier to assess. The availability heuristic, specifically, suggests that the ease with which an [Page 103]event can be brought to mind or imagined is used as a proxy for probability, as when you assess the probability of an event by considering how easy it is to retrieve similar events from memory. One could estimate, for example, the risk of dying from pneumonia by assessing the ease with which one can come up with examples of people that have died from pneumonia. If it is difficult to come up with such examples, the risk of dying from pneumonia is presumably fairly low.

Although availability may be a useful guide to probability in many real-life circumstances, because the ease of retrieval or imagination is often affected also by factors other than probability or frequency, use of the heuristic may produce a number of biases, or cognitive illusions, in probability judgment. These biases may arise in two slightly different ways, either as a consequence of biases in the external flow of information itself or because of the properties of encoding and retrieval relevant to human memory.

External Origins of Availability Bias

The reliance on availability may in part produce biased probability or frequency judgments because of biases in the external information flow a person encounters in his or her environment, which make the ease of retrieval a misleading cue to the population frequencies. For example, people's perception of the frequency of different death causes correlates more strongly with the frequencies reported in media than with the true fatality frequencies. People thus tend to overestimate the rate of violent and dramatic deaths that receive the most attention by the media (e.g., murder, car accidents) whereas they underestimate the risk of more mundane causes of death (e.g., pneumonia, cardiovascular disease). One may overestimate the prevalence of one's own opinions and values in the population, simply because one tends to socialize specifically with people who have similar opinions and values. In a variety of domains, people are exposed to selective information and to the extent that people are unaware of this fact, reliance on availability yields biased judgments.

...