What is the availability heuristic?
The availability heuristic has two readings, one about how many examples come to mind and one about how easily they arrive. A 1991 experiment was designed to tell them apart.
Answer to the question
What is the availability heuristic?
The availability heuristic is the habit of judging how common or likely something is by how easily examples come to mind. Amos Tversky and Daniel Kahneman described it in 1973. A 1991 experiment by Norbert Schwarz and colleagues found that the ease of recalling examples, not their number, drove the judgement.
Key takeaways
- Tversky and Kahneman (1973) proposed that people judge frequency and probability by how easily relevant instances come to mind, and that this is generally correlated with real frequency but produces systematic biases.
- Schwarz and colleagues (1991) found that people who recalled twelve examples of their own assertive behaviour rated themselves as less assertive than people who recalled six.
- In the same 1991 paper, the effect disappeared when participants were told that ease of recall was irrelevant to the judgement.
- Lichtenstein and colleagues (1978) traced distorted estimates of causes of death to uneven exposure to information, ease of remembering and ease of imagining.
- The two readings agree that people answer from memory, not from a count, and differ on whether quantity or ease does the work.
The availability heuristic is the habit of judging how common something is by how easily examples of it come to mind. The research behind it supports two readings: people may lean on how many instances they recall, or on how easy the recalling felt. Amos Tversky and Daniel Kahneman proposed the first, and Norbert Schwarz and colleagues later set the two apart.
Where the Frequency Account Began
The account appeared in Cognitive Psychology as Availability: A Heuristic for Judging Frequency and Probability. It starts from an ordinary problem. People rarely have a count of how often a class of events occurs, yet they are asked, and ask themselves, to judge frequency and probability all the time. The authors proposed that people answer by consulting memory. The more easily relevant instances come to mind, the more frequent or probable the class seems.
The abstract adds two qualifications that popular summaries often drop. Availability is generally correlated with how often things actually occur, so the availability heuristic is not a random guess. But availability is also affected by other factors, and relying on it therefore produces systematic biases. The authors demonstrated those biases in judgements of how often classes of words occur, of combinatorial outcomes and of repeated events. They also explained the impression that two things go together as an effect of associative bonds formed between pairs of stimuli.
Kahneman gave the availability heuristic its widest audience in his book Thinking, Fast and Slow. There he places it among the cases in which a hard question about frequency is quietly replaced by an easier question about how readily examples arrive. The 1973 paper is the source of the availability heuristic, and Tversky is its first author.
The same book's account of how wording changes a choice is the subject of What is the framing effect.
Number of Examples Against Ease of Recall
The 1973 account left a question open. Does a judgement under the availability heuristic follow the number of instances a person recalls, or the feeling of how easy the recall was? In daily life the two usually travel together, because a category with many instances yields many examples quickly. Schwarz and his co-authors separated them in 1991, in a paper whose title says what it did, Ease of Retrieval as Information: Another Look at the Availability Heuristic.
Participants recalled either six or twelve instances of their own assertive behaviour, or of their own unassertive behaviour, and then rated themselves. Under a reading based on quantity, twelve recalled instances should make people rate themselves as more assertive than six. The abstract reports the reverse. Those who recalled twelve rated themselves as less assertive than those who recalled six, and the same held for unassertive behaviour. Twelve instances are hard to produce and six are easy, so the ease of the task carried the judgement.
A second manipulation points the same way. When participants were told that the ease of recall was irrelevant to the judgement, the effect disappeared. People appear to have treated the feeling of difficulty as information about themselves, and a reason to discount the feeling removed its influence. That is the sense of the paper's title.
Kahneman reports this result in Thinking, Fast and Slow as part of his account of the availability heuristic. What drives the judgement can be how hard retrieval feels, not the content retrieved.
Causes of Death and Careful Wording
A third study shows the availability heuristic at work on larger questions. In 1978 Sarah Lichtenstein, Paul Slovic and three colleagues asked people to estimate how often various causes of death occur. The judgements formed a consistent picture, but a distorted one. People overestimated small frequencies and underestimated large ones. At any given true frequency, they also exaggerated the risk of some causes and understated others.
The authors traced these errors to several factors. They named uneven exposure to information about different hazards, how easily events are remembered and how easily they can be imagined. Because exposure and memory are listed together, the availability heuristic appears in this paper as one contributor to the pattern and not its only explanation. The paper also considers ways to improve frequency judgements and what the results mean for the management of public hazards.
A news-driven version of the same mechanism, with the evidence on whether it moves purchases, is set out in Does the Availability Heuristic Sell. The role of whatever happens to be at the front of memory in unplanned shopping is covered in Why Impulse Buys Happen Without a List.
Three points follow for anyone describing the availability heuristic.
1. It is a claim about judgement, not a claim that memory is generally unreliable. The 1973 abstract itself says availability is generally correlated with real frequency.
2. A statement that people overestimate whatever the news covers is an application of the idea and needs evidence of its own. The lethal events paper treats exposure to information as one factor among several.
3. A careful account says which reading is meant: the number of examples recalled, or the ease with which they arrived.
In the one task built to separate the number of examples from the ease of producing them, ease carried the judgement. The 1978 paper adds that what is easy to remember also depends on how much information about a hazard reaches a person in the first place. Claims about what the idea explains in advertising, in news coverage or in public policy each need evidence of their own, and the three studies above do not supply it for them.
Common questions
What is the availability heuristic?
It is the habit of judging how common or likely something is by how easily examples come to mind, described by Tversky and Kahneman in 1973.
Who proposed the availability heuristic?
Amos Tversky and Daniel Kahneman, in a 1973 paper in Cognitive Psychology. Tversky is the first author.
What is an example of the availability heuristic?
In Schwarz and colleagues' 1991 experiment, people who recalled twelve examples of their own assertive behaviour rated themselves as less assertive than people who recalled six. Recalling twelve was harder, and the difficulty shaped the rating.
Does the availability heuristic always produce errors?
No. The 1973 abstract says availability is generally correlated with how often things occur. Errors arise when other factors also affect how easily examples come to mind.
Sources
Kahneman, D. (2011). Thinking, Fast and Slow. Seed book; vault hub Books/Thinking Fast and Slow.
Tversky, A., & Kahneman, D. (1973). “Availability: A heuristic for judging frequency and probability.” Cognitive Psychology, 5(2), 207-232. doi:10.1016/0010-0285(73)90033-9 https://doi.org/10.1016/0010-0285(73)90033-9
Schwarz, N., Bless, H., Strack, F., Klumpp, G., Rittenauer-Schatka, H., & Simons, A. (1991). “Ease of retrieval as information: Another look at the availability heuristic.” Journal of Personality and Social Psychology, 61(2), 195-202. doi:10.1037/0022-3514.61.2.195 https://doi.org/10.1037/0022-3514.61.2.195
Lichtenstein, S., Slovic, P., Fischhoff, B., Layman, M., & Combs, B. (1978). “Judged frequency of lethal events.” Journal of Experimental Psychology: Human Learning and Memory, 4(6), 551-578. doi:10.1037/0278-7393.4.6.551 https://doi.org/10.1037/0278-7393.4.6.551