| The availability heuristic is an oversimplified rule of thumb,
or heuristic, which occurs when people estimate the probability of an outcome
based on how easy that outcome is to imagine. As such, vividly described, emotionally-charged possibilities will be perceived as
being more likely than those that are harder to picture or are difficult to understand, resulting in a corresponding cognitive bias.
For example, most people think that dying from a shark attack is more likely than dying from being hit by falling airplane
parts, yet the opposite is true by a factor of 30. Perhaps this is because sharks are inherently terrifying or because shark
attacks receive more media coverage. Many people seem to fear plane
crashes, yet you are far more likely to be harmed in a car accident on
the way to the airport. Similarly, much more money is spent on fighting terrorism than on preventing car
crashes, yet the latter kill many more people per year:
- Car crashes don't get attention because they occur in ones and twos. If people died at the same rate but in one horrifying
crash a month that killed 3,500 people, then President Bush and Congress would
speedily make auto safety a priority and save thousands of lives a year. As Transportation Secretary
Norman Mineta has said: "If we had 115 people die a day in aviation
crashes, we wouldn't have a plane in the sky." [1] (http://www.nytimes.com/2004/03/13/opinion/13KRIS.html)
One important corollary finding to this heuristic is that people asked to
imagine an outcome immediately view it as more likely than those that were not.
In one experiment that occurred before the 1976 US Presidential election, participants were asked simply to imagine Gerald Ford winning the upcoming election. Those who were asked to do this
subsequently viewed Ford as being significantly more likely to win the upcoming the election, and vice versa for participants
that had been asked to imagine Jimmy Carter. Analogous results were found
with vivid versus pallid descriptions of outcomes in other experiments.
This tendency is often seen in politics. For example, when justifying an invasion of Iraq in the 2003 State of the Union Address, President George W. Bush said: "Imagine those 19 hijackers with other weapons and other plans, this time armed by
Saddam Hussein. It would take one vial, one canister, one crate slipped
into this country to bring a day of horror like none we have ever known." Politicians will also frequently speak about the
experience of a single person whom their policies would ostensibly help rather than appeal to base rate statistics.
An opposite effect of this bias, called denial, occurs when an outcome is so upsetting that the very act of thinking
about it leads to an increased refusal to believe it might occur. In this case, being asked to imagine the outcome actually made
participants view it as less likely.
Taking advantage of the availability heuristic in reasoning may commit the fallacy of misleading vividness. See also the Texas sharpshooter fallacy.
This phenomenon was first reported by psychologists Amos Tversky and Daniel
Kahneman, who also identified the representativeness heuristic. To see how availability differs from related terms vividness and salience, see availability, salience and
vividness.
References
- Tversky, A., & Kahneman, D. (1973). Availability: a heuristic for judging frequency and probability. Cognitive
Psychology, 5, 207-232.
- Tversky, A. & Kahneman, D. (1974). Judgment under uncertainty: Heuristics and biases. Science, 185,
1124-1130.
- Combs, B. & Slovic, P. (1979). Newspaper coverage of causes of death. Journalism Quarterly, 56, 837-843.
- Carroll, J. S. (1978). The effect of imagining an event on expectations for the event: An interpretation in terms of the
availability heuristic. Journal of Experimental Social Psychology, 14, 88-96.
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