Availability heuristic
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 injuries sustained from 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 a dark fate is much more likely to befall you in an
automobile accident on the way to the
airport.
Or, in another example, much more money is spent on fighting
terrorism than on automobile safety yet the latter kills many more people per year:
- Vehicle fatalities 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]
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, subjects 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 subjects 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 subjects 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.
References
- Tversky, A. & Kahneman, D. (1974). Judgment under uncertainty: Heuristics and biases. Science, 185, 1124-1130.\n* Combs, B. & Slovic, P. (1979). Newspaper coverage of causes of death. Journalism Quarterly, 56, 837-843.\n* 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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