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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.

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. And, that which was vividly described is viewed as more likely than that which was wrought a pallid decription. This tedency 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."

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.

This phenomenon was first reported by psychologists Amos Tversky and Daniel Kahneman.