Availability heuristic

{{{ExpPsy}} The availability heuristic is a rule of thumb, heuristic, or cognitive bias, where people base their prediction of an outcome on the vividness and emotional impact rather than on actual probablity.

An everyday example would be the statement: "Sorry I'm late—I hit every red light on the way here." Here the aggravation of the red lights made them seem more prevelant than they actually were.

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 vivid and salience, see availability, salience and vividness.

Overview
One important corollary finding to this heuristic is that people asked to imagine an outcome tend to immediately view it as more likely than people that were not asked to imagine the specific outcome. If group A was asked to imagine a specific outcome and then asked if it was a likely outcome, and group B was asked whether the same specific outcome was likely without being asked to imagine it first, the members of group A tend to view the outcome as more likely than the members of group B, thereby demonstrating the tendency toward using an availability heuristic as a basis for logic.

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 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.

Availability effects in lethal events
When asked to rate the probability of a variety of causes of death people tend to rate more "newsworthy" events as more likely. People often rate the chance of death by plane crash higher after plane crashes, and death by natural disaster as too likely only because these events are more reported than more common causes of death. Other rare forms of death are also seen as more common then they really are because of their inherent drama: shark attacks, terrorism, etc.

Denial as a reverse availablity heuristic
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.