The availability heuristic, identified by Tversky and Kahneman (1973), is a mental shortcut for estimating frequency or probability based on the ease with which relevant instances come to mind. Events that are easily recalled (available) are judged as more common or likely than events that are difficult to recall. Since ease of recall often correlates with actual frequency, the heuristic is usually accurate. But it can be systematically biased by factors that affect availability without affecting actual frequency.
Sources of Bias
Several factors inflate availability without reflecting true frequency. Recency: recent events are more available than remote ones (a recent plane crash makes flying feel more dangerous). Salience: vivid, dramatic events are more available than mundane ones (terrorist attacks vs. diabetes). Media coverage: heavily reported events are judged as more frequent. Personal experience: experienced events feel more common than statistically equivalent events known only abstractly. Emotional intensity: emotionally arousing events are more easily recalled.
The availability heuristic contributes to systematic risk misperception. After highly publicized shark attacks, beach attendance drops despite the tiny statistical risk. After natural disasters, insurance purchases spike temporarily. Medical diagnoses are influenced by recent cases: a doctor who recently treated a rare disease is more likely to consider it for subsequent patients. These biases have implications for policy: public risk perception (and thus policy priorities) often reflects availability rather than actual risk magnitude.