Availability bias is a human cognitive bias that causes us to overestimate probabilities of events associated with memorable or vivid occurrences. Because memorable events are further magnified by coverage in the media, the bias is compounded on the society level. Two prominent examples would be estimations of the probability of plane accidents, and the abduction of children. Both events are quite rare, but the vast majority of the population wildly overestimates their probability, and behaves accordingly. They are falling prey to the availability bias.

In reality, you're much more likely to die from an auto accident than a plane accident, and your child is more likely to die in an accident than get abducted. But the majority thinks the inverse, because the less likely events are more "available" - more memorable. If you look at the literature or even just the interactions of daily life, you will find thousands of examples of availability bias in action.

Availability bias is at the root of many other human biases and culture-level effects. For instance, medieval medicine was probably barely more effective than leaving a malady alone to heal on its own, but because the times where the therapy "worked" are more available in the minds of many, practicing medicine was generally considered effective whether or not it really was.

The study of availability bias was pioneered by psychologists Amos Tversky and Daniel Kahneman, who founded the field of "heuristics and biases" and developed a model called prospect theory to explain systematic bias in human decision-making. Daniel Kahneman subsequently won the 2002 Nobel Prize in Economics for his work, despite having never taken an economics class. His long-time partner in the research of heuristics and biases, including the availability bias, Amon Tversky, died in 1996.

A concept intimately connected to availability bias is that of base-rate neglect. Base-rate neglect refers to integrating irrelevant information into a probability judgment, biasing it from the natural base rate. An example would be letting someone into a college just based on an interview, when empirical studies have shown that past performance and grades are the best possible indicator of future performance, and that interviews merely cloud the assessment. But because we like "seeing things for ourselves," the interviews are likely to continue to take place, even in the absence of any support for their effectiveness.