The availability bias is the human tendency to think that examples of things that come readily to mind are more representative than is actually the case. The psychological phenomenon is just one of a number of cognitive biases that hamper critical thinking and, as a result, the validity of our decisions.
The availability bias results from a cognitive shortcut known as the availability heuristic, defined as the reliance on those things that we immediately think of to enable quick decisions and judgments. That reliance helps us avoid laborious fact-checking and analysis but increases the likelihood that our decisions will be flawed.
Naturally, the things that are most memorable can be brought to mind most quickly. However, there are a number of factors that influence how well we remember things. For example, we tend to remember things that we observed ourselves more easily than things that we only heard about. So, for example, if we personally know of several startups and all of them are successful, we are likely to overestimate the percentage of startups that succeed, even if we have read statistics to the contrary.
Similarly, people remember vivid events like plane crashes and lottery wins, leading some of us to overestimate the likelihood that our plane will crash or, more optimistically -- but equally erroneously -- that we will win the lottery. In these cases, the availability bias leads some people to avoid flying at all costs and leads others to rely on a big lottery win as a retirement plan.
Other cognitive biases include the confirmation bias, which involves giving undue credence to materials that support our own beliefs and attitudes, and the self-serving bias, which involves putting a positive spin on our own activities and interpreting ambiguous data in a way that suits our own purposes.
Cognitive biases are among a number of types of errors that humans are prone to. Awareness of the tendency to make such errors is one of the first steps required to improve our capacity for critical thinking.