After the tragic events of September 11th 2001 when planes were flown into the World Trade Center towers, the number of people travelling by air in America fell significantly. This is a classic example of a widely publicised and sensational event affecting behaviour by skewing the perceived odds of such an incident happening. It's an example of availability bias, one of several cognitive biases affecting judgement – in this case, statistical judgement.
Easy to recall occurrences are often used to estimate probabilities, possibly in the absence of any other data. This availability heuristic can lead to bias in statistical estimates. A commonplace example is a person who claims smoking isn't so dangerous to one's health because his grandfather smoked 40 cigarettes a day and lived till he was 90. Indeed, there will be some people who don't die from a life-long smoking habit, but what can't be easily brought to mind are the thousands of people who have died as a result of smoking.
Consider this question: are there more words which begin with the letters ‘r’ and ‘k’ than words where ‘r’ and ‘k’ appear as the third letter? Most people will judge that there are more words beginning with ‘r’ or ‘k’ because they are easier to think of; but there are actually more words with them as the third letter.
Availability bias can still occur even when sufficient data is supplied to judge probability correctly. This was demonstrated in a study that presented subjects with lists of well-known men and women. The subjects had to decide which gender occurred most on a list. Some lists contained men relatively more famous than the women, and vice versa. The subjects wrongly thought that the gender occurring most on a list was the one where they were more famous, because the more famous people were easier to bring to mind.
The media is a major source of availability bias. Relatively rare yet sensational events are widely, and repeatedly, reported and thus easy to remember – who can forget the repeated footage of planes crashing into the twin towers? In the wake of 9/11 many people in America chose to drive instead of fly. However, flying was still safer than car travel and the result was additional road deaths. People tend to overestimate the odds of a sensational event, and correspondingly underestimate the odds of a mundane event like a car crash.
Clearly availability bias is difficult to overcome, and can have potentially fatal consequences when people alter their behaviour after incorrectly judging probabilities which have been skewed by it. It may also lead or contribute to a moral panic. Recently the media have focused much attention on knife murders. High profile coverage of these crimes may fuel fear and exaggerate the perceived chances of one becoming a victim.
