Which is greater: the number of six-letter English words having n as their fifth letter or the number of six-letter English words ending in ing? Most people choose the group of words ending in ing. Why? Because words ending in ing are easier to think of than generic six-letter words having n as their fifth letter. But you don’t have to survey the Oxford English Dictionary—or even know how to count—to prove that guess wrong: the group of six-letter words having n as their fifth letter words includes all six-letter words ending in ing. Psychologists call that type of mistake the availability bias because in reconstructing the past, we give unwarranted importance to memories that are most vivid and hence most available for retrieval.

The nasty thing about the availability bias is that it insidiously distorts our view of the world by distorting our perception of past events and our environment. For example, people tend to overestimate the fraction of homeless people who are mentally ill because when they encounter a homeless person who is not behaving oddly, they don’t take notice and tell all their friends about that unremarkable homeless person they ran into. But when they encounter a homeless person stomping down the street and waving his arms at an imaginary companion while singing “When the Saints Go Marching In,” they do tend to remember the incident. How probable is it that of the five lines at the grocery-store checkout you will choose the one that takes the longest? Unless you’ve been cursed by a practitioner of the black arts, the answer is around 1 in 5. So why, when you look back, do you get the feeling you have a supernatural knack for choosing the longest line? Because you have more important things to focus on when things go right, but it makes an impression when the lady in front of you with a single item in her cart decides to argue about why her chicken is priced at $1.50 a pound when she is certain the sign at the meat counter said $1.49.

One stark illustration of the effect the availability bias can have on our judgment and decision making came from a mock jury trial. In the study the jury was given equal doses of exonerating and incriminating evidence regarding the charge that a driver was drunk when he ran into a garbage truck. The catch is that one group of jurors was given the exonerating evidence in a “pallid” version: “The owner of the garbage truck stated under cross-examination that his garbage truck was difficult to see at night because it was gray in color.” The other group was given a more “vivid” form of the same evidence: “The owner of the garbage truck stated under cross-examination that his garbage truck was difficult to see at night because it was gray in color. The owner remarked his trucks are gray ‘because it hides the dirt. What do you want, I should paint ’em pink?’” The incriminating evidence was also presented in two ways, this time in a vivid form to the first group and in a pallid version to the second. When the jurors were asked to produce guilt/innocence ratings, the side with the more vivid presentation of the evidence always prevailed, and the effect was enhanced when there was a forty-eight-hour delay before rendering the verdict (presumably because the recall gap was even greater).Excerpted from ‘The Drunkard’s Walk – How Randomness Rules our Lives’ by Leonard Mlodinow