The Availability Heuristic
In almost all cases, we want our conclusions to rest not just on one observation, but on patterns of observations. Is last-minute cramming an effective way to prepare for exams?Does your car start more easily if you pump the gas while turning the key? Do you get sick less often if you take vitamin tablets? In each case, you could reach a conclusion based on just one experience (one exam, or one flu season), but that’s a risky strategy because that experience might have been a fluke of some sort, or in some way atypical. Thus, what you really want is a summary of multiple experiences, so that you can draw conclusions only if there’s a consistent pattern in the evidence.
Generally, this summary of multiple experiences will require a comparison among frequency estimates—assessments of how often you’ve encountered a particular event orobject. How often have you crammed for an exam and done well? How often have you crammed and done poorly? How many people do you know who take vitamins and still get sick? How many stay healthy?
In this way, frequency estimates are central for judgment, but there’s an obvious problem here: Most people don’t keep ledgers recording the events of their lives, and so they have no objective record of what happened each time they started their car or how many of their friends take vitamins. What do people do, then, when they need frequency estimates? They rely on a simple strategy: They try to think of spe-cific cases relevant to their judgment—exams that went well after cramming, or frustrating mornings when the car just wouldn’t start. If these examples come easily to mind, people conclude that the circumstance is a common one; if the examples come to mind slowly or only with great effort, people conclude that the circumstance is rare.
This strategy is referred to as the availability heuristic, because the judgment uses availability (i.e., how easily the cases come to mind) as the basis for assessing frequency (how common the cases actually are in the world). For many frequency estimates this strategy works well, because objects or events that are broadly frequent in the world are likely to be frequent in our personal experience and are therefore well represented in our memories. On this basis, “easily available from memory” is often a good indicator of “frequent in the world.”
But there are surely circumstances in which this strategy is misleading. In one study, participants were asked this question: “Considering all the words in the lan-guage, does R occur more frequently in the first position of the word (rose, robot,rocket) or in the third position (care, strive, tarp)?” Over two-thirds of the participantssaid that R is more common the first position—but actually the reverse is true, by a wide margin.
What caused this error? Participants made this judgment by trying to think of words in which R is the first letter, and these came easily to mind. They next tried to think of words in which it’s the third letter, and these came to mind only with some effort. They then interpreted this difference in ease of retrieval (i.e., the difference in availability) as if it reflected a difference in frequency—and so drew the wrong conclusion. As it turns out, the difference in retrieval merely shows that our mental dictionary, roughly like a printed one, is organized according to the starting sound of each word. This arrange-ment makes it easy to search memory using a word’s “starting letter” as the cue; a search based on a word’s third letter is more difficult. In this fashion, the organization of memory creates a bias in what’s easily available; this bias, in turn, leads to an error in frequency judgment (Tversky & Kahneman, 1973).
In this task, it seems sensible for people to use a shortcut (the heuristic) rather than some more laborious strategy, such as counting through the pages in a dictionary. The latter strategy would guarantee the right answer but would surely be far more work than the problem is worth. In addition, the error in this case is harmless—nothing hinges on these assessments of spelling patterns. The problem, though, is that people rely on the same shortcut—using availability to assess frequency—in cases that are more con-sequential. For example, many friendships break up because of concerns over fairness: “Why am I always the one who does the dishes?” Or “Why is it that you’re usually the one who starts our fights, but I’m always the one who reaches out afterward?” These questions hinge on frequency estimates—and use of the availability heuristic routinely leads to errors in these estimates (M. Ross & Siccoly, 1979). As a result, this judgment heuristic may leave us with a distorted perception of some social relations—in a way that can undermine some friendships!
As a different example, what are the chances that the stock market will go up tomor-row or that a certain psychiatric patient will commit suicide? The stockbrokers and psy-chiatrists who make these judgments regularly base their decisions on an estimate of probabilities. In the past, has the market generally gone up after a performance like today’s? In the past, have patients with these symptoms generally been dangerous to themselves? These estimates, too, are likely to be based on the availability heuristic. Thus, for example, the psychiatrist’s judgment may be poor if he vividly remembers a particular patient who repeatedly threatened suicide but never harmed himself. This easily available recollection may bias the psychiatrist’s frequency judgment, leading to inadequate precautions in the present case.