Thinking and Deciding

HEURISTICS AND BIASES IN PROBABILITY 155
Availability is also affected by personal experience. When we do something with
another person, we may remember our own point of view more vividly than we remember
the point of view of the other person — a point of view we may not even
experience. Ross and Sicoly (1979) asked husbands and wives to estimate the extent
to which they were responsible for a number of activities, such as cleaning the
house, making breakfast, and causing arguments. Ratings of the two spouses for the
same activity tended to differ. Each spouse tended to think that he or she was more
responsible than the other spouse thought. As a result, the total responsibility added
up to more than 100%. For example, if a husband thought that he was 60% responsible
for causing arguments, and the wife thought that she was 60% responsible, that
would add up to 120%. This effect held for the negative items (such as causing arguments)
as well as the positive ones. In a second study, the same effect was found
when basketball players were asked to estimate the extent to which the members of
their own team, rather than their opponents, were responsible for “turning points” in
their games.
Availability of examples can be affected by mood, and this effect, in turn, can affect
probability or frequency judgments. E. J. Johnson and Tversky (1983) gave college
students descriptions of various events, written like newspaper stories, to read.
They then asked the students to estimate the frequencies with which 50,000 people
would experience certain dangers (such as traffic accidents, fire, and leukemia)
within a one-year period. When the newspaper story concerned the violent death of
a male undergraduate — for example, in a fire — estimates of all risks increased,
whether or not they were similar or identical to the event in question. The same
kinds of increases occurred when the story was simply a sad one about a young man
who had just broken up with his girlfriend while undergoing other stresses from his
family and his job. When the story was a happy one about a young man who experienced
positive events — getting into medical school and doing well on a difficult
examination — frequency estimates decreased.
The kinds of availability problems described so far cannot be called “irrational”
when they occur in everyday life. It would, in most cases, take an unreasonable
amount of effort to avoid the effects of memory organization or of the information
we are given by others. In many cases, these effects could be avoided only with
the use of systematic data analysis of the sort done by scientists. Most of the time,
availability is probably as good a guide as we can expect, when systematic data are
not available. In extremely important decisions, however, such as those that affect
many people, it is worthwhile to take every precaution we can imagine to avoid error.
In such decisions, the danger of availability effects is a good reason for us to rely on
systematic data collection and analysis — rather than our own hunches — when
numerical data are available.
tree that he could check without the help of a mechanic, he finally called a towing service. As the car
was about to be towed away, he noticed that the car was not his. His car started without trouble, after he
remembered where he had parked it.