Thinking and Deciding

HEURISTICS AND BIASES IN PROBABILITY 157
more likely than a UNC victory in fencing. Of course, 44% plus 75% is larger than
100%, and only one such game would be played. Thus, familiarity with basketball
led subjects to think of the basketball event as more likely than the fencing event, no
matter which basketball event was described.
Hindsight bias
Another cause of distortion that is observed in everyday reasoning can be more easily
avoided. This is illustrated in the phenomenon of hindsight bias. Fischhoff (1975)
asked subjects to read true historical accounts of incidents with which they were unfamiliar,
drawn from history books as well as personal psychological case histories.
One scenario concerned the battle between the British forces and the Gurkhas (from
Nepal) on the northern frontier of Bengal in 1814. Subjects were asked to assign
numerical probabilities to the major possible outcomes: British victory; Gurkha victory;
military stalemate with no peace settlement; or military stalemate with a peace
settlement. The history provided was consistent with any one of these outcomes.
Some subjects were told the outcome: The British won. These subjects were
asked to rate the probability of the various outcomes as they would have if they had
not been told the true outcome. The mean probability that these subjects assigned
to the true outcome was .57. Other subjects were not told the outcome. They rated
the probability of a British victory as only .34. Evidently, subjects who were told the
outcome could not avoid what they knew in hindsight.
Similar results have been obtained from many other kinds of studies. Fischhoff
(1977) give subjects two-choice questions concerning general knowledge, like those
used in the studies of confidence described earlier in this chapter. Some subjects
were told the answers and were asked what probabilities they would have assigned
to these answers had they not been given them. Once again, these probabilities were
higher than those assigned to the same items by other subjects who had not been
given the answers. It appears that people tend to underestimate how much they have
learned from being told something. They tend to think they knew it all along.
Slovic and Fischhoff (1977) presented subjects with descriptions of scientific
experiments. In one experiment some blood from a rat that had just given birth was
injected into another female rat. The question was whether the second rat would
exhibit maternal behavior. Some subjects, the hindsight group, were told that the
first time this experiment was done, the rat did exhibit the behavior. They were
asked to estimate the probability that all of the next 10 rats would replicate this initial
result. Other subjects, the foresight group, were asked how they would answer this
question if the experiment worked on the first rat. The hindsight group gave higher
proportions. For this experiment, the hindsight subjects gave a mean probability of
.44, and the foresight subjects gave a mean probability of .30.
In a second study, subjects were asked to give reasons why the study worked out
the way it did and how they would explain the result if it came out the other way.
These instructions reduced the effect. The same kind of instructions were effective
in reducing hindsight bias among a group of neuropsychologists who were asked