Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

188 RAYMOND S. NICKERSON
notes. As the participants made their choices,
they were given feedback according to a
predetermined schedule that was independent of
the choices they made, but that insured that the
feedback some participants received indicated
that they performed far above average on the
task while that which others received indicated
that they performed far below average.
Following completion of Ross et al.'s (1975)
task, researchers informed participants of the
arbitrary nature of the feedback and told them
that their rate of "success" or "failure" was
predetermined and independent of their choices.
When the participants were later asked to rate
their ability to make such judgments, those who
had received much positive feedback on the
experimental task rated themselves higher than
did those who had received negative feedback,
despite being told that they had been given
arbitrary information. A follow-up experiment
found similar perseverance for people who
observed others performing this task (but did not
perform it themselves) and also observed the
debriefing session.
Nisbett and Ross (1980) pointed out how a
confirmation bias could contribute to the perseverance
of unfounded beliefs of the kind
involved in experiments like this. Receiving
feedback that supports the assumption that one
is particularly good or particularly poor at a task
may prompt one to search for additional
information to confirm that assumption. To the
extent that such a search is successful, the belief
that persists may rest not exclusively on the
fraudulent feedback but also on other evidence
that one has been able to find (selectively) in
support of it.
It is natural to associate the confirmation bias
with the perseverance of false beliefs, but in fact
the operation of the bias may be independent of
the truth or falsity of the belief involved. Not
only can it contribute to the perseverance of
unfounded beliefs, but it can help make beliefs
for which there is legitimate evidence stronger
than the evidence warrants. Probably few beliefs
of the type that matter to people are totally
unfounded in the sense that there is no
legitimate evidence that can be marshalled for
them. On the other hand, the data regarding
confirmation bias, in the aggregate, suggest that
many beliefs may be held with a strength or
degree of certainty that exceeds what the
evidence justifies.
Own-Judgment Evaluation
Many researchers have done experiments in
which people have been asked to express their
degree of confidence in judgments that they
have made. When participants have expressed
confidence as probability estimates or as ratings
that, with some plausible assumptions, can be
transformed into probability estimates, it has
been possible to compare confidence with
performance on the primary task. Thus researchers
can determine for each confidence judgment
the percentage of the correct items on the
primary task to which that judgment was
assigned. Plots of actual percent correct against
percent correct "predicted" by the confidence
judgments are often referred to as calibration
curves; perfect calibration is represented by the
unit line, which indicates that for a given
confidence level, X, the proportion of all the
judgments with that level that were correct was X.
In general, people tend to express a higher
degree of confidence than is justified by the
accuracy of their performance on the primary
task, which is to say that calibration studies have
typically shown overconfidence to be more
common than underconfidence (Einhorn &
Hogarth, 1978; Fischhoff, 1982; Lichtenstein &
Fischhoff, 1977; Lichtenstein, Fischhoff, &
Phillips, 1977; Pitz, 1974; Slovic, Fischhoff, &
Lichtenstein, 1977). Kahneman and Tversky
(1973) refer to the confidence that people feel
for highly fallible performance as the illusion of
validity. Being forced to evaluate one's views,
especially when that includes providing reasons
against one's position, has reduced overconfidence
in some instances (Fischhoff, 1977; Hoch,
1984, 1985; Koriat, Lichtenstein, & Fischhoff,
1980; Tetlock & Kim, 1987). But generally
overconfidence has only been reduced, not
eliminated, and providing reasons against one's
position is not something that most people do
spontaneously.
One explanation of overconfidence starts with
the assumption that people tend to be good
judges of their knowledge as it relates to
situations they are likely to encounter in
everyday life. This explanation of overconfidence
also notes that a minimum requirement
for observing good calibration in experimental
situations is that the questions people are to
answer and that one to be used to judge the
probability of the correctness of their answers