Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

198 RAYMOND S. NICKERSON
potent as many lay people and most motivational
theorists presume them to be. (p. 233)
This argument is especially interesting in the
present context, because it invokes a form of
confirmation bias to account for the tendency of
some investigators to attribute certain behaviors
to motivational causes and to ignore what, in
Nisbett and Ross's view, are equally likely
alternative explanations.
The role of motivation in reasoning has been
a subject of debate for some time. Kunda (1990)
noted that many of the phenomena that once
were attributed to motivational variables have
been reinterpreted more recently in cognitive
terms; according to this interpretation, conclusions
that appear to be drawn only because
people want to draw them may be drawn
because they are more consistent with prior
beliefs and expectancies. She noted too that
some theorists have come to believe that
motivational effects are mediated by cognitive
processes. According to this view, "[p]eople
rely on cognitive processes and representations
to arrive at their desired conclusions, but
motivation plays a role in determining which of
these will be used on a given occasion" (Kunda,
1990, p. 480).
Kunda defended this view, arguing that the
evidence to date is consistent with the assumption
that motivation affects reasoning, but it
does so through cognitive strategies for accessing,
constructing, and evaluating beliefs:
Although cognitive processes cannot fully account for
the existence of self-serving biases, it appears that they
play a major role in producing these biases in that they
provide the mechanisms through which motivation
affects reasoning. Indeed, it is possible that motivation
merely provides an initial trigger for the operation of
cognitive processes that lead to the desired conclusions,
(p. 493)
The primary cognitive operation hypothesized
to mediate motivational effects is the
biased searching of memory. Evidence of
various types converges, she argued, on the
conclusion that "goals enhance the accessibility
of those knowledge structures—memories, beliefs,
and rules—that are consistent with desired
conclusions" (p. 494); "Motivation will cause
bias, but cognitive factors such as the available
beliefs and rules will determine the magnitude
of the bias" (p. 495).
Several of the accounts of confirmation bias
that follow stress the role of cognitive limita-
tions as causal factors. It is possible, however,
and probable, in my view, that both motivational
and cognitive factors are involved and that each
type can mediate effects of the other.
Information-Processing Bases
for Confirmation Bias
The confirmation bias is sometimes attributed
in part to the tendency of people to gather
information about only one hypothesis at a time
and even with respect to that hypothesis to
consider only the possibility that the hypothesis
is true (or only the possibility that it is false) but
not to consider both possibilities simultaneously
(Tweney, 1984; Tweney & Doherty, 1983).
Doherty and Mynatt (1986) argued, for example,
that people are fundamentally limited to
think of only one thing at a time, and once
having focused on a particular hypothesis, they
continue to do so. This, they suggested, explains
why people often select nondiagnostic over
diagnostic information in Bayesian decision
situations. Suppose that one must attempt to
decide which of two diseases, A or B, a patient
with Symptoms X and Y has. One is informed of
the relative frequency of Symptom X among
people who have Disease A and is then given the
choice of obtaining either of the following items
of information: the relative frequency of people
with A who have Symptom Y or the relative
frequency of people with B who have symptom
X. Most people who have been given choices of
this sort opt for the first; they continue to focus
on the hypothesis that the patient has A, even
though learning the relative frequency of Y
given A does not inform the diagnosis, whereas
learning the relative frequency of X given B
does.
Assuming a restricted focus on a single
hypothesis, it is easy to see how that hypothesis
might become strengthened even if it is false. An
incorrect hypothesis can be sufficiently close to
being correct that it receives a considerable
amount of positive reinforcement, which may be
taken as further evidence of the correctness of
the hypothesis in hand and inhibit continued
search for an alternative. In many contexts
intermittent reinforcement suffices to sustain the
behavior that yields it.
People also can increase the likelihood of
getting information that is consistent with
existing beliefs and decrease the likelihood of