Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

176 RAYMOND S. NICKERSON
assumption that people can and do engage in
case-building unwittingly, without intending to
treat evidence in a biased way or even being
aware of doing so, is fundamental to the
concept.
The question of what constitutes confirmation
of a hypothesis has been a controversial matter
among philosophers and logicians for a long
time (Salmon, 1973). The controversy is exemplified
by Hempel's (1945) famous argument
that the observation of a white shoe is
confirmatory for the hypothesis "All ravens are
black," which can equally well be expressed in
contrapositive form as "All nonblack things are
nonravens." Goodman's (1966) claim that
evidence that something is green is equally good
evidence that it is "grue"—grue being defined
as green before a specified future date and blue
thereafter—also provides an example. A large
literature has grown up around these and similar
puzzles and paradoxes. Here this controversy is
largely ignored. It is sufficiently clear for the
purposes of this discussion that, as used in
everyday language, confirmation connotes evidence
that is perceived to support—to increase
the credibility of—a hypothesis.
I also make a distinction between what might
be called motivated and unmotivated forms of
confirmation bias. People may treat evidence in
a biased way when they are motivated by the
desire to defend beliefs that they wish to
maintain. (As already noted, this is not to
suggest intentional mistreatment of evidence;
one may be selective in seeking or interpreting
evidence that pertains to a belief without being
deliberately so, or even necessarily being aware
of the selectivity.) But people also may proceed
in a biased fashion even in the testing of
hypotheses or claims in which they have no
material stake or obvious personal interest. The
former case is easier to understand in commonsense
terms than the latter because one can
appreciate the tendency to treat evidence
selectively when a valued belief is at risk. But it
is less apparent why people should be partial in
their uses of evidence when they are indifferent
to the answer to a question in hand. An adequate
account of the confirmation bias must encompass
both cases because the existence of each is
well documented.
There are, of course, instances of one wishing
to disconfirm a particular hypothesis. If, for
example, one believes a hypothesis to be untrue,
one may seek evidence of that fact or give undue
weight to such evidence. But in such cases, the
hypothesis in question is someone else's belief.
For the individual who seeks to disconfirm such
a hypothesis, a confirmation bias would be a
bias to confirm the individual's own belief,
namely that the hypothesis in question is false.
A Long-Recognized Phenomenon
Motivated confirmation bias has long been
believed by philosophers to be an important
determinant of thought and behavior. Francis
Bacon (1620/1939) had this to say about it, for
example:
The human understanding when it has once adopted an
opinion (either as being the received opinion or as
being agreeable to itself) draws all things else to
support and agree with it. And though there be a greater
number and weight of instances to be found on the
other side, yet these it either neglects and despises, or
else by some distinction sets aside and rejects; in order
that by this great and pernicious predetermination the
authority of its former conclusions may remain
inviolate.. . . And such is the way of all superstitions,
whether in astrology, dreams, omens, divine judgments,
or the like; wherein men, having a delight in
such vanities, mark the events where they are fulfilled,
but where they fail, although this happened much
oftener, neglect and pass them by. (p. 36)
Bacon noted that philosophy and the sciences
do not escape this tendency.
The idea that people are prone to treat
evidence in biased ways if the issue in question
matters to them is an old one among psychologists
also:
If we have nothing personally at stake in a dispute
between people who are strangers to us, we are
remarkably intelligent about weighing the evidence and
in reaching a rational conclusion. We can be convinced
in favor of either of the fighting parties on the basis of
good evidence. But let the fight be our own, or let our
own friends, relatives, fraternity brothers, be parties to
the fight, and we lose our ability to see any other side of
the issue than our own. .. . The more urgent the
impulse, or the closer it comes to the maintenance of
our own selves, the more difficult it becomes to be
rational and intelligent. (Thurstone, 1924, p. 101)
The data that I consider in what follows do
not challenge either the notion that people
generally like to avoid personally disquieting
information or the belief that the strength of a
bias in the interpretation of evidence increases
with the degree to which the evidence relates
directly to a dispute in which one has a personal