Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

202 RAYMOND S. NICKERSON
negative manner; from a logical point of view,
they should have used potentially disconfirming
test cases, but they failed to think to do so.
Evans (1989) cited the results of several
efforts by experimenters to modify hypothesistesting
behavior on derivatives of Wason's
number-set task by instructing participants
about the importance of seeking negative or
disconfirming information. He noted that although
behavioral changes were sometimes
induced, more often performance was not
improved. This too was taken as evidence that
results that have been attributed to confirmation
bias do not stem from a desire to confirm, but
rather from the difficulty people have in thinking
in explicitly disconfirmatory terms.
Evans used the same argument to account for
the results typically obtained with Wason's
(1966, 1968) selection task that have generally
been interpreted as evidence of the operation of
a confirmation bias. Participants select named
items in this task, according to this view,
because these are the only ones that come to
mind when they are thinking about what to do.
The bias that is operating is not that of wanting
to confirm a tentative hypothesis but that of
being strongly inclined to think only of information
that is explicitly provided in the problem
statement. In short, Evans (1989) distinguished
between confirmatory behavior, which he acknowledges,
and confirmatory intentions, which
he denies. The "demonstrable deficiencies in the
way in which people go about testing and
eliminating hypotheses," he contended, "are a
function of selective processing induced by a
widespread cognitive difficulty in thinking
about any information which is essentially
negative in its conception" (p. 63). The
tendency to focus on positive information and
fail to consider negative information is regarded
not as a conscious cognitive strategy but as the
result of preattentive processes. Positive is not
synonymous with confirmatory, but as most
studies have been designed, looking for positive
cases is tantamount to looking for confirmation.
The idea that people have a tendency to focus
more on positive than on negative information,
like the idea of a confirmation bias, is an old
one. An observation by Francis Bacon (1620/
1939) can again illustrate the point: "It is the
peculiar and perpetual error of the human
understanding to be more moved and excited by
affirmatives than negatives; whereas it ought
properly to hold itself indifferently disposed
towards both alike" (p. 36). Evans's (1989)
argument that such a bias can account, at least in
part, for some of the phenomena that are
attributed to a confirmation bias is an intuitively
plausible one. A study in which Perkins et al.,
(1983) classified errors of reasoning made in
informal arguments constructed by over 300
people of varying age and educational level
supports the argument. Many of the errors
Perkins et al. identified involved participants'
failure to consider lines of reasoning that could
be used to defeat or challenge their conclusions.
I will be surprised if a positivity bias turns out
to be adequate to account for all of the ways in
which what has here been called a confirmation
bias manifests itself, but there are many
evidences that people find it easier to deal with
positive information than with negative: it is
easier to decide the truth or falsity of positive
than of negative sentences (Wason, 1959,1961);
the assertion that something is absent takes
longer to comprehend than the assertion that
something is present (Clark, 1974); and inferences
from negative premises require more time
to make or evaluate and are more likely to be
erroneous or evaluated incorrectly than are those
that are based on positive premises (Fodor,
Fodor, & Garrett, 1975). How far the idea can be
pushed is a question for research. I suspect that
failure to try to construct counterarguments or to
find counterevidence is a major and relatively
pervasive weakness of human reasoning.
In any case, the positivity bias itself requires
an explanation. Does such a bias have some
functional value? Is it typically more important
for people to be attuned to occurrences than to
nonoccurrences of possible events? Is language
processed more effectively by one who is
predisposed to hear positive rather than negative
assertions? Is it generally more important to be
able to make valid inferences from positive than
from negative premises? It would not be
surprising to discover that a positivity bias is
advantageous in certain ways, in which case
Bacon's dictum that we should be indifferently
disposed toward positives and negatives would
be wrong.
There is also the possibility that the positivity
bias is, at least in part, motivationally based.
Perhaps it is the case that the processing of
negative information generally takes more effort
than does the processing of positive information