Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

186 RAYMOND S. NICKERSON
though it may well be seen (erroneously) as so
by those who display it. Inasmuch as an
observation can be confirming in a psychological
sense (i.e., interpreted as confirming evidence)
independently of whether it is logically
confirmatory, perhaps the confirmation bias
should be thought of as a tendency to seek
evidence that increases one's confidence in a
hypothesis regardless of whether it should. The
distinction between logical confirmation and
psychological confirmation deserves greater
emphasis than it has received.
Although researchers have interpreted the
considerable collection of results that have been
obtained from experiments with variations of
the selection task as generally reflective of a
confirmation bias, alternative hypotheses have
also been proposed. L. J. Cohen (1981) argued
that it is not necessary to suppose that people do
anything more in testing a conditional rule than
check whether the consequent holds true whenever
the antecedent does. The fact that people
sometimes select the card representing Q (4 in
the case of the present example) as well as the
one representing P (A) is because they falsely
perceive ifP then Q to be equivalent to ifQ then
P (conversion error); and the fact that they do
not select the card representing not-Q (7 in the
present example) is attributed to a failure to
apply the law of the contrapositive.
Several investigators have argued that people
interpret the conditional relationship ifP then Q
as equivalent to the biconditional relationship if
and only if P then Q, or that they fail to
distinguish between ifP then Q and ifQ then P,
especially as the if-then construction is used in
everyday language (Chapman & Chapman,
1959; L. J. Cohen, 1981; Henle, 1962; Legrenzi,
1970; Politzer, 1986; Revlis, 1975a, 1975b).
Others have assumed the operation of a
matching bias whereby people tend to focus on
(and select) cards explicitly named in the
statement of the rule to be tested (Evans, 1972;
Evans & Lynch, 1973; Hoch & Tschirgi, 1983).
Evans (1989) has suggested that people make
their selections, without thinking much about
them, on the basis of a "preconscious heuristic
judgment of relevance, probably linguistically
determined" (p. 108).
Recent theorizing about the selection task has
given rise to several new explanations of how
people interpret it and deal with it, especially as
presented in various concrete frames of refer-
ence. An idea common to several of these
explanations is that people's performance of the
task is indicative of behavior that has proved to
be adaptively effective in analogous real-world
situations. Cosmides (1989), for example, has
argued that the typical results are consistent with
the idea that people are predisposed to look for
cheating on a social contract and are particularly
skilled at detecting it. Consequently, they do
well on selection tasks in which checking the
not-Q condition is analogous to looking for an
instance of violating a tacit social agreement,
like failing to meet an obligation. Gigerenzer
and Hug (1992) have defended a similar point of
view.
Liberman and Klar (1996) proposed an
alternative explanation of the results of research
on cheating detection. They argued that a
cheating-detection perspective is neither necessary
nor sufficient to yield the P and not-Q
selection; they contend that the logically correct
choice will be likely if the conditional ifP then
Q is understood to represent a deterministic (as
opposed to a probabilistic) unidirectional relationship
between P and Q, if what constitutes a
violation of the rule is clear, and if it is
understood that the task is to look for such a
violation.
Other investigators have begun to view the
selection task as more a problem of data
selection and decision making than of logical
inference (Kirby, 1994a, 1994b; Manktelow &
Over, 1991, 1992). Oaksford and Chater (1994),
for example, have proposed a model of the task
according to which people should make selections
so as to maximize their gain in information
regarding the tenability of the hypothesis that
the conditional ifP then Q is true relative to the
tenability that it is false. I have done an analysis
that leads to a similar conclusion, at least when
the task is perceived as that of deciding whether
the conditional is true in a general sense, as
opposed to being true of four specific cards
(Nickerson, 1996).
Wason's selection task has proved to be one
of the most fertile paradigms in experimental
psychology. It would be surprising if all the
results obtained with the many variations of the
task could be accounted for by a single simple
hypothesis. I believe that, taken together, the
results of experimentation support the hypothesis
that one of the several factors determining
performance on this task is a confirmation bias