Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

194 RAYMOND S. NICKERSON
how they approach their task (D. Kuhn et al.,
1994), jurors often come to favor a particular
verdict early in the trial process (Devine &
Ostrom, 1985). The final verdicts that juries
return are usually the same as the tentative ones
they initially form (Kalven & Zeisel, 1966;
Lawson, 1968). The results of some mock trials
suggest that deliberation following the presentation
of evidence tends to have the effect of
making a jury's average predeliberation opinion
more extreme in the same direction (Myers &
Lamm, 1976).
The tendency to stick with initial tentative
verdicts could exist because in most cases the
initial conclusions stand up to further objective
evaluation; there is also the possibility, however,
that the tentative verdict influences jurors'
subsequent thinking and biases them to look for,
or give undo weight to, evidence that supports it.
This possibility gains credence from the finding
by Pennington and Hastie (1993) that participants
in mock-jury trials were more likely to
remember statements consistent with their
chosen verdict as having been presented as trial
evidence than statements that were inconsistent
with this verdict. This was true both of
statements that had been presented (hits) and of
those that had not (false positives).
Science
Polya (1954a) has argued that a tendency to
resist the confirmation bias is one of the ways in
which scientific thinking differs from everyday
thinking:
The mental procedures of the trained naturalist are not
essentially different from those of the common man,
but they are more thorough. Both the common man and
the scientist are led to conjectures by a few observations
and they are both paying attention to later cases
which could be in agreement or not with the conjecture.
A case in agreement makes the conjecture more likely,
a conflicting case disproves it, and here the difference
begins: Ordinary people are usually more apt to look
for the first kind of cases, but the scientist looks for the
second kind. (p. 40)
If seeking data that would disconfirm a
hypothesis that one holds is the general rule
among scientists, the history of science gives us
many exceptions to the rule (Mahoney, 1976,
1977; Mitroff, 1974). Michael Faraday was
likely to seek confirming evidence for a
hypothesis and ignore such disconfirming evidence
as he obtained until the phenomenon
under study was reasonably well understood, at
which time he would begin to pay more
attention to disconfirming evidence and actively
seek to account for it (Tweney & Doherty,
1983). Louis Pasteur refused to accept or
publish results of his experiments that seemed to
tell against his position that life did not generate
spontaneously, being sufficiently convinced of
his hypothesis to consider any experiment that
produced counterindicative evidence to be
necessarily flawed (Farley & Geison, 1974).
When Robert Millikan published the experimental
work on determining the electric charge of a
single electron, for which he won the Nobel
prize in physics, he reported only slightly more
than half (58) of his (107) observations, omitting
from publication those that did not fit his
hypothesis (Henrion & Fischhoff, 1986).
It is not so much the critical attitude that
individual scientists have taken with respect to
their own ideas that has given science the
success it has enjoyed as a method for making
new discoveries, but more the fact that individual
scientists have been highly motivated to
demonstrate that hypotheses that are held by
some other scientist(s) are false. The insistence
of science, as an institution, on the objective
testability of hypotheses by publicly scrutable
methods has ensured its relative independence
from the biases of its practitioners.
Conservatism among scientists. The fact
that scientific discoveries have often met
resistance from economic, technological, religious,
and ideological elements outside science
has been highly publicized. That such discoveries
have sometimes met even greater resistance
from scientists, and especially from those whose
theoretical positions were challenged or invalidated
by those discoveries, is no less a fact if
less well known (Barber, 1961; Mahoney, 1976,
1977). Galileo, for example, would not accept
Kepler's hypothesis that the moon is responsible
for the tidal motions of the earth's oceans.
Newton refused to believe that the earth could
be much older than 6,000 years on the strength
of the reasoning that led Archbishop Usher to
place the date of creation at 4,004 BC. Huygens
and Leibniz rejected Newton's concept of
universal gravity because they could not accept
the idea of a force extending throughout space
not reducible to matter and motion.
Humphrey Davy dismissed John Dalton's
ideas about the atomic structure of matter as