Thinking and Deciding

EFFECTS OF PRIOR BELIEF 197
evidence failed to distinguish the evidence for a theory from a restatement of the
theory itself. For example, a subject who believed that school failure was caused by
problems at home, when asked for evidence, said that “when the mother and father
are divorced they [the children] can have psychological problems, you know, and
they can’t really function in school” (Kuhn, 1991, p. 66). Interestingly, most of the
subjects said that the “evidence” they provided did prove the correctness of their
theory, and their level of confidence was unrelated to the quality of this evidence.
Many subjects were also unsuccessful in generating alternative theories to their own.
The most common failure was that the response overlapped or coincided with the
subject’s own theory, often even using the same words. Some subjects denied that
anyone would propose an alternative or indicated that, although they could not think
of an alternative, they would reject it if it were presented to them. Likewise, some
subjects rejected the possibility of any counterargument — for example, because
both the antecedent (such as family problems) and consequent (school failure) were
present. Finally, “in extreme cases ...the subject exhibits a kind of proprietorship
over the theory that undermines its independent existence, rendering it incontestable.
To challenge the theory is to challenge the subject’s own self” (p. 144).
At the end of the interview, subjects were asked to interpret two kinds of evidence
on the crime and school issues. Underdetermined evidence simply presented
a scenario about one individual person, without mentioning any causal factors. The
overdetermined evidence reported a study of twenty-five prisoners or pupils, with no
control group, in which one expert found evidence for each of three possible causes
(for example, a school psychologist found evidence of learning problems in children
who failed at school).
Despite the fact that both sets of evidence were essentially useless, many subjects
tended to interpret them as supporting their theories. For the underdetermined evidence,
many subjects simply asserted that their theory was correct for the case. For
the overdetermined evidence, subjects tended to focus on the parts consistent with
their view. Most subjects (especially the noncollege group) interpreted the evidence
as agreeing with or supporting their theories. As in the study of food and colds, subjects
often answered questions about the evidence as if they had been asked about the
truth of their own theories. If this response is typical of subjects’ thinking, then they
are unable to evaluate evidence independently of their theories. If the evidence disagrees
with the theory, they will not note the disagreement. Instead, they will either
neglect the disagreement or eventually change their theory without acknowledging
the role of the evidence. This failure is important: In order for people to learn to
evaluate evidence correctly, it helps to be able to say what the evidence would imply
in the absence of any prior commitment to a particular theory.
Subjects were also interviewed directly about their theories of knowledge. They
were asked how confident they were in their own theories; whether experts could or
did know the truth; and whether different theories could be correct for different people.
Responses to these questions seemed to reflect three kinds of implicit theories
of knowledge. “Absolutist” theories hold that experts can be certain of the truth and
that the subject could be certain of the truth too. “Multiplist” theories of knowledge