Thinking and Deciding

188 JUDGMENT OF CORRELATION AND CONTINGENCY
disease, subjects attend most consistently to the present/present cell (see, for example,
Arkes and Harkness, 1983; Schustack and Sternberg, 1981; Shaklee and
Mims, 1982; Shaklee and Tucker, 1980). Many subjects, however, behave like the
“more sophisticated laypeople” described by Nisbett and Ross, paying attention to
the present/absent cell (that is, symptom present and disease absent) as well as the
present/present cell. In essence, when these subjects want to know whether the symptom
(S) predicts the disease (D), they pay attention most often to the probability of
D given S, that is, p(D|S), but they neglect p(D| ∼S). Other subjects compare
the present/present cell to the absent/present cell. In essence, these subjects attend to
p(S|D) and neglect p(S|∼D), the probability that subjects would have the symptom
even without the disease.
A consequence of the lack of attention to the “symptom absent” or to the “disease
absent” part of the table is that people who have the chance do not inquire about
the half of the table to which they do not attend. For example, Beyth-Marom and
Fischhoff (1983), following Doherty, Mynatt, Tweney, and Schiavo, 1979) asked
subjects to determine whether “Mr. Maxwell,” a fictitious person whom, they were
to imagine, they had just met at a party, was a professor. The subjects were told that
Mr. Maxwell was either a professor or an executive and that he was a member of the
Bear’s Club. The subjects were asked which of several additional pieces of information
they would want to have in order to determine his profession. They were asked,
for example, whether they would rather know “what percentage of the professors at
the party are members of the Bear’s Club” or “what percentage of the executives at
the party are members of the Bear’s Club.” Although 89% of the subjects thought the
first item was relevant (because it came from the “disease present” part of the table
— the “disease” in question being “professor”), only 54% thought that the second
item was (because it came from the “disease absent” part of the table). In fact, both
pieces are relevant (as is the percentage of professors at the party).
In a similar study, Doherty and Falgout (1986) gave subjects an opportunity to
keep or throw away various pieces of data in making an inference about whether
cloud seeding causes rain. Subjects varied greatly in their choice of data, but the
dominant pattern was to keep data only from the present/present and present/absent
cells, that is, those cells in which cloud seeding was done (and rain occurred or did
not occur).
Kuhn, Phelps, and Walters (1985) asked children and adults to judge whether a
fictional product called EngineHelp makes cars run well. The experimenter told the
subjects, “Six people I talked to said they use EngineHelp and they all said their cars
run well.” A majority of the children, and one-third of the adults, concluded from
this single cell that EngineHelp did help. Most of the others said, correctly, that there
was not enough information. Then subjects were told, “Two other people I talked to
said that they use EngineHelp and that their cars run poorly.” The responses were
about the same as after the first cell, even though subjects still knew nothing about
the absent/absent and absent/present cells.
Attentional bias can be understood as failure to look for evidence against an initial
possibility, or as failure to consider alternative possibilities. If there are many