Thinking and Deciding

92 LOGIC
The same subject did not choose the 6 card for either rule. Again, for the negative
rule, the subject gave a correct justification for not choosing the 6 card: “Any letter
may be on the other side; therefore [there is] no way of knowing if the statement is
true.” (If only the subject had applied this argument to the 3 card!) In the affirmative
condition, however, the subject justified not choosing the 6 card as follows: “If numbers
are fairly random, then there may be any letter on the other side, thereby giving
no indication unless the letter is B.” Again, the subject could have chosen this card
and justified it in the same way as the 3 card was justified for the negative rule.
In sum, subjects’ justifications tend to be correct when they have chosen the correct
cards, and incorrect when they have chosen the incorrect cards. Yet the justifications
seem to play little role in determining the choice itself, for the choice remains
much the same whether the rule is stated affirmatively or negatively. It would appear
that the justification is indeed after the fact, that it is the result of a search for
evidence in favor of a decision already made.
Content effects
Performance in the four-card task is affected by the content of the rules. (See Evans,
1982, ch. 9, for a review.) The clearest effects concern bureaucratic rules. Johnson-
Laird, Legrenzi, and Legrenzi (1972) showed adult subjects in England a drawing
of sealed and open stamped envelopes and asked them, “Which of the following
envelopes must you examine to test the rule: If a letter is sealed, then it has a 5d
stamp on it?” 6
These subjects had an easy time figuring out that one had to check the closed envelope
and the 4d envelope in order to test this rule. However, when Griggs and Cox
(1982) attempted to replicate this experiment using college students in the United
States, there was no difference between the envelope condition and a control condition
with abstract materials (like the original four-card task described earlier). The
American students were helped, however, when they were asked to imagine themselves
as police officers whose job it was to test the rule “If a person is drinking beer,
then the person must be over nineteen years of age.” In this case, most students understood
that the people to check are those drinking beer (to find their age) or those
nineteen or under (to see whether they are drinking beer).
These results indicate that the mere concreteness of the envelope condition did
not help to make the problem easier. Rather, thinking was helped either by familiarity
6 A “5d” stamp is a stamp worth five pence.