strikingly illustrated by one of the examples ofbase-rate neglect. Casscells
et al. (1978) posed staVand students at Harvard Medical School with the
question: what is the probability of a patient who tests positive for a
certain disease actually having that disease, given that the prevalence of the
disease in the general population is 1/1,000, and that the false-positive rate
for the test is 5 per cent. Almost half of these highly trained people
completely ignored the information about the base rate and said that the
probability was 95 per cent. (Some way out, as the correct answer is 2 per
cent! And on these sorts of judgements, life-and-death decisions get made!)
This suggests some cause for concern about the way general defects in
human reasoning may aVect the decision-making even of those who have
been given expensive professional training.
There is one particular sort of test which has become a dominant
paradigm in theWeld of research on reasoning, much as the false-belief test
has in research on mind-reading. It may be that this is because the test is
uniquely well-designed as a way of probing the way conditional reasoning
works. Or it may be that it just makes things easy for psychological
experimenters because it is simple to carry out, suitable for the usual
undergraduate body of subjects, and readily lends itself to variations which
ring the changes. We have also heard it described as ‘an obsession’. But in
any case, it has deWnitely produced results which call for explanation, and
which are illustrative of many of the issues arising in the debate concerning
human rationality.
This test is the Wason selection task. In its basic form it goes like this.
Subjects are presented with four cards, (a), (b), (c) and (d), as shown in
Wgure 5.1 below, of which they can only see one face. They are told that
each card has a letter on one side and a number on the other. They are then
given instructions such as the following:
Indicate which cards you need to turn over in order to decide whether it
is true for these cards thatIf there is an A on one side of the card, then
there is a 5 on the other. You should select only those cards which it is
essential to turn over.
(If you like, pause to consider which cards you would select. Alternatively,
get a group you can test, to see if previous results are replicated.)
About 75 to 90 per cent of subjects, given this style of presentation, fail
to make the correct selection. This is because only about 10 per cent select
the (d) card. Most people pick card (a) and card (c), or only card (a). The
correct selection is card (a) and card (d), because only these two cards are
potential falsiWers of the conditional, ‘If there is an A on one side of the
card, then there is a 5 on the other.’ Whatever might be on the other sides
of the (b) and (c) cards is irrelevant to whether that conditional is true or
not.
Some psychological evidence 109