Thinking and Deciding

Chapter 2
The study of thinking
Descriptive, normative, and prescriptive
Here is a problem: “All families with six children in a city were surveyed. In seventytwo
families, the exact order of births of boys (B) and girls (G) wasGBGBBG.
What is your estimate of the number of families surveyed in which the exact order
ofbirthswasBGBBBB?”
Many people give figures less than seventy-two as their answers, even if they
believe that boys and girls are equally likely (Kahneman and Tversky, 1972). Apparently
they feel that the second sequence, which contains only one girl, is not
typical of the sequences they expect. In fact, if you believe that boys and girls are
equally likely, your best guess should be exactly seventy two. This is because the
probability of each sequence is 1/2 · 1/2 · 1/2 · 1/2 · 1/2 · 1/2 or 1/64, thesame
in both cases. In other words, the two sequences are equally likely. The births are
independent: The probability of each one is independent of what came before. In
this case, multiplication of probabilities yields the probability that the particular sequence
will happen. This multiplication rule is a design, which we can understand in
terms of its purpose, which we can think of, for now, as corresponding to the correct
proportions of possible sequences.
What makes this problem tricky is that the first sequence looks more like the kind
of sequence you might expect, because it has an equal number of boys and girls, and
the sexes alternate fairly frequently within the sequence.
This problem can help us illustrate three general models, 1 or approaches to the
study of thinking, which I shall call descriptive models, prescriptive models, and
normative models.
1 The term “model” comes from the idea that one way to understand something is to build a model of it.
In this sense, the game of Monopoly is a model of real estate investment. In this book, the term “model”
is used loosely to mean “theory” or “proposal.” Sometimes, however, the models will be more detailed —
for example, computer models or mathematical models.
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