Thinking and Deciding

THE PSYCHOLOGY OF HYPOTHESIS TESTING 171
were greatest. In formulating this view of concept learning, the researchers were
implicitly making a distinction between normative (ideal — no memory limits) and
prescriptive views of how the task ought to be done, and they suggested (again, implicitly)
that people generally follow the prescriptive model.
The view of concept learning that lay behind this work has since been largely
discredited. Putnam (1975) has pointed out that the concept of “water” has little
to do with the features that we usually associate with water, such as being liquid,
transparent, freezing at 32 degrees Fahrenheit, and boiling at 212 degrees Fahrenheit,
having a density of 1.00, and so on. As noted earlier, if a substance were found that
had all of these properties but had the chemical formula XYZ rather than H2O, we
would say that it was not water, because we take the chemical formula as definitive,
not the observable properties. Similarly, if a fruit looked and tasted like an orange but
came from a tree that was grown from an apple seed, we would say that the fruit was
an (unusual) apple, not really an orange. Concepts about natural kinds (categories
that exist in nature) are shaped by scientific theories.
Similarly, Wittgenstein (1958) has pointed out that many other concepts that we
have seem to lack defining (deterministic) features altogether. Take, for example, the
concept of a game. There is essentially nothing that is true of all games — no defining
features. (For every feature you can think of, such as being done for fun, there are
games that do not have it, such as jousting.) Games are not united by any features,
but rather by what Wittgenstein called a family resemblance. Each game shares some
features with some other games. Of course, within games, many concepts do have
simple definitions. For example, a strike, in baseball, has a disjunction of features.
Either the batter swings and misses, the batter fouls (except after two strikes), or the
ball goes through the strike zone.
In sum, the idea that concepts have defining features applies only to man-made
categories such as “forward pass,” “resident alien,” and perhaps “past participle.”
The work of Bruner and his colleagues on strategies for discovering such concepts,
therefore, seems to tell us little about the learning of concepts in general. It still may
say something, however, about the testing of hypotheses in other situations, such as
science and diagnosis.
Congruence bias
Although Bruner, Goodnow, and Austin did not set out to look for flaws in human
reasoning, they did make one observation that inspired others to look for such flaws
(p. 86):
Human subjects — and the same may be true of other species as well —
prefer a direct test of any hypothesis they may be working on. [Suppose
that] a subject is faced with deciding whether a white door or a black
door is the correct entrance to a reward chamber and adopts the hypothesis
that the white door is correct. There are two ways of testing this