Thinking and Deciding

CLASSIFICATION OF BIASES 55
table, only Section I is properly about the role of heuristics. Sections II and III focus
on other kinds of explanations of biases. The meaning of the terms in this table is
the subject of most of the rest of this book, so the reader might refer back to this
table as a way of thinking about the organization of the subsequent material. This
organization is different from the organization of chapters, which follows the type of
normative models at issue.
The classification of biases in this table is meant to be suggestive, not final. Research
might reveal that some biases are in two different categories, or a different
category from that shown here. Some biases violate more than one normative model,
but I have given only one in the table. When the normative model listed is “right
answer,” the subjects are asked a particular question, such as the number of balls in a
jar, and the bias is that they make errors consistently in one direction away from the
correct answer.
To anticipate with just a little explanation of the three sections, Section I deals
with attending to one attribute when other attributes are relevant. For example, in the
bias called single-mindedness, people pay attention only to a single goal, ignoring
the effects of options on other goals, such as choosing a university on the basis of
academic quality alone, ignoring other issues that matter such as cost and location.
All the explanations in this section can be seen as heuristics.
Section I.A. deals with biases in which the attribute in question captures our
attention because it is the result of recent or immanent events. For example, in the
availability effect people estimate the risk of death in airplanes to be higher just after
they have heard about an airplane crash. They are attending to their vivid memory
and ignoring (perhaps) what they have heard about airplanes being safer than cars.
Section I.B. covers biases in which one attribute is used because it is usually a
good indicator for another attribute. In the probability problem with the sequence of
births, the appearance of typicality is usually a good indicator of probability, but it
isn’t in this case. The biases in this section are based on counterexamples to correlations
that are often reliable. These biases are the ones most closely related to failures
of understanding as discussed earlier.
Section I.C. concerns cases in which people isolate, or focus on, one attribute
even though it is not particularly salient or not particularly useful as an indicator.
They seem less aware of other attributes that are relevant according to a normative
model. For example, in the illusion of control, people judge their control over a
positive event (getting a reward) in terms of the number of successes that follow one
type of act, such as pressing the right-hand button, ignoring the question of whether
this action is necessary, for example, whether the positive event might be just as
frequent if they pressed the left-hand button or did nothing at all. These kinds of
biases are often nonobvious because they require some familiarity with the normative
model itself. In this case, the normative model specifies how control requires the
ability to prevent an event as well as to produce it.
Section II is concerned with biases that result from effects of goals or desires on
beliefs. Normatively, beliefs are about truth, about correspondence with the world as
it is. We use these beliefs, together with our goals, to make decisions. We can achieve