Daniel Kahneman Nobel Lecture

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increases the aggregate value, but the combination rule is non-additive (typi-

cally sub-additive).

3

A category or set which is sufficiently homogeneous to have a prototype

can also be described by its prototype attributes. Where extensional attributes

are akin to a sum, prototype attributes are averages. As the display of lines in

Figure 3 illustrated, prototype attributes are often highly accessible. This ob-

servation is well-documented. Whenever we look at, or think about, an en-

semble or category that has a prototype, information about the prototype be-

comes accessible. The classic discussion of basic-level categories included

demonstrations of the ease with which features of the prototype come to

mind (Rosch & Mervis, 1975). Even earlier, Posner and Keele (1968, 1970)

had reported experiments in which observers were exposed on many trials to

various distortions of a single shape. The prototype shape was never shown,

but observers erroneously believed that it had been presented often. More re-

cently, several studies in social psychology have shown that exposure to the

name of a familiar social category increases the accessibility of the traits that

are closely associated with its stereotype (see Fiske, 1998). 

Because of their high accessibility, the prototype attributes are natural can-

didates for the role of heuristic attributes. A prototype heuristic is the label for

the process of substituting an attribute of a prototype for an extensional at-

tribute of its category (Kahneman & Frederick, 2002). The original instance

of a prototype heuristic was the use of representativeness in category predic-

tion. The probability of Linda being a bank teller is an extensional variable,

but her resemblance to a typical bank teller is a prototype attribute. 

Two tests of prototype heuristics

Because extensional and prototypical attributes are governed by characteris-

tically different rules, the substitution of a prototype attribute for an exten-

sional attribute entails two testable biases: extension neglect and violations of

monotonicity. Tests of the two hypotheses are discussed in turn.

Tests of extension neglect

Doubling the frequencies of all values in a set will not affect prototype 

attributes, because measures of central tendency depend only on relative 

frequencies. In contrast, the value of an extensional attribute will increase

monotonically with extension. The hypothesis that judgments of a target 

attribute are mediated by a prototype heuristic gains support if the judgments

are insensitive to variations of extension. 

The proposition that extension is neglected in a particular judgment has

the character of a null hypothesis: it is strictly true only if all individuals in the

3

If the judgment is monotonically related to an additive scale (such as the underlying count of

the number of birds), the formal structure is known in the measurement literature as an “exten-

sive structure” (Luce, Krantz, Suppes & Tversky, 1990, Chapter 3). There also may be attributes

that lack any underlying additive scale, in which case the structure is known in the literature as a

“positive concatenation structure” (Luce et al., 1990, Chapter 19, vol. III, p. 38).

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sample are completely insensitive to variations of extension. The hypothesis

will be rejected, in a sufficiently large study, if even a small proportion of par-

ticipants show some sensitivity to extension. The chances of some individuals

responding to extension are high a priori, because educated respondents are

generally aware of the relevance of this variable (Kahneman & Frederick,

2002). Everyone agrees that WTP for saving birds should increase with the

number of birds saved, that extending a painful medical procedure by an ex-

tra period of pain makes it worse, and that evidence from larger samples is

more reliable. Complete extension neglect is therefore an unreasonably strict

test of prototype heuristics. Nevertheless, this extreme result can be obtained

under favorable conditions, as the following examples show:

• The study of Tom W. (see Figure 8) illustrated a pattern of base-rate neglect

in categorical prediction. This finding is robust when the task requires a

ranking of multiple outcomes (Kahneman & Tversky, 1973). As noted in

the preceding section, the sophisticated participants in this experiment

were aware of the base-rates and were capable of using this knowledge in

their predictions – but the thought of doing so apparently occurred to al-

most none of them. Kahneman and Tversky also documented almost com-

plete neglect of base-rates in an experiment (the engineer/lawyer study)

in which base-rates were explicitly stated. However, the neglect of explicit

base-rate information in this design is a fragile finding (see Kahneman &

Frederick, 2002; Koehler, 1996, Evans, Handley, Over, & Perham, 2002).

• Participants in a study by Desvousges et al., (1993) indicated their willing-

ness to contribute money to prevent the drowning of migratory birds. The

number of birds that would be saved was varied for different sub-samples.

The estimated amounts that households were willing to pay were $80, $78

and $88, to save 2,000, 20,000, or 200,000 birds, respectively. Frederick and

Fischhoff (1998) reviewed numerous other demonstrations of scope neglect

in studies of willingness to pay (WTP) for public goods. For example,

Kahneman and Knetsch found that survey respondents in Toronto were

willing to pay almost as much to clean up the lakes in a small region of

Ontario or to clean up all the lakes in that province (reported by

Kahneman, 1986). 

• In a study described by Redelmeier and Kahneman (1996), patients un-

dergoing colonoscopy reported the intensity of pain every 60 seconds dur-

ing the procedure (see Figure 9), and subsequently provided a global evalu-

ation of the pain they had suffered. The correlation of global evaluations

with the duration of the procedure (which ranged from 4 to 66 minutes in

that study) was .03. On the other hand global evaluations were correlated

(r =.67) with an average of the pain reported on two occasions: when pain

was at its peak, and just before the procedure ended. For example, patient

A in Figure 9 reported a more negative evaluation of the procedure than

patient B. The same pattern of duration neglect and Peak/End evaluations

has been observed in other studies (Fredrickson & Kahneman, 1993; see

Kahneman, 2000b, 2000c for a discussion). 

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