Daniel Kahneman Nobel Lecture

periences (joint evaluation), they did not notice that the long episode con-

tained all the pain of the short one, and then some extra pain. Most respon-

dents would have made a different choice if they had understood the struc-

ture of the options. 

The normative logic of belief and choice is extensional, and it requires ap-

propriate valuation of extensional attributes, which include both probability

and utility. The examples that were discussed in this section demonstrate a

pervasive departure from extensional logic, in the intuitive evaluation of both

evidence and outcomes. The substitution of prototype attributes for exten-

sional attributes appears to be a general characteristic of System 1, which is in-

compatible with both Bayesian beliefs and utility maximization. 

CONCLUSIONS

The starting point of the present analysis was the observation that complex

judgments and preferences are called ‘intuitive’ in everyday language if they

come to mind quickly and effortlessly, like percepts. Another basic observa-

tion was that judgments and intentions are normally intuitive in this sense,

but can be modified or overridden in a more deliberate mode of operation.

The labels ‘System 1’ and ‘System 2’ were associated with these two modes of

cognitive functioning. 

The preceding sections elaborated a single generic proposition: “Highly ac-

cessible impressions produced by System 1 control judgments and prefer-

ences, unless modified or overridden by the deliberate operations of System

2.” This template sets an agenda for research: to understand judgment and

choice we must study the determinants of high accessibility, the conditions

under which System 2 will override or correct System 1, and the rules of these

corrective operations. Much is known about each of the three questions.

First, consider the ways in which the concept of accessibility was used here.

Framing effects were attributed to the fact that alternative formulations of the

same situation make different aspects of it accessible. The core idea of

prospect theory, that the normal carriers of utility are gains and losses, in-

voked a general principle that changes are relatively more accessible than ab-

solute values. Judgment heuristics were explained as the substitution of a

highly accessible heuristic attribute for a less accessible target attribute.

Finally, the proposition that averages are more accessible than sums unified

the analysis of prototype heuristics. A recurrent theme was that different as-

pects of problems are made accessible in between-subjects and in within-sub-

ject experiments, and more specifically in separate and joint evaluations of

stimuli. In all these cases, the discussion appealed to rules of accessibility that

are independently plausible and sometimes quite obvious.

The status of accessibility factors in psychological theorizing is, in princi-

ple, similar to the status of perceptual grouping factors. In both cases there is

no general theory, only a list of powerful empirical generalizations that pro-

vide a sound basis for experimental predictions and for models of higher-

level phenomena. Unlike Gestalt principles, which were catalogued a long

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time ago, a comprehensive list of the factors that influence accessibility is yet

to be drawn. The list will be long, but many of its elements are already known.

For example, it is safe to assume that similarity is more accessible than proba-

bility, that changes are more accessible than absolute values, and that aver-

ages are more accessible than sums. Furthermore, each of these assumptions

can be verified independently by multiple operations, including measure-

ments of reaction time, susceptibility to interference by secondary tasks, and

asymmetric priming. Assumptions about accessibility are incompletely theo-

rized, but they need not be vague and they can do genuine explanatory work.

The present discussion of accessibility effects has been restricted to the dif-

ferential accessibility of attributes (dimensions) on which judgment objects

vary, such as length or price, similarity and probability, (Kahneman &

Frederick, 2002). A similar analysis could be applied to the accessibility of

particular values of attributes, such as ‘six feet’ or ‘two dollars’. Highly acces-

sible values are generally overweighted, and when considered as possible an-

swers to a question they become potent anchors (Epley & Gilovich, 2002;

Strack & Mussweiler, 1997; Chapman & Johnson, 2002). The effects of

salience and anchoring play a central role in treatments of judgment and

choice. Indeed, anchoring effects are among the most robust phenomena of

judgment, and overweighting of salient values is likely to be the mechanism

that explains why low-probability events sometimes loom large in decision

making. The analysis of accessibility could readily be extended to deal with

these observations.

The claim that cognitive illusions will occur unless they are prevented by

System 2 sounds circular, but it is not. Circular inferences are avoidable be-

cause the role of System 2 can be independently verified in several ways. For

example, the assumption that System 2 is vulnerable to interference by com-

peting activities suggests that manifestations of intuitive thought that are nor-

mally inhibited may be expressed when people are placed under cognitive

load. Another testable hypothesis is that intuitive judgments that are sup-

pressed by System 2 still have detectable effects, e.g., in priming subsequent

responses. 

Principles of accessibility determine the relative power of the cues to which

the monitoring functions of System 2 respond. For example, we know that

differences between options are more salient in joint than in separate evalua-

tion, and that any variable which is manipulated in a factorial design will at-

tract some attention. Other cues can be found in the wording of problems

and in the context of previous tasks. Many apparent inconsistencies in the lit-

erature on judgment heuristics are easily resolved within this framework

(Kahneman & Frederick, 2002). A judgment bias that appears in some situa-

tions but not in others usually provides information about the factors that

control corrective operations. As already noted, the attribution of the vari-

ability of intuitive judgments to System 2 is a source of readily testable hy-

potheses. It suggests, for example, that intelligence will be correlated with

susceptibility to biases only in problems that provide relatively weak cues to

the correct solution. In the absence of cues, there is no opportunity for intel-

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