Daniel Kahneman Nobel Lecture

System 2, a meta-cognitive appreciation of one’s ability to think incompatible

thoughts about the same thing.

As this discussion illustrates, much is known about the determinants of ac-

cessibility, but there is no general theoretical account of accessibility and no

prospect of one emerging soon. In the context of research in judgment and

decision making, however, the lack of a theory does little damage to the use-

fulness of the concept. For most purposes, what matters is that empirical gen-

eralizations about the determinants of accessibility are widely accepted – and,

of course, that there are procedures for testing their validity. For example, the

claims about differential accessibility of different attributes in Figures 2 and 3

appealed to the consensual judgments of perceivers, but claims about acces-

sibility are also testable in other ways. In particular, judgments of relatively in-

accessible properties are expected to be substantially slower and more sus-

ceptible to interference by concurrent mental activity. Some tasks can be

performed even while retaining several digits in memory for subsequent re-

call, but the performance of more effortful tasks will collapse under cognitive

load. 

Considerations of accessibility and analogies between intuition and per-

ception play a central role in the programs of research that I will briefly re-

view in what follows. Framing effects in decision making (Section 3) arise

when different descriptions of the same problem highlight different aspects

of the outcomes. The core idea of prospect theory (Section 4) is that changes

and differences are much more accessible than absolute levels of stimulation.

Judgment heuristics, which explain many systematic errors in beliefs and

preferences are explained in Section 5 by a process of attribute substitution:

people sometimes evaluate a difficult attribute by substituting a more accessi-

ble one. Variations in the ability of System 2 to correct or override intuitive

judgments are explained by variations in the accessibility of the relevant rules

(Section 6). Diverse manifestations of the differential accessibility of averages

and sums are discussed in Section 7.

2. FRAMING EFFECTS

In Figure 2, the same property (the total height of a set of blocks) is highly ac-

cessible in one display and not in another, although both displays contain the

same information. This observation is entirely unremarkable – it does not

seem shocking that some attributes of a stimulus are automatically perceived

while others must be computed, or that the same attribute is perceived in one

display of an object but must be computed in another. In the context of deci-

sion making, however, similar observations raise a significant challenge to the

rational-agent model. The assumption that preferences are not affected by

variations of irrelevant features of options or outcomes has been called ex-

tensionality (Arrow, 1982) and invariance (Tversky & Kahneman, 1986).

Invariance is an essential aspect of rationality, which is violated in demonstra-

tions of framing effects such as the Asian disease problem (Tversky &

Kahneman, 1981): 

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Problem 1

– The Asian Disease

Imagine that the United States is preparing for the outbreak of an un-

usual Asian disease, which is expected to kill 600 people. Two alterna-

tive programs to combat the disease have been proposed. Assume that

the exact scientific estimates of the consequences of the programs are

as follows:

If Program A is adopted, 200 people will be saved

If Program B is adopted, there is a one-third probability that 600 people

will be saved and a two-thirds probability that no people will be saved

Which of the two programs would you favor?

In this version of the problem, a substantial majority of respondents favor

program A, indicating risk aversion. Other respondents, selected at random,

receive a question in which the same cover story is followed by a different de-

scription of the options:

If Program A’ is adopted, 400 people will die

If Program B’ is adopted, there is a one-third probability that nobody

will die and a two-thirds probability that 600 people will die

A clear majority of respondents now favor program B’, the risk-seeking op-

tion. Although there is no substantive difference between the versions, they

evidently evoke different associations and evaluations. This is easiest to see in

the certain option, because outcomes that are certain are over-weighted rela-

tive to outcomes of high or intermediate probability (Kahneman & Tversky,

1979). Thus, the certainty of saving people is disproportionately attractive,

and the certainty of deaths is disproportionately aversive. These immediate af-

fective responses respectively favor A over B and B’ over A’. As in Figures 2a

and 2b, the different representations of the outcomes highlight some fea-

tures of the situation and mask others. 

The question of how to determine whether two decision problems are ‘the

same’ or different does not have a general answer. To avoid this issue, Tversky

and I restricted framing effects to discrepancies between choice problems

that decision makers, upon reflection, consider effectively identical. The

Asian disease problem passes this test: respondents who are asked to compare

the two versions almost always conclude that the same action should be taken

in both. Observers agree that it would be frivolous to let a superficial detail of

formulation determine a choice that has life and death consequences.

In another famous demonstration of an embarrassing framing effect,

McNeill, Pauker, Sox and Tversky (1982) induced different choices between

surgery and radiation therapy, by describing outcome statistics in terms of

survival rates or mortality rates. Because 90% short-term survival is less threat-

ening than 10% immediate mortality, the survival frame yielded a substan-

tially higher preference for surgery. The framing effect was no less pro-

nounced among experienced physicians than it was among patients.

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