Daniel Kahneman Nobel Lecture

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of course, by the actual properties of the object of judgment: it is easier to

see a tower in Figure 2a than in Figure 2b, because the tower in the latter is

only virtual. Physical salience also determines accessibility: if a large green

letter and a small blue letter are shown at the same time, ‘green’ will come

to mind first. However, salience can be overcome by deliberate attention: an

instruction to look for the smaller letter will enhance the accessibility of all

its features. Motivationally relevant and emotionally arousing stimuli spon-

taneously attract attention. All the features of an arousing stimulus become

accessible, including those that are not linked to its motivational or emo-

tional significance. This fact is known, of course, to the designers of bill-

boards.

The perceptual effects of salience and of spontaneous and voluntary at-

tention have counterparts in the processing of more abstract stimuli. For ex-

ample, the statements ‘Team A beat team B’ and ‘Team B lost to team A’ con-

vey the same information. Because each sentence draws attention to its

subject, however, the two versions make different thoughts accessible.

Accessibility also reflects temporary states of priming and associative activa-

tion, as well as enduring operating characteristics of the perceptual and cog-

nitive systems. For example, the mention of a familiar social category tem-

porarily increases the accessibility of the traits associated with the category

stereotype, as indicated by a lowered threshold for recognizing manifesta-

tions of these traits (Higgins, 1996; for a review, see Fiske, 1998). And the

“hot” states of high emotional and motivational arousal greatly increase the

accessibility of thoughts that relate to the immediate emotion and current

needs, and reduce the accessibility of other thoughts (George Loewenstein,

1996).

Some attributes, which Tversky and Kahneman (1983) called natural as-

sessments, are routinely and automatically registered by the perceptual system

or by System 1, without intention or effort. Kahneman and Frederick (2002)

compiled a list of natural assessments, with no claim to completeness. In ad-

dition to physical properties such as size, distance and loudness, the list in-

cludes more abstract properties such as similarity (e.g., Tversky & Kahneman,

1983), causal propensity (Kahneman & Varey, 1990; Heider, 1944; Michotte,

1963), surprisingness (Kahneman & Miller, 1986), affective valence (e.g.,

Bargh, 1997; Cacioppo, Priester, & Berntson, 1993; Kahneman, Ritov, &

Schkade, 1999; Slovic, Finucane, Peters, & MacGregor, 2002; Zajonc, 1980),

and mood (Schwarz & Clore, 1983). Accessibility itself is a natural assessment

– the routine evaluation of cognitive fluency in perception and memory (e.g.,

Jacoby & Dallas, 1981; Johnson, Dark, & Jacoby, 1985; Schwarz & Vaughn,

2002; Tversky & Kahneman, 1973).

1

1

The availability heuristic is based on an assessment of accessibility, in which frequencies or prob-

abilities are judged by the ease with which instances come to mind. Tversky and I were responsib-

le for this terminological confusion (Tversky and Kahneman, 1973).

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Figure 4 illustrates the effect of context on accessibility. An ambiguous

stimulus that is perceived as a letter in a context of letters is seen as a number

in a context of numbers. The figure also illustrates another point: the ambi-

guity is suppressed in perception. This aspect of the demonstration is spoiled

for the reader who sees the two versions in close proximity, but when the two

lines are shown separately, observers will not spontaneously become aware of

the alternative interpretation. They ‘see’ the interpretation that is the most

likely in its context, but have no subjective indication that it could be seen dif-

ferently. Similarly, in bi-stable pictures such as the mother/daughter figure or

the Necker cube, there is no perceptual representation of the instability. And

almost no one (for a report of a tantalizing exception, see Wittreich, 1961) is

able to see the Ames room as anything but rectangular, even when fully in-

formed that the room is distorted, and that the photograph does not provide

enough information to specify its true shape. As the transactionalists who

built the Ames room emphasized, perception is a choice of which we are not

aware, and we perceive what has been chosen.

The unpredictability that is perceived as inherent to some causal systems is

psychologically distinct from epistemic uncertainty, which is attributed to

one’s own ignorance (Kahneman & Tversky, 1982b). Competing propensities

are often perceived – as they are when we watch a close horse race. And coun-

terfactual alternatives to what happened are also perceived – we can see a

horse that was catching up at the finish as ‘almost winning the race’

(Kahneman & Varey, 1990). In contrast to competing propensities, however,

competing interpretations of reality appear to suppress each other: we do not

see each horse in a close finish as both winning and losing. Epistemic uncer-

tainty and ambiguity are not natural assessments. 

Uncertainty is poorly represented in intuition, as well as in perception.

Indeed, the concept of judgment heuristics was invented to accommodate the

observation that intuitive judgments of probability are mediated by attributes

such as similarity and associative fluency, which are not intrinsically related to

uncertainty. The central finding in studies of intuitive decisions, as described

by Klein (1998), is that experienced decision makers working under pressure,

such as captains of firefighting companies, rarely need to choose between op-

tions because in most cases only a single option comes to their mind. The op-

tions that were rejected are not represented. Doubt is a phenomenon of

Figure 4.

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