Daniel Kahneman Nobel Lecture

changes in the general climate of psychological opinion. It is worth noting

that in the early 1970’s the idea of purely cognitive biases appeared novel and

distinctive, because the prevalence of motivated and emotional biases of judg-

ment was taken for granted by the social psychologists of the time. There fol-

lowed a period of intense emphasis on cognitive processes, in psychology gen-

erally and in the field of judgment in particular. It took another thirty years to

achieve what now appears to be a more integrated view of the role of affect in

intuitive judgment.

5. THE ACCESSIBILITY OF CORRECTIVE THOUGHTS

The present treatment assumes that System 2 is involved in all voluntary ac-

tions – including overt expressions of the intuitive judgments that originated

in System 1. This assumption implies that errors of intuitive judgment involve

failures of both systems: System 1, which generated the error, and System 2

which failed to detect and correct it (Kahneman & Tversky, 1982a). To illus-

trate this point, Kahneman and Frederick (2002) revisited the perceptual

analogy that Tversky and Kahneman (1974) had used to explain how judg-

ment heuristics generate biases: blur is a good cue to the distance of moun-

tains, but reliance on this cue causes predictable errors of distance estimation

on sunny or hazy days. The analogy was apt, but the analysis of the perceptu-

al example neglected an important fact. Observers know, of course, whether

the day is sunny or hazy, and they could use this knowledge to counteract the

bias – but most often they do not. Contrary to what the early treatment im-

plied, the use of blur as a cue does not inevitably lead to bias in the judgment

of distance – the illusion could just as well be described as a failure to assign

adequate negative weight to ambient haze. The effect of haziness on impres-

sions of distance is a failing of System 1: the perceptual system is not designed

to correct for this variable. The effect of haziness on judgments of distance is a

separate failure of System 2. Analogous failures can be identified in other er-

rors of intuitive judgment. 

It is useful to consider how System 2 might have intervened in the prob-

lems of Tom W. and Linda that were described in an earlier section.

“Tom W. does look like a library science person, but there are many

more graduate students in Humanities and Social Sciences. I should ad-

just my rankings accordingly.” “Linda cannot be more likely to be a fem-

inist bank teller than to be a bank teller. I must rank these two out-

comes accordingly”

These hypothetical samples of reasoning illustrate two ways in which intuitive

judgments can be corrected. In the Tom W. example, the individual becomes

aware of a factor that was not part of the intuitive judgment, and makes an ef-

fort to adjust accordingly. In the Linda example, the individual recognizes

that the question can be answered by applying a decisive logical rule, which

makes intuitions to the contrary irrelevant. Both would come under the

rubric of “statistical heuristics”, which people are sometimes capable of de-

471

---

ploying in their reasoning about uncertain events (Nisbett, Krantz, Jepson, &

Kunda, 1983/2002).

Neither of these examples of reasoning exceeds the intellectual reach of

the graduate students at major universities whose rankings were shown in

Figure 8. However, the data indicate that very few respondents actually came

up with corrections. The puzzle is the same as in the blur illusion: why did

these people not put their knowledge to good use? In the context of the pre-

sent treatment, the question can be rephrased: Why did the statistical heuris-

tics not become accessible when they were needed?

An important part of the answer is that attribute substitution is a silent

process: the respondents who judge probability as if they had been asked to

judge representativeness are not self-conscious about what they are doing.

The substitute attribute is pertinent to the task, and its value comes to mind

with little or no effort and with high confidence. There is therefore little rea-

son for respondents to question their judgment, perhaps even less than in the

bat-and-ball problem that was mentioned earlier. In contrast, the accessibility

of statistical heuristics is often low, but it can be enhanced in at least two ways:

by increasing the vigilance of the monitoring activities, or by providing

stronger cues to the relevant rules. 

A substantial research program was mounted by Nisbett, Krantz and their

colleagues to investigate the factors that control the accessibility of statistical

heuristics (Nisbett et al., 1983/2002). For example, Nisbett et al. studied for-

mally identical problems in several domains. They found that statistical rea-

soning was most likely to be evoked in the context of games of chance, occa-

sionally evoked in situations involving sports, but relatively rare when the

problems concerned the psychology of individuals. They also showed that the

explicit mention of a sampling procedure facilitated statistical thinking

(Nisbett  et al., 1983; see also Gigerenzer, Hell, & Blank, 1988). Zukier and

Pepitone (1984) found that respondents were more likely to use base-rate in-

formation when instructed to think as statisticians than when instructed to

emulate psychologists. Agnoli and Krantz (1989) found that brief training in

the logic of sets improved performance in a simple version of the Linda prob-

lem. Considerations of accessibility are evidently relevant to the activation of

statistical reasoning, not only to attribute substitution. 

Nisbett, Krantz and their colleagues drew a sharp distinction between their

statistical heuristics and the intuitive heuristics, which they described as

“rapid and more or less automatic judgmental rules of thumb” (2002, p. 510).

In the same vein, the present treatment assigns the competing heuristics to

different cognitive systems. Attribute substitution has been described as an

operation of System 1, which occurs automatically and effortlessly. In con-

trast, the statistical heuristics illustrate the rule-governed reasoning of System

2 (Sloman, 1996), which is deliberate and demands some effort. It is worth

noting that the intervention of System 2 and the application of statistical

heuristics and other rules do not guarantee a correct response. The rules that

people apply in deliberate reasoning are sometimes false.

An implication of the view of intuition that has been proposed here is that

472

---