Unpacking, repacking, and anchoring: advances in support theory.

Support theory represents probability judgment in terms of the support, or strength of evidence, of the focal relative to the alternative hypothesis. It assumes that the judged probability of an event generally increases when its description is unpacked into disjoint components (implicit subadditivity). This article presents a significant extension of the theory in which the judged probability of an explicit disjunction is less than or equal to the sum of the judged probabilities of its disjoint components (explicit subadditivity). Several studies of probability and frequency judgment demonstrate both implicit and explicit subadditivity. The former is attributed to enhanced availability, whereas the latter is attributed to repacking and anchoring.

On the reality of cognitive illusions.

The study of heuristics and biases in judgement has been criticized in several publications by G. Gigerenzer, who argues that "biases are not biases" and "heuristics are meant to explain what does not exist" (1991, p. 102). The article responds to Gigerenzer's critique and shows that it misrepresents the authors' theoretical position and ignores critical evidence. Contrary to Gigerenzer's central empirical claim, judgments of frequency--not only subjective probabilities--are susceptible to large and systematic biases. A postscript responds to Gigerenzer's (1996) reply.

Confidence and accuracy in trait inference: judgment by similarity.

We examined the confidence and accuracy with which people make personality trait inferences and investigate some consequences of the hypothesis that such judgments are based on similarity or conceptual relatedness. Given information concerning a target person's standing on three global personality dimensions, American and Israeli subjects were asked to estimate the target's self-ratings of 50 trait adjectives and to express their confidence by setting a 90 percent uncertainty range around each estimate. The estimates were positively correlated with the actual ratings obtained from subjects who had evaluated themselves in terms of the 50 traits, but were far too extreme. Furthermore, confidence was negatively correlated with accuracy: People's estimates were most inaccurate and made with greatest certainty when the trait in question was highly similar to the information provided as a basic for judgment. We suggest that intuitive personality judgments overestimate the coherence of the structure underlying trait constructs.

On the belief that arthritis pain is related to the weather.

There is a widespread and strongly held belief that arthritis pain is influenced by the weather; however, scientific studies have found no consistent association. We hypothesize that this belief results, in part at least, from people's tendency to perceive patterns where none exist. We studied patients (n = 18) for more than I year and found no statistically significant associations between their arthritis pain and the weather conditions implicated by each individual. We also found that college students (n = 97) tend to perceive correlations between uncorrelated random sequences. This departure of people's intuitive notion of association from the statistical concept of association, we suggest, contributes to the belief that arthritis pain is influenced by the weather.

Probability judgement in medicine: discounting unspecified possibilities.

Research in cognitive psychology has indicated that alternative descriptions of the same event can give rise to different probability judgments. This observation has led to the development of a descriptive account, called support theory, which assumes that the judged probability of an explicit description of an event (that lists specific possibilities) generally exceeds the judged probability of an implicit description of the same event (that does not mention specific possibilities). To investigate this assumption in medical judgment, the authors presented physicians with brief clinical scenarios describing individual patients and elicited diagnostic and prognostic probability judgments. The results showed that the physicians tended to discount unspecified possibilities, as predicted by support theory. The authors suggest that an awareness of the discrepancy between intuitive judgments and the laws of chance may provide opportunities for improving medical decision making.

Reason-based choice.

This paper considers the role of reasons and arguments in the making of decisions. It is proposed that, when faced with the need to choose, decision makers often seek and construct reasons in order to resolve the conflict and justify their choice, to themselves and to others. Experiments that explore and manipulate the role of reasons are reviewed, and other decision studies are interpreted from this perspective. The role of reasons in decision making is considered as it relates to uncertainty, conflict, context effects, and normative decision rules.

Thinking through uncertainty: nonconsequential reasoning and choice.

When thinking under uncertainty, people often do not consider appropriately each of the relevant branches of a decision tree, as required by consequentialism. As a result they sometimes violate Savage's sure-thing principle. In the Prisoner's Dilemma game, for example, many subjects compete when they know that the opponent has competed and when they know that the opponent has cooperated, but cooperate when they do not know the opponent's response. Newcomb's Problem and Wason's selection task are also interpreted as manifestations of nonconsequential decision making and reasoning. The causes and implications of such behavior, and the notion of quasi-magical thinking, are discussed.

Differential weighting of common and distinctive components.

We investigated possible explanations of the finding that the relative weight (W) of common components in similarity judgments is higher for verbal than for pictorial stimuli. A serial presentation of stimulus components had no effect on verbal stimuli; it increased the impact of both common and distinctive components of pictorial stimuli but did not affect their relative weight. On the other hand, W was increased by manipulations that reduced the cohesiveness of composite pictures, such as separating, scrambling, and mixing their components. Furthermore, W was decreased by manipulations that enhanced the cohesiveness of composite verbal stimuli by imposing structure on their components. Verbal and pictorial representations of the same stimuli yielded no systematic differences in W.

On the elicitation of preferences for alternative therapies.

We investigated how variations in the way information is presented to patients influence their choices between alternative therapies. Data were presented summarizing the results of surgery and radiation therapy for lung cancer to 238 ambulatory patients with different chronic medical conditions and to 491 graduate students and 424 physicians. We asked the subjects to imagine that they had lung cancer and to choose between the two therapies on the basis of both cumulative probabilities and life-expectancy data. Different groups of respondents received input data that differed only in whether or not the treatments were identified and whether the outcomes were framed in terms of the probability of living or the probability of dying. In all three populations, the attractiveness of surgery, relative to radiation therapy, was substantially greater when the treatments were identified rather than unidentified, when the information consisted of life expectancy rather than cumulative probability, and when the problem was framed in terms of the probability of living rather than in terms of the probability of dying. We suggest that an awareness of these effects among physicians and patients could help reduce bias and improve the quality of medical decision making.

Representations of qualitative and quantitative dimensions.

Geometric representations of psychological dimensions are analyzed and compared to an alternative set-theoretical approach. Judgments of similarity between forms and figures reveal the following effects: (a) qualitative attributes are curved relative to quantitative attributes, contrary to intradimensional subtractivity; (b) quantitative attributes augment differences in qualitative attributes, contrary to interdimensional additivity; (c) adding a new dimension with a fixed value increases similarity, contrary to translation invariance. The implications of these results to multidimensional representations of proximity data are discussed.

The framing of decisions and the psychology of choice.

The psychological principles that govern the perception of decision problems and the evaluation of probabilities and outcomes produce predictable shifts of preference when the same problem is framed in different ways. Reversals of preference are demonstrated in choices regarding monetary outcomes, both hypothetical and real, and in questions pertaining to the loss of human lives. The effects of frames on preferences are compared to the effects of perspectives on perceptual appearance. The dependence of preferences on the formulation of decision problems is a significant concern for the theory of rational choice.

Judgment under Uncertainty: Heuristics and Biases.

This article described three heuristics that are employed in making judgements under uncertainty: (i) representativeness, which is usually employed when people are asked to judge the probability that an object or event A belongs to class or process B; (ii) availability of instances or scenarios, which is often employed when people are asked to assess the frequency of a class or the plausibility of a particular development; and (iii) adjustment from an anchor, which is usually employed in numerical prediction when a relevant value is available. These heuristics are highly economical and usually effective, but they lead to systematic and predictable errors. A better understanding of these heuristics and of the biases to which they lead could improve judgements and decisions in situations of uncertainty.