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We report the results of experiments on economic decisions with two populations, one of healthy elderly individuals (average age 82) and one of younger students (average age 20). We examine confidence, decisions under uncertainty, differences between willingness to pay and willingness to accept and the theory of mind (strategic thinking).Our findings indicate that the older adults' decision behavior is similar to that of young adults, contrary to the notion that economic decision making is impaired with age.Choices over lotteries do not reflect the age differences previously reported in the

The psychological literature has identifi ed a number of heuristics which individuals may use in making judgements or choices under uncertainty. Mathematically equivalent problems may be treated differently depending upon details of the decision setting (Gigerenzer et al. (1988), Hinz et al. (1988), Birnbaum and Mellers (1983), Ginossar and Trope (1987)) or upon how the decisions are framed (Tversky and Kahneman (1986)). The results presented in this paper are consistent with those fi ndings and are unsettling. In equivalent problems subjects appear to adopt different strategies in response to observing different data. All problems were inference problems about populations represented by bingo cages and all randomization was operational and observed by the subjects. Thus one cannot explain the change of decision strategy by appeal to changing reference points nor should difference between surface and deep structure of problems apply (Wagenaar et al. (1988)). A striking observation from the experiments is the result of employing financial incentives. Some experiments included financial incentives for accuracy and some did not. In the latter experiments the number of nonsense or incoherent responses increased by a factor of three. The majority of subjects in both treatments behaved reasonably, but of those lacking fi nancial incentives a larger proportion gave obviously absurd responses. This suggests that data from decision experiments in which no financial incentives were should be treated as possibly contaminated and statistical methods robust against outliers employed.

Economists and psychologists have recently been developing new theories of decision making under uncertainty that can accommodate the observed violations of standard statistical decision theoretic axioms by experimental subjects. We propose a procedure which finds a collection of decision rules that best explain the behavior of experimental subjects. The procedure is a combination of maximum likelihood estimation of the rules together with an implicit classification of subjects to the various rules, and a penalty for having too many rules. We apply our procedure to data on probabilistic updating by subjects in four different universities. We get remarkably robust results which show that the most important rules used by the subjects (in order of importance) are Bayes ' s rule, a representativeness rule (ignoring the prior), and to a lesser extent, conservatism (over-weighting the prior).