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The psychological experiments on reasoning seem to support the pessimistic conclusion that humans are vulnerable to systematic and widespread irrationality. According to the experimental results, people exhibit poor reasoning performance in a number of tasks, including logical, probabilistic, and decision-making tasks; the conclusion drawn is that people do not reason rationally in those circumstances.

The evidence indicates that people make inferential errors. The errors seem to be due to lack of knowledge of certain inductive rules or an inability to apply them. If so, then people are not fully rational in that their inferences fall short of the best available normative standards. (Thagard & Nisbett 1983, 257)

We argue that the deviations of actual behavior from the normative model are too widespread to be ignored, too systematic to be dismissed as random error and too fundamental to be accommodated by the theory of rational choice and then show that the most basic rules of the theory are commonly violated by decision makers. (Tversky & Kahneman 1986, 252)

The argument for pessimism goes as follows. We provisionally answered the philosophical questions by adopting the standard picture of rationality according to which to be rational is to reason in accordance with the rules of logic, probability, and decision-making (Section 1.3). Then, we moved onto the psychological questions and saw that human reasoning systematically deviates from the rules of logic, probability, and decision-making (Section 1.4). Putting the philosophical and psychological discussions together, we reach a pessimistic conclusion about human rationality.

We now turn to the objections to pessimism, in particular the objections from the ecological rationality programme led by Gigerenzer. As Kahneman and Tversky note, Gigerenzer’s objections include two claims: ‘a conceptual argument against our use of the term “bias”’ and ‘an empirical claim about the “disappearance” of the patterns of judgment that we had documented’ (Kahneman & Tversky 1996, 582).

Gigerenzer’s ‘empirical claim’ is related to the psychological part of the argument for pessimism. Gigerenzer interprets the psychological studies differently, emphasizing the fact that reasoning performance can be improved when the problems are formulated in a different way. As we saw above, in the frequency version of the experiment run by Fiedler (1988), participants’ performances improved significantly. Notice that the proponents of the heuristics and biases programme and the proponents of the ecological rationality programme both recognize the possible performance improvements. Gigerenzer tends to stress the fragility of biases; he says that the biases ‘disappear’ (Gigerenzer 1991). Kahneman and Tversky, in contrast, tend to stress the robustness of the biases; they say that biases can be ‘reduced by targeted interventions but cannot be made to disappear’ (Kahneman & Tversky 1996, 589). But it is far from obvious that this is more than a difference in emphasis or rhetoric. Indeed, Kahneman and Tversky write: ‘There is less psychological substance to [Gigerenzer’s] disagreement with our position than meets the eye’ (Kahneman & Tversky 1996, 589; see also Samuels, Stich, & Bishop 2002).

A more substantial and philosophically interesting disagreement concerns Gigerenzer’s ‘conceptual argument’, which is related to the philosophical part of the argument for pessimism. He rejects the standard picture of rationality, or at least the way in which the standard picture is used in the argument for pessimism. He does not think that reasoning performance should be evaluated in terms of the rules of logic, probability, and decision-making. Among Gigerenzer’s conceptual or philosophical objections, we focus on three main objections, which we discuss in turn in the next section.