Introduction

The experimental study of human rationality – that is, of validity in deductive or probabilistic reasoning – has become entangled during the past decade or so in a web of paradox. On the one hand, reputable investigators tell us that “certain psychological discoveries have bleak implications for human rationality” (Nisbett & Borgida 1975), or that “for anyone who would wish to view man as a reasonable intuitive statistician, such results are discouraging” (Kahneman & Tversky 1972b) or that “people systematically violate principles of decision-making when judging probabilities, making predictions, or otherwise attempting to cope with probabilistic tasks” and they “lack the correct programs for many important judgmental tasks” (Slovic, Fischhoff & Lichtenstein 1976). On the other hand, those investigators are reminded that people could not even drive automobiles unless they could assess uncertainties fairly accurately (Edwards 1975). The ordinary person is claimed to be prone to serious and systematic error in deductive reasoning, in judging probabilities, in correcting his biases, and in many other activities. Yet, from this apparently unpromising material – indeed, from the very same students who are the typical subjects of cognitive psychologists' experiments – sufficient cadres are recruited to maintain the sophisticated institutions of modern civilisation. Earlier decades, in an era of greater optimism, may well have overestimated the natural reasoning powers of human beings. But there seems now to be a risk of underestimating them.