The rationale for the present volume is simple: The great majority of the everyday reasoning, including that of expert groups engaged in their professions, is informal. By contrast, most of the studies of human inference reported by psychologists in the literature are of formal reasoning. This discrepancy provides considerable cause for concern and not only because cognitive psychology should have some practical application. Excessive focus on formal reasoning tasks has also, in our view, inhibited the development of good theories of human reasoning.
What Is Informal Reasoning, and Why Do We Need to Study It?
Psychological studies of formal reasoning have fallen largely into two domains: deductive reasoning and statistical inference. These two endeavours have much in common and some researchers work in both areas. In both cases, participants are presented with what problem-solving researchers call well-defined problems. A well-defined problem can be solved by use of the information provided and no other; in fact, the correct solution to these problems often requires the reasoner to use only the information provided in the premises, and to avoid adding background information and knowledge to the problem domain.
Instead, a correct solution is achieved by applying a normatively appropriate rule of inference. Normative systems are often applied to formal reasoning problems in order to define solutions as right or wrong, such that these problems are then construed as tests of correct and fallacious reasoning. Hence, these problems are designed to measure the extent to which participants bring to the laboratory an understanding and ability to apply - the relative normative principles.
In the case of deductive reasoning research, the relevant normative system is formal logic. Participants are given some premises and asked whether a conclusion follows. Under strict deductive reasoning instructions, they are told (a) to assume that the premises are true and (b) to draw or approve only conclusions that necessarily follow. As observed elsewhere (Evans, 2002), this widely used method was developed over 40 years ago when belief in logic as a normative and descriptive system for human reasoning was veiy much higher than it is today. In spite of the method, much evidence has emerged to support the conclusion that pragmatic factors play a large part in human reasoning. We say "in spite of" because standard deductive instructions aim to suppress precisely those factors that dominate informal reasoning: the introduction of prior belief and the expression of uncertainty in premises and conclusions.
In research on statistical inference, a similar story is found. People are asked to make statistical inference on the basis of well-defined problems, in which relevant probabilities or frequency distributions are provided, and their answers are assessed for correctness against the norms provided by the probability calculus. Research in this tradition has been mostly conducted by researchers in the "heuristics and biases" tradition inspired by the work of Danny Kahneman and Amos Tversky (Gilovich, Griffin, & Kahneman, 2002; Kahneman, Slovic, & Tversky, 1982). This results in an arguably negative research strategy that is similar to much work on deductive reasoning. That is, researchers show primarily what people cannot do (conform to the principles of logic or probability theory) and only secondarily address what people actually do.
Indeed, one of the most common explanations for why intelligent, educated individuals often fail to reason normatively is that they use informal reasoning processes to solve formal reasoning tasks. For example, notwithstanding instructions to the contraiy, reasoners often supplement the information they are provided with background knowledge and beliefs, and make inferences that are consistent with, rather than necessitated by, the premises. If this is the case, it is reasonable to suggest that we study these processes directly, by giving our participants tasks that allow them to express these types of behaviours freely, rather than indirectly, via the observation of poor performance on a formal task. …