Reasoning and judgment under uncertainty are often based on a limited number of simplifying heuristics rather than formal logic or rule-based argumentation. Heuristics are low-effort mental shortcuts, which save time and effort, and usually result in accurate judgment, but they can also lead to systematic errors and biases when applied inappropriately. In the past 40 years hundreds of papers have been published on the topic of heuristics and biases in judgment and decision making. However, we still know surprisingly little about the development and the cognitive underpinnings of heuristics and biases. The main aim of my thesis is to examine these questions. Another aim is to evaluate the applicability of dual-process theories of reasoning to the development of reasoning. Dual-process theories claim that there are two types of process underlying higher order reasoning: fast, automatic, and effortless (Type 1) processes (which are usually associated with the use of reasoning heuristics), and slow, conscious and effortful (Type 2) processes (which are usually associated with rule-based reasoning). This thesis presents eight experiments which investigated the development of reasoning heuristics in three different populations: typically developing children and adolescents between the age of 5 and 16, adolescents with autism, and university students. Although heuristic reasoning is supposed to be basic, simple, and effortless, we have found evidence that responses that are usually attributed to heuristic processes are positively correlated with cognitive capacity in the case of young children (even after controlling for the effects of age). Moreover, we have found that adolescents with autism are less susceptible to a number of reasoning heuristics than typically developing children. Finally, our experiments with university students provided evidence that education in statistics increases the likelihood of the inappropriate use of a certain heuristic (the equiprobability bias). These results offer a novel insight into the development of reasoning heuristics. Additionally, they have interesting implications for dual-process theories of reasoning, and they can also inform the debates about the rationality of reasoning heuristics and biases.

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