BEHAVIORAL AND BRAIN SCIENCES (2011) 34, 57–111 doi:10.1017/S0140525X10000968 Why do humans reason? Arguments for an argumentative theory Hugo Mercier Philosophy, Politics and Economics Program, University of Pennsylvania, Philadelphia, PA 19104 [email protected] http:// sites.google.com/site/hugomercier/ Dan Sperber Jean Nicod Institute (EHESS-ENS-CNRS), 75005 Paris, France; Department of Philosophy, Central European University, Budapest, Hungary [email protected] http:// www.dan.sperber.fr Abstract: Reasoning is generally seen as a means to improve knowledge and make better decisions. However, much evidence shows that reasoning often leads to epistemic distortions and poor decisions. This suggests that the function of reasoning should be rethought. Our hypothesis is that the function of reasoning is argumentative. It is to devise and evaluate arguments intended to persuade. Reasoning so conceived is adaptive given the exceptional dependence of humans on communication and their vulnerability to misinformation. A wide range of evidence in the psychology of reasoning and decision making can be reinterpreted and better explained in the light of this hypothesis. Poor performance in standard reasoning tasks is explained by the lack of argumentative context. When the same problems are placed in a proper argumentative setting, people turn out to be skilled arguers. Skilled arguers, however, are not after the truth but after arguments supporting their views. This explains the notorious confirmation bias. This bias is apparent not only when people are actually arguing, but also when they are reasoning proactively from the perspective of having to defend their opinions. Reasoning so motivated can distort evaluations and attitudes and allow erroneous beliefs to persist. Proactively used reasoning also favors decisions that are easy to justify but not necessarily better. In all these instances traditionally described as failures or flaws, reasoning does exactly what can be expected of an argumentative device: Look for arguments that support a given conclusion, and, ceteris paribus, favor conclusions for which arguments can be found. Keywords: argumentation; confirmation bias; decision making; dual process theory; evolutionary psychology; motivated reasoning; reason-based choice; reasoning Inference (as the term is most commonly understood in Evans et al. 1993; Johnson-Laird 2006; Oaksford & psychology) is the production of new mental represen- Chater 2007; Rips 1994), there is very little discussion of tations on the basis of previously held representations. the why-question. How come? It may be that the function Examples of inferences are the production of new of reasoning is considered too obvious to deserve much beliefs on the basis of previous beliefs, the production of expectations on the basis of perception, or the production of plans on the basis of preferences and beliefs. So under- HUGO MERCIER is a postdoctoral fellow at the Univer- stood, inference need not be deliberate or conscious. It is sity of Pennsylvania. His work has focused on the at work not only in conceptual thinking but also in percep- theme of the present article – reasoning and argumen- tion and in motor control (Kersten et al. 2004; Wolpert & tation. He is working on a series of articles that cover Kawato 1998). It is a basic ingredient of any cognitive this issue from different perspectives – developmental, system. Reasoning, as commonly understood, refers to a cross-cultural, political, and historical. very special form of inference at the conceptual level, DAN SPERBER is a French social and cognitive scientist. where not only is a new mental representation (or con- He is professor of philosophy and cognitive science at the clusion) consciously produced, but the previously held Central European University, Budapest, and directeur representations (or premises) that warrant it are also con- de recherche emeritus at the Institut Jean Nicod, sciously entertained. The premises are seen as providing (CNRS, ENS, and EHESS, Paris). He is the author of reasons to accept the conclusion. Most work in the psy- Rethinking Symbolism (1975), On Anthropological chology of reasoning is about reasoning so understood. Knowledge (1985), and Explaining Culture (1996); the Such reasoning is typically human. There is no evidence co-author with Deirdre Wilson of Relevance: Communi- that it occurs in nonhuman animals or in preverbal cation and Cognition (1986 – Second Revised Edition, children.1 1995); the editor of Metarepresentations: A Multidisci- plinary Perspective (2000); the co-editor with David How do humans reason? Why do they reason? These Premack and Ann James Premack of Causal Cognition: two questions are mutually relevant, since the mechanisms A Multidisciplinary Debate (1995), and, with Ira for reasoning should be adjusted to its function. While the Noveck, of Experimental Pragmatics (2004). how-question has been systematically investigated (e.g., # Cambridge University Press 2011 0140-525X/11 $40.00 57 Electronic copy available at: http://ssrn.com/abstract=1698090 Mercier & Sperber: Why do humans reason? attention. According to a long philosophical tradition, because of its intuitive force. Such beliefs, held with reasoning is what enables the human mind to go beyond awareness of one’s reasons to hold them, are better mere perception, habit, and instinct. In the first, theoreti- described not as intuitive but as reflective beliefs cal section of this article we sketch a tentative answer to (Sperber 1997). Our consciously held reason for accepting the how-question and then focus on the why-question: a reflective belief may be trust in its source (the professor, We outline an approach to reasoning based on the idea the doctor, the priest). Our reasons may also have to do that the primary function for which it evolved is the pro- with the content of the belief: We realize, for example, duction and evaluation of arguments in communication. that it would be inconsistent on our part to hold to our pre- In sections 2–5, we consider some of the main themes vious beliefs and not accept some given new claim. Far and findings in the experimental literature on reasoning from denying that we may arrive at a belief through and show how our approach helps make better sense of reflecting on our reasons to accept it, we see this as reason- much of the experimental evidence and hence gains ing proper, the main topic of this article. What character- empirical support from it. izes reasoning proper is indeed the awareness not just of a conclusion but of an argument that justifies accepting that conclusion. We suggest, however, that arguments 1. Reasoning: Mechanism and function exploited in reasoning are the output of an intuitive infer- ential mechanism. Like all other inferential mechanisms, 1.1. Intuitive inference and argument its processes are unconscious (as also argued by Since the 1960s, much work in the psychology of reasoning Johnson-Laird 2006, p. 53; and Jackendoff 1996) and its has suggested that, in fact, humans reason rather poorly, conclusions are intuitive. However, these intuitive con- failing at simple logical tasks (Evans 2002), committing clusions are about arguments; that is, about represen- egregious mistakes in probabilistic reasoning (Kahneman tations of relationships between premises and conclusions. & Tversky 1972; Tversky & Kahneman 1983), and being The intuitive inferences made by humans are not only subject to sundry irrational biases in decision making about ordinary objects and events in the world. They can (Kahneman et al. 1982). This work has led to a rethinking also be about representations of such objects or events of the mechanisms for reasoning, but not – or at least, not (or even about higher-order representations of represen- to the same degree – of its assumed function of enhancing tations). The capacity to represent representations, and human cognition and decision making. The most impor- to draw inferences about them, is a metarepresentational tant development has been the emergence of dual- capacity with formal properties relevant to the mental process models that distinguish between intuitions and computations involved (Recanati 2000; Sperber 2000b). reasoning (or system 1 and system 2 reasoning) (Evans Several mental mechanisms use this metarepresentational 2007; Johnson-Laird 2006; Kahneman 2003; Kahneman capacity. In particular, humans have a mechanism for & Frederick 2002; 2005; Sloman 1996; Stanovich 2004). representing mental representations and for drawing Here we outline our own dual-process approach: We intuitive inferences about them. This Theory of Mind contend in particular that the arguments used in reasoning mechanism is essential to our understanding of others are the output of a mechanism of intuitive inference and of ourselves (Leslie 1987; Premack & Woodruff (Mercier & Sperber 2009; Sperber 1997; 2001). 1978). Humans also have a mechanism for representing A process of inference is a process, the representational verbal representations and for drawing intuitive inferences output of which necessarily or probabilistically follows about them. This pragmatic mechanism is essential to our from its representational input. The function of an inferen- understanding of communicated meaning in context tial process is to augment and correct the information (Grice 1975; Sperber & Wilson 2002). available to cognitive system. An evolutionary approach We want to argue that there is yet another intuitive suggests that inferential processes,