Mercier and Sperber (M&S) are insightful in proposing a strong link between reasoning and argumentation. Understanding the argumentative context sheds light on the processes of reasoning. However, empirical research on the psychology of argumentation contradicts several of their key claims. Contrary to their position, reasoning biases are common even in the context of argumentation, the confirmation bias is not a feature of argumentation and actually weakens arguments, and people cling to claims less rigidly than is tacitly assumed by the authors.

M&S's review of the literature on the psychology of argumentation is surprisingly sparse. Unfortunately, the data suggest that people are subject to fallacies and suboptimal performance in generating, comprehending, and evaluating arguments. Kuhn (Reference Kuhn2001) found that 4-year-old children are able to distinguish between evidence and explanation. However, children are often poor at generating arguments (Means & Voss Reference Means and Voss1996). In adolescents, national educational assessments find that only about 15% of 12th graders are adequately skilled in developing written arguments (Beatty Reference Beatty, Reese, Persky and Carr1996; Greenwald et al. Reference Greenwald, Persky, Campbell and Mazzeo1999). Anne Britt and colleagues conducted a series of experiments on the comprehension, evaluation, and generation of argumentative texts by college students. Among their findings, only about 35% of participants were able to identify the main claims and reasons in arguments, 37% failed to reject unsupported arguments, 32% failed to reject unwarranted arguments, 48% included other-side information in their arguments, and 65% wrote essays that did not include a single reason to support their claims (Britt & Kurby Reference Britt and Kurby2005; Britt & Larson Reference Britt and Larson2003; Britt et al. Reference Britt, Kurby and Wolfe2005; Larson et al. Reference Larson, Britt and Larson2004). Britt et al. (Reference Britt, Kurby, Dandotkar and Wolfe2008) found that university students have difficulty precisely recalling the main predicate of argument claims, but less difficulty recalling the predicates of comparable narrative statements. Wolfe et al. (Reference Wolfe, Albrecht and Britt2007) found that highly implausible reasons and warrants (for example, Paul should walk to the store “because walking is the absolute best exercise that will ever exist, and exercising can lead to immortality”) yielded higher agreement than the same claims without support. Argumentation is a fundamental skill that permeates human thinking (Voss & Van Dyke Reference Voss and Van Dyke2001). However, M&S have painted an unduly optimistic portrait of our argumentation abilities.

Some of what M&S describe as “confirmation bias” is sometimes called the “myside bias” (Baron Reference Baron1995; Perkins et al. Reference Perkins, Farady, Bushey, Voss, Perkins and Segal1991; Toplak & Stanovich Reference Toplak and Stanovich2003; Wolfe & Britt Reference Wolfe and Britt2005; Reference Wolfe and Britt2008; Wolfe et al. Reference Wolfe, Britt and Butler2009a). Although some authors use the terms interchangeably, confirmation bias typically refers to a biased search for or weighing of evidence, whereas myside bias refers to biases in generating reasons or arguments (Wolfe & Britt Reference Wolfe and Britt2008). M&S state that the confirmation bias “is a consequence of the function of reasoning and hence a feature of reasoning when used for the production of arguments” (sect. 3, para. 1, emphasis theirs). My colleagues and I have conducted a series of experiments on argumentation, and the evidence differs in key respects from their assertions (Wolfe & Britt Reference Wolfe and Britt2005; Reference Wolfe and Britt2008; Wolfe et al. Reference Wolfe, Britt and Butler2009a).

Wolfe & Britt (Reference Wolfe and Britt2008) had participants write argumentative essays under different conditions. Some were assigned to write essays for and others against an unpopular proposition. Participants had access to a number of pro and con online texts, and we also examined their search behavior. We found that the myside bias was pervasive. However, it was not linked to participant's personal opinions. People exhibited the myside bias when arguing for the side with which they personally disagreed just as often as for the side with which they agreed. We have replicated this finding, yet also find significant correlations between opinion strength and myside bias on nonargumentative reasoning tasks (Wolfe & Boone, under review; Wolfe & Britt Reference Wolfe and Britt2008). Moreover, participants exhibiting the myside bias in their arguments were not biased in their search. They sought out both pro-side and con-side texts.

The myside bias is indeed a bias – even in the context of argumentation. To illustrate, a content analysis of published authentic arguments found that most writers included other-side information in their arguments, commonly for the purpose of rebuttal (see Wolfe & Britt Reference Wolfe and Britt2008). In laboratory experiments, presenting and rebutting other-side information consistently leads to better ratings of agreement, argument quality, and impressions of authors than does excluding other-side information (Wolfe & Boone, under review; Wolfe et al. Reference Wolfe, Britt and Butler2009a). The myside bias weakens arguments measurably.

The factors predicting the myside bias in written essays are individual differences in beliefs about argumentation. Evidence stems from successful tutorials that significantly reduce the myside bias in generating and evaluating arguments (Wolfe et al. Reference Wolfe, Britt and Butler2009a; Reference Wolfe, Britt, Petrovic, Albrecht and Kopp2009b), answers to the open-ended question “what makes a good argument?” (Wolfe & Britt Reference Wolfe and Britt2008), and reliable individual difference measures (Wolfe & Boone, under review; Wolfe & Britt Reference Wolfe and Britt2009). The context of argumentation changes the nature of the myside bias, but one-sided argumentation is problematic and not an inherent feature of argumentation.

A tacit assumption in M&S's account is that people have strong, stable preferences and unwavering commitments to claims. Argumentation is seen as a form of rationalization used to convince others of claims derived from intuitive processes about which people are only dimly aware. Yet, starting with early research on informal reasoning (Perkins et al. Reference Perkins, Allen, Hafner and Maxwell1983), we have learned that positions are often fluid and tentative. As reasoning progresses, those positions undergo changes. We typically argue about matters that are “debatable,” where reasonable people arrive at different conclusions and are open to persuasion. I believe that reasoning and argumentation are interrelated skills drawing on many of the same cognitive processes – two sides of the same coin. Dual-process theories suggest that people lack access to many of our own cognitive processes. Thus, when we have tentative intuitions that are not well understood, we use many of the same processes to convince ourselves that in other contexts we use to convince other people. However flawed these processes may be, argumentation would be ineffective if we were not also able to reason for ourselves.