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.

Our recent work suggests that (1) the purpose of human conscious thought is participation in social and cultural groups, and (2) logical reasoning depends on conscious thought. These mesh well with the argument theory of reasoning. In broader context, the distinctively human traits are adaptations for culture and inner processes serve interpersonal functions.

Mercier and Sperber (M&S) argue that reasoning has evolved primarily as an adjunct to persuasive communication rather than as a basis for consequential choice. Recent research on decision-related regret suggests that regret aversion and concomitant needs for justification may underpin a complementary mechanism that can, if appropriately deployed, convert M&S's facile arguer into an effective decision maker, with obvious evolutionary advantages.

Mercier and Sperber (M&S) sketch a bleak picture of logical reasoning in classic, nonargumentative tasks. I argue that recent processing data indicate that despite people's poor performance they at least seek to adhere to traditional logical norms in these tasks. This implies that classic reasoning tasks are less artificial–and logical reasoning less exceptional–than M&S's framework suggests.

The biological function of human reasoning abilities cannot be to improve shared knowledge. This is at best a side effect. A more plausible function of argumentation, and thus of reasoning, is to advertise one's ability to detect lies and errors. Such selfish behavior is closer to what we should expect from a naturally selected competence.

There is indeed extensive evidence that people perform fairly poorly in reasoning tasks and that they often construct arguments for intuitively cued responses. Mercier & Sperber (M&S) may also be right to claim that reasoning evolved primarily as argumentation. However, if it did, the facility became exapted to the function of supporting uniquely human abilities for reflective thinking and consequential decision making.

Mercier and Sperber (M&S) suggest that human reasoning is reflective and has evolved to support social interaction. Cognitive agents benefit from being able to reflect on their beliefs whether they are acting alone or socially. A formal framework for argumentation that has emerged from research on artificial cognitive systems that parallels M&S's proposals may shed light on mental processes that underpin social interactions.

This commentary does three things. First, it offers further support for the view that explicit reasoning evolved for public argumentation. Second, it suggests that promoting effective communication may not be the only, or even the main, function of public argumentation. Third, it argues that the data Mercier and Sperber (M&S) cite are compatible with the view that reasoning has subsequently been co-opted to play a role in individual cognition.

Mercier and Sperber (M&S) claim that the main function of reasoning is to generate support for conclusions derived unconsciously. An alternative account holds that reasoning has a deliberative function even though it is an internalized analogue of public discourse. We sketch this alternative and compare it with M&S's in the light of the empirical phenomena they discuss.

This commentary suggests that the general population has much less reasoning skill than is claimed by Mercier & Sperber (M&S). In particular, many studies suggest that the skills of understanding, evaluating, and producing arguments are generally poor in the population of people who have not had specific training.

Logical consistency demands that Mercier and Sperber's (M&S's) argumentative theory of reasoning apply to their own reasoning in the target article. Although they hint that their argument applies to professional reasoners such as scientists and philosophers, they do not develop this idea. In this commentary, I discuss the applicability of argumentative theory to science and philosophy, emphasizing the perils of moral reasoning.

While impressed by much of what Mercier & Sperber (M&S) offer through their argumentative hypothesis, we question whether the specific competencies entailed in each system are adequate. In particular, whether system 2 might not require independent reasoning capabilities. We explore the adequacy of the explanations offered for confirmation bias and the Wason selection task.

It warrants examining how well people can come to argue under supportive conditions, not only what they do under ordinary conditions. Sustained engagement of young people in dialogic argumentation yields more than the temporary “contextual effect” that Mercier & Sperber (M&S) identify in the target article. If such engagement were to become the norm, who can say what the argumentive potential of future generations is?

If the target article represents the summary findings of the field, reasoning research is deeply flawed. The vision is too narrow and seems to fall into biological determinism. Humans use reasoning in effective ways apparently not studied by researchers, such as reasoning for action. Moreover, as the brain develops through adulthood and from experience so do reasoning capabilities.

This commentary agrees with Mercier and Sperber's (M&S's) thesis on the argumentative function of reasoning but suggests that an account of argument strength is required. A Bayesian account of argument strength (Hahn & Oaksford 2007) shows how the deployment of deductive fallacies, weak inductive arguments, and judgment fallacies such as base-rate neglect, can all be rationally defended in the right argumentative context.

It is unclear how an argumentative environment would select for better reasoning given three general findings. First, argument rationality typically fails to persuade poor reasoners. Second, reasoned argumentation competes with more persuasive and less rational arguments for limited cognitive resources. Third, those poor at reasoning fail to distinguish between valid and invalid arguments. Reasoning, therefore, is poorly designed for argument.

A consideration of selection pressures on the psychology of argument suggests that fixing the truth value of claims is not the primary criterion for argument generation or evaluation. Instead, argument psychology is designed to change representations in other minds as a way to negotiate conflicts of interest and as a way to signal social coordination.

Mercier and Sperber's (M&S's) theory of reasoning cannot predict reasoning in the absence of an argumentative context. Applying the theory to hypothesis testing behavior, I propose that hypothesis testing is often motivated by determining the true inference and that reasoning models should account for utilities (affected by various motives, including the wish to convince) of reasoning outcomes.

Mercier and Sperber (M&S) are correct that reasoning and argumentation are closely related. But they are wrong in arguing that this relationship is one of evolutionary adaptation. In fact, persuasive reasoning that is not veridical can be fatal to the individual and to the propagation of his or her genes, as well as to the human species as a whole.

Mercier & Sperber (M&S) claim that the phenomenon of belief bias – which they consider to be an archetypal manifestation of a general confirmation bias in human reasoning – provides fundamental support for their argumentative theory and its basis in intuitive judgement. We propose that chronometric evidence necessitates a more nuanced account of belief bias that is not readily captured by argumentative theory.