Why do humans reason? Arguments for an argumentative theory

Response/Mercier & Sperber: Why do humans reason?were addressed to people who were wholly unable toevaluate them from a sound epistemic perspective.R2.1. The double-sided argumentative functionof reasoningWhy does reasoning exist at all, given that it is a relativelyhigh-cost mental activity with a relatively high failure rate?To answer this question, we proposed to step back fromthe study of individual cognitive processes and to look atthe evolution of human communication. Humans areimmersed in a flow of socially transmitted informationand are highly dependent on it. For communication tohave evolved, it had to be advantageous to both communicatorsand receivers (who are, of course, the same individualsbut acting in two different capacities). What makescommunication advantageous to receivers is that it providesthem with rich information that they could not, ornot easily, have obtained on their own. For this, the informationthey receive has to be genuine information; that is,close enough to truth. What makes communication advantageousto communicators is that it allows them to achievesome desirable effect in the receivers. For this, the informationthey emit has to be conducive to this effect,whether it is true or false.Dessalles, who has himself developed a perspective insome respects comparable to ours, understands us to claimthat “the biological function of reasoning is to achieveshared knowledge optimization” (para. 2) and that this isdone not at the individual but at the group level. We doargue that the main function of reasoning is indeedsocial but by serving the social interests of individualsrather than the collective interests of the group.To reap the benefits of communication while limitingthe risk of being misled, receivers must exercise what wehave called epistemic vigilance (Sperber et al. 2010).There is no fail-safe algorithm to sort genuine from spuriousinformation; hence, we argue, various cost-effectiveheuristics that may contribute to approximating such asorting are likely to have evolved. The main heuristic ofepistemic vigilance consists in assessing the trustworthinessof communicators. Thus, we agree with Opfer &Sloutsky that “children detect trustworthiness longbefore they detect argument inconsistency” (para. 4)(e.g., see Mascaro & Sperber 2009). But if detecting thetrustworthiness of communicators were the only heuristicused, then receivers would end up rejecting a goodamount of genuine and relevant information when theylack sufficient ground to accept it on trust. For instance,few if any readers of our article would accept its conclusionsjust out of trust in its authors! To be more effective,epistemic vigilance must be exercised not onlytowards the source of information but also towards itscontent. Independently of its source, a message mayhave a greater or a lesser believability. This believabilityis assessed by considering its coherence with backgroundknowledge. Coherence checking, we argue, is the secondmajor heuristic used in filtering communicated information,and is at the basis of reasoning proper.Coherence checking starts as a method for receivers tofilter information; it ends up being exploited also by communicatorswho engage in coherence displays in order tohave their messages accepted. Just as receivers wouldfilter out some genuine information if they relied only onthe trustworthiness of the source, communicators wouldfail to communicate some believable messages if theyrelied only on their own authority. Arguing consists in displayingcoherence-based reasons for the acceptance of agiven message. It is, in essence, an “honest display” strategyopened to evaluation and aimed at the audience’s epistemicconcerns. Of course, what is displayed may be aninvalid argument made in the service of deception andin the hope that its invalidity won’t be detected. Contraryto what Dessalles attributes to us, we do not believe thatcommunicators argue in order “to correct or updateothers’ beliefs” (para. 3) when it is not to their advantage.They argue for whatever it is advantageous to them to havetheir audience believe. Often enough, for instance, whencommunicating to coordinate action, communicator andaudience have convergent interests in sharing true information,but this is far from being always the case.In an evolutionary perspective, receivers’ coherencechecking creates selective pressure for communicators’coherence displays in the form of arguments, which inturn creates selective pressure for adequate evaluation ofarguments on the part of receivers. At least in some culturalcontexts, this results in a kind of arms race towardsgreater sophistication in the production and evaluationof arguments. Of course, argumentation can be misusedand abused – for instance, by arguing above the head ofone’s audience (Sperber 2009) or by lacing argumentswith appeals to emotion. Doing so, however, is morelikely to serve the interests of the communicator thanthose of the audience. Contrary to what Opfer &Sloutsky maintain, “hot” persuasion is advantageous tocommunicators only to the extent that receivers yield toit, but it is not advantageous to receivers who care to bewell informed. For this they had better reason, as Pettyand Cacioppo (whom Opfer & Sloutsky oddly cite insupport of their claim) have shown in numerous experimentsthat demonstrate precisely this point: Whenpeople are motivated to reason, they do a better job ataccepting only sound arguments, which is quite generallyto their advantage (e.g., see Petty et al. 1981).R2.2. Other functions of reasoning?Several commentators, while agreeing that argumentationmay be an important function of reasoning, suggest that itmay serve other functions, as well: either social functionsother than the production and evaluation of arguments(Baumeister, Masicampo, & DeWall [Baumeisteret al.]; Dessalles; Frankish; Pietraszewski) orfunctionscontributing to individual cognition (Evans; Frankish;Godfrey-Smith & Yegnashankaran). We recognize thepossibility. Our claim is that argumentation is the main functionof reasoning, and we tried to demonstrate this claim byshowing the signature of this function in the way reasoningactually functions, and in particular in what has been seenas flaws and that we argue are features of reasoning. Anyevolved mechanism can be put to a variety of uses: Themouth can be used to play a wind instrument, the skin canbe used to display tattoos, and the sense of smell can beused to develop wine expertise. Whether these additionaluses become functions in the biological sense, or evenmain functions (in which case Gould and Vrba’s term exaptationmight be properly applied) depends on the relative96 BEHAVIORAL AND BRAIN SCIENCES (2011) 34:2