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Why we reason: intention-alignment and the genesis of human rationality


Why do humans reason? Many animals draw inferences, but reasoning—the tendency to produce and respond to reason-giving performances—is biologically unusual, and demands evolutionary explanation. Mercier and Sperber (Behav Brain Sci 34:57–111, 2011) advance our understanding of reason’s adaptive function with their argumentative theory of reason (ATR). On this account, the “function of reason is argumentative… to devise and evaluate arguments intended to persuade.” ATR, they argue, helps to explain several well-known cognitive biases. In this paper, I develop a neighboring hypothesis called the intention alignment model (IAM) and contrast it with ATR. I conjecture that reasoning evolved primarily because it helped social hominins more readily and fully align their intentions. We use reasons to advance various proximal ends, but in the main, we do it to overwrite the beliefs and desires of others: to get others to think like us. Reason afforded our ancestors a powerful way to build and maintain the shared outlooks necessary for a highly collaborative existence. Yes, we sometimes argue so as to gain argumentative advantage over others, or otherwise advantage ourselves at the expense of those we argue with, but more often, we reason in ways that are mutually advantageous. In fact, there are excellent reasons for thinking this must be so. IAM, I suggest, neatly explains the available evidence, while also providing a more coherent account of reason’s origins.

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  1. Please note the distinction made in first article note, between basic intuitive inference and reasoning proper.

  2. Aristotle (2001) held that we are by nature rational animals, that the purpose of reasoning is to generate new knowledge, and that, consequently, we best serve our inherent telos by living the contemplative life—a tidily self-serving conclusion for any philosopher!

  3. Aquinas (1999), of course, did not so much hatch this view as systematize a set of ideas that had shaped the institutions of Medieval Europe for centuries.

  4. In graduate school, I subjected the hypothesis that argumentative posturing carries significant reproductive rewards to (informal) empirical testing. I found that “show(ing) off (my) competence for dealing with anomalies in information” (Dessalles 2011) had a subtractive effect on reproductive opportunity. This makes me skeptical of the idea that ancestral geeks cashed in their nerd-cred to reproduce more frequently than their dialectically challenged contemporaries.

  5. “Groupish” is the term psychologist Haidt (2012) uses to characterize a set of moral emotions that function to enhance group solidarity: loyalty, gratitude, sympathy and the like. We needn’t assume that the members of the clan reason or even think explicitly in these terms; they simply want to be together.

  6. Imagine several hominins each deciding, individually and for self-interested reasons, to shun an advantage-taking free rider, and the shunned individual starving for lack of assistance. On this dynamic, selfishness is selected against without any group being selected against.

  7. To say nothing of kin selection and reciprocity, which likely also played a role. The point is that one needn’t go anywhere near group selection to make these claims.

  8. Biologists have observed other primates making decisions about which way the troop should go by tentatively “voting” with their bodies. If a critical mass trends left, those with a preference for going right will often capitulate by joining the majority. Note the disinclination to set off on one’s own.

  9. The odds of surviving might be enhanced, for example, by keeping mum about a fruit tree discovered while scouting.

  10. This demonstrates a contemporary penchant for groupish thinking—a phenomenon Sugden (2003) proposes a theory of “team agency” to explain. I borrow the term “team reasoning” from Sugden. Haidt (2012) and Tomasello (2008) present ample evidence that the underlying groupish instincts predate the emergence of language, making it likely that they were already “on board” when Zog first employed gestural reasoning to shape the group’s collective intent. Reason dawned on a world already shaped by collective intent.

  11. Provided, of course, that the giving of reasons counts as a kind of reasoning. But who can deny that?

  12. Research on the cognitive aptitudes of the great apes makes plain that we humans differ markedly from our nearest relatives in our penchant for intention sharing: where we share states of mind compulsively and readily, chimps and other great apes do so only rarely and with difficulty. Tomasello (2008) argues that this penchant underlies human language, but it also seems to undergird much of human culture, technology, Haidt’s “moral matrices” (2012), Gottschall’s storytelling impulse (2012), Wade’s faith instinct (2010), and much else that is distinctively human. My claim here is that the faculty of reason played—and continues to play—a critical role in the fabrication and maintenance of the common ground on which we humans erect much of our lives.

  13. By calling manipulative reasoning “salesman reasoning,” I don’t mean to imply that sales are not often, or even typically, win–win. The stereotype of a used car salesman may be unfair, but it gives the phrase the needed connotation.

  14. Under certain conditions, collaborative reasoning “leads to big improvements in performance” (M&S, pp. 62–63). When study participants worked in groups, for example, solution rates on the Wason selection task rose from around 10 % to around 80 % (Wason 1966). In this task, four cards are laid before the subject, each showing a letter or a number. The cards, for example, might display 4, E, 7 and K. The subject is then asked which cards must be turned over to determine the truth of the rule “If there is a vowel on one side of the card, then there is an even number on the other side.” Individuals frequently fail to recognize that the card with a “7” on it could falsify the rule, so must be turned over. When reasoning together in groups, however, people usually get the answer correct.

  15. The actual phrase M&S use is “unique integrative perspective” (72)—an apt description, to be sure—but the context of the claim makes it apparent that they view their theory as uniquely integrative—a claim that becomes false once a yet more integrative theory comes along.

  16. “Species defining” in the sense that several human traits that are commonly spoken of as distinctively human—such as language, culture, technological innovation, robust intentionality-sharing and self-consciousness—may have been made possible by the prior emergence of basic reasoning aptitudes. The claim is conjectural, but worth further exploration.

  17. Importantly, I do not disavow “function”’s colloquial usage in this context, for it is not incorrect to speak of intention alignment as the primary utility or purpose of reasoning. This raises philosophical issues best saved for another time.

  18. The question of what exactly reasons function to align intentions with is an important one, and I take it up presently.

  19. Obviously, a creature can alter the contents of a conspecific’s mind unintentionally by simply entering the latter’s field of vision. The interesting kind of “mind writing” involves intentional or deliberate alteration via some kind of signaling.

  20. It is tempting to call the communicative use of reason “proper,” and its strategic use “improper.” The norms of philosophical ethics permit such editorializing, but at present, the norms of scientific discourse do not. The ethical implications of IAM are significant, and well worth exploring.

  21. See Tomasello (2008) and Haidt (2012, p. 204 ff).

  22. Bees, of course, are known to use a kind of dance to share information about the location of a food source. This surely counts as a kind of intention sharing—or would so count, if we could be sure that a bee’s nervous system supported something properly described as a mind.

  23. These difficulties stem from the evident fact that much reason-giving is much simpler and more elemental than the phenomenon we call “argumentation.” Once the contrast has been made between a reason’s functioning in an argumentative capacity, and it’s functioning in a simple, intention-aligning capacity, the argumentative theory becomes hard to defend, because “argumentation” rightly refers to a complex social practice built atop basic reason-giving propensities. Also, ATR implies that reasoning is largely “argumentative” or agonistic in character, when we have seen that, in the main, it must be mostly alignment-oriented.

  24. See, for example, Zak’s (2012) discussion, which touches on famous experiments by Milgram (1974).

  25. There may well be better explanations of our susceptibility to arguments by authority, but on the face of it, the intention alignment model explains it rather well. Again, my aim here is not to decide the question, but make a preliminary case that IAM belongs in the discussion.

  26. Or a belief, at any rate, that we can indulge in without damage to our reputation: because a ready justification is often sufficient to demonstrate a good-faith effort at accountability, it often provides adequate reputational cover.

  27. See footnote 15.

  28. An established conclusion is rightly understood to be available for premising. When this conclusion is only newly established, this represents an expansion of common ground. Of course, if the conclusion was accepted by both parties all along, the argument serves only to confirm (rather than create) an alignment of intentions.

  29. There is no suggestion here of cosmic purpose, just of evolved functioning being adaptive.

  30. One needn’t take sides in the Grice–Sperber debate to note that, whatever its shortcomings, Grice’s cooperative principle points to a significant pattern of relevance to the intention alignment hypothesis.


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Correspondence to Andy Norman.

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Following Mercier and Sperber (2011, p. 58), I distinguish between basic intuitive inference, which is immediate, largely unconscious, and not uncommon among nonhuman animals, and reasoning proper, which involves the deliberate use of reasons to change minds. See also (Kahneman 2011).

I am especially indebted to Michael Tomasello for inspiring early portions of this work. I am grateful also to Lee McIntyre, Clay Naff, David Danks, Kevin Zollman, Adam Bjorndahl and Hugo Mercier for useful feedback.

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Norman, A. Why we reason: intention-alignment and the genesis of human rationality. Biol Philos 31, 685–704 (2016).

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  • Reasoning
  • Evolution
  • Cooperation
  • Bias
  • Cognitive bias
  • Intention
  • Intention-sharing