It is often suggested that reason is what makes us human. But if reason is so useful, why doesn't it evolve in other animals too? And why do we so often use our reasoning to produce nonsensical conclusions? These are the questions that cognitive scientists Hugo Mercier and Dan Sperber set out to solve in their book "The Enigma of Reason", which takes a look at the evolution and workings of reason. Here, they discuss their argument that reason helps humans exploit their social environments by helping us justify our beliefs and actions to others.

Why did you decide to address the subject of why reason evolved?

Hugo: Since my undergrad, I was interested in evolutionary approaches to the human mind. Dan Sperber, with whom I wanted to work, had previously put forward the intriguing suggestion that the function of human reason was not to think better but to produce arguments in order to convince others.

Dan: This was just a sketchy hypothesis. During his PhD, Hugo fleshed it out, reviewed the literature, and conducted new experiments to test it. It took us several more years of common work to come to a novel and, we hope, illuminating account of reason.

How has the human capacity for reason been misunderstood in the past?

There have been two main misunderstandings about human reason, one bearing on what it is, the other on what it is for.

Reason is often seen as a very general capacity to solve problems, make better decisions, and arrive at sounder beliefs. A modern instantiation of this view takes the form of the System 1 / System 2 distinction made popular by Daniel Kahneman. In this view of the mind, System 1 corresponds roughly to our intuitions, which function well most of the time, but are subject to systematic mistakes. System 2, by contrast, corresponds to our capacity to reason in a rule-governed way, which enables us to fix System 1’s mistakes.

The first problem with this view is that it’s not clear how reason so understood could work. How could a single mechanism be responsible for fixing the rest of the mind? How could it be superior to the knowledge and experience encapsulated in all our other intuitions?

The second problem is that, if such a mechanism that can fix everything else could somehow have evolved, why would it have evolved only in humans? Why aren’t other cognitively complex animals also endowed with reason or at least some rudimentary form of it?

We suggest that reason is very much like any other cognitive mechanism—it is itself a form of intuition. Like other intuitions, it is a specialised mechanism. The specificity of reason is to bear... on reasons. Reason delivers intuitions about relationships between reasons and conclusions: some reasons are intuitively better than others. When you want to convince someone, you use reason to construct arguments. When someone wants to convince you of something, you use reason to evaluate their arguments. We are swayed by reasons that are intuitively compelling and indifferent to reasons that are intuitively too weak. Reason, then, does not contrast with intuition as would two quite different systems. Reason, rather, is just a higher order mechanism of intuitive inference.

Most of the time, we operate without thinking of reasons. When you drive to work in the morning, you are not thinking of reasons for every turn, every push on the gas pedal, every though that comes in our mind as you listen to the radio, etc. And these intuitions that drive the vast majority of our behaviours and inferences function remarkably well; they are not intuitions about reasons.

Why do we sometimes bother reasoning then? We suggest that the selective pressure behind the evolution of reason is not for solitary reasoners to improve on their thoughts and decisions, but for making social interaction more efficient. Reason has evolved chiefly to serve two related social purposes. Thanks to reason, people can provide justifications for their words and deeds and thereby mutually adjust their expectations. Thanks to reason, people can devise arguments to convince others. And, thanks to reason, people can evaluate the justifications and arguments offered by others and accept or reject them accordingly.

We all take these uses of reason for granted, but imagine how difficult even the most mundane interactions would be if we couldn’t exchange justifications and arguments. We would constantly run the risk of misjudging others and of being misjudged, and we would get stuck as soon as a disagreement emerges. You are driving with a colleague, and know that there is roadwork along the usual route, so you take a longer way. If you can’t explain why you chose this roundabout itinerary, your colleague will think you have no sense of direction. Or he’s the one driving, and you want to convince him to take the longer route. If he doesn’t trust your sense of direction over his, and you can’t defend your suggestion, there’s no way to change his mind, and you’ll end up stuck with the roadwork.

How would you define rationality?

There are many different senses of “rationality.” Two of them are quite useful. Cognition works by using a variety of inputs (from perception, communication, and memory) to draw inferences about how things are and how to act. In a wide sense, rationality is just the property exhibited by well-functioning inferential systems, whether those of flies, octopi, or humans.

In a narrower sense, rationality is the property of reasons that are intuitively recognisable as good reasons, and, by extension, of the opinions, decisions, actions or policies that can be justified or argued for by using such good reasons. Just as the use of reasons in justification and argumentation plays a major role in human interaction, appeal to reason-based rationality is a central feature of our attempts to come to terms with one another.

What is the function of irrationality?

If rationality in the wide sense correspond to effective inference, and rationality in the narrow sense correspond to effective uses of reason proper, then irrationality can no more have a function than any other form of cognitive or bodily impairment. On the other hand, irrationality can be put to use in a number of more or less disingenuous ways: as an defense for oneself, as means to attack others, as an excuse for letting go, and so on. These uses of irrationality may themselves, on occasion, be quite rational.

What place do flaws in reasoning, like confirmation bias, have in your view of reason?

The so-called confirmation bias consists in a strong tendency to find evidence and arguments in support of our preexisting opinions or hunches and to ignore counter-evidence or counter-arguments. It is better called the “myside bias” since we demonstrate it only in our own favour: we are not disposed, even if asked to do so, to “confirm” ideas that we do not share.

The myside bias, we argue, makes sense in interactive contexts. If you want to justify yourself, be your own advocate, not your own prosecutor. If you want to convince others of some opinion you already hold, look for the strengths, not the weaknesses of your viewpoint. Contrary to the dominant view, the myside bias is not a bug, but an adaptive feature of reason.

It is an important and original part of our hypothesis that the myside bias is characteristic of the production of reasons, but not of their evaluation. People must be much more objective in evaluate the reasons provided by other than in producing their own. This might seem surprising, but the evidence suggests that this is what people actually do. By and large, they respond well to good arguments, even if this means revising their own beliefs.

How can your view of the evolutionary/social function of reason be applied practically?

To make the best of our capacity to reason, we should keep in mind that it typically yields its best results in social settings. When we reason on our own, our natural inclination is to keep finding arguments for our point of view—because of the myside bias. As a result, we are unlikely to change our initial point of view—whether it is correct or not—and we might end up becoming overconfident. By contrast, if we discuss with people who disagree with us, but share some overarching goal—to make a good decision or have more accurate opinions—we are better equipped to evaluate their arguments and they are better equipped to evaluate ours. As a result, the myside biases of the different parties may be held in check, potentially yielding an efficient division of cognitive labuor.

We live in an era when "reason" seems far removed from much political discourse ("alternative facts" and "fake news"). How would you explain this trend?

One element of explanation is that many of the arguments we run in the political realm into are not really meant to convince anyone, but simply to bolster the views of like-minded people. As a result, they can spread without being properly evaluated.

Ditto increasing political polarisation in the US, UK, and elsewhere in Europe: why is this happening if our capacity for reason has a social function?

For reason to function well in a social setting, both disagreement and a common interest in reaching better knowledge and decisions is critical. When people who share a deeply entrenched opinions discuss together, arguments supporting one and the same point of view are likely to pile up, largely unexamined. In such conditions, people typically end up developing even more extreme views.

The question is, then, why do people keep reasoning on their own or with like-minded people? We think that the main impetus for such reasoning is the anticipation of being challenged or of having the opportunities of challenging one’s adversaries. People are typically rehearsing arguments and justifications to be used in such confrontations. A typical cue that such rehearsing may be useful is the knowledge that there are people around who strongly disagree with us. When we learn of different political views on TV, the newspaper, the Internet, etc., it is difficult not to spontaneously think of arguments defending our own views. The issue, however, is that we rarely end up actually talking with the people whose contrary views we are exposed to (and even less often discussing in an open-minded, constructive manner). So we don’t know how they would have answered our arguments. The myside bias is left to run amok.

The psychology of human reasoning, however, is only one relevant consideration among many in addressing the broad and complex historical, social, and cultural issues raised by the remarkable current political situation. In our book, we just aimed, more modestly - if this is the right word - to solve the challenge that human reason poses to scientific psychology.