Laurie Santos is an associate professor of Psychology and the director of the Comparative Cognition Laboratory and the Canine Cognition Center. She is also the instructor of “Sex, Evolution and Human Nature,” one of Yale’s most popular courses with almost 600 students. Santos’s research focuses primarily on comparing the behavior and cognitive abilities of non-humans to those of humans in order to better understand the way the human mind works. Last month, together with co-author and postdoctoral researcher at Yale’s Department of Psychology Alexandra Rosati, Santos published a comprehensive review of the evolutionary roots of human decision-making in the Annual Review of Psychology. The News sat down with Santos to discuss the choice biases humans share with their primate relatives and what that says about the origins of human irrationality from a psychological and evolutionary perspective.

Q

In your review, you examined the foundation of biases in human decision-making through studying similar “errors” committed by non-human primates. How do you think studying animals can help shed light on human irrationality?

A

I think studying animals can shed light in a couple different ways. One is that when we study how people make decisions or how people make errors when they make decisions, we don’t really know where the errors come from. Maybe they are learned, maybe they have to do with some of the specific experiences we have as humans, but maybe there is something deeper there. So I think studying animals can often tell us whether or not there is something deeper than we expected. Perhaps there are evolutionary reasons for some of the strategies we use. Particularly from studying primates, we get some hints on how old different biases are and where the biases come from.

Q

What are some of the most striking examples of decision-making biases in non-human primates?

A

One that we studied in our lab is the bias known as loss aversion, which is this idea that you pay more attention to changes that go in the negative direction than similarly sized changes in the positive direction. Loss aversion is a big bias in human decision-making — you see it in the housing market, you see it in how people invest, and so on. We were interested in studying that in capuchin monkeys, which we did using a token economy, where the monkeys made decisions trading different tokens with humans. What we found was that the monkeys, too, showed this loss aversion. Even though they got the same amount of food either way, they didn’t like receiving less than they expected.

A

Another one that we saw in the capuchin monkeys was that they also seemed to rationalize their decisions after the fact. This is something humans do. You make a choice between A and B. Let’s say you decided to pick A. In addition to liking A better, you also derogate B to justify your decision. The capuchin monkeys also seemed to do that. If you give them a choice between two M&M colors, say red and green, they make their choice, but then what you find is that, whichever one they reject, they continue rejecting that one even further in the future, as though they are rationalizing their choice. You only see this effect when the monkeys make a choice. When you give them one of the two [colors], the effect disappears. The interpretation is that they might, like us, show cognitive dissonance, where they are trying to reduce their dissonance in order to accept the choices that they made before.

Q

Given that human beings and non-human primates share a lot of fallacies in decision-making, what does that say about the evolutionary significance of irrationality? Why do we share many of these biases with our non-human primate relatives?

A

This idea of shared errors presents a bit of a puzzle. Usually when you see a strategy that’s shared between humans and primates, you assume that the strategy has been around for a long time. When you think about errors, then, it’s kind of curious why you expect them to be there for so long if they’re not really good strategies. Say that systematically you make worse decisions if you’re loss averse. If that’s true, why has [loss aversion] been maintained in our primate lineage for so long? I think the answer to the puzzle is two-fold. One is that maybe these biases are rational — the best we can do given our constraints. The second possibility is that the strategies are good for something, it’s just that we don’t use them for the right things anymore. In the domain of loss aversion, some have suggested that it is a good strategy for keeping track of who has what resources in a situation concerning fairness. Maybe it’s a healthy strategy to have in one context. Another possibility is that the strategies are very useful, but we cannot take them out of context. I think there are hints that some of the biases make sense for [non-human primates], but not for us. Take inter-temporal choice [choosing smaller rewards that come sooner over larger rewards that come later] — yes we can set up a situation for [non-human primates], but in the wild, later is never really guaranteed. It is really risky, and later might be a shorter time horizon than we can have with human savings and so on. So it’s possible that those strategies are built for situations we faced as ancestral primates, but we don’t use them anymore. There are different situations now.

Q

Throughout your research, have you been able to identify certain mechanisms in decision-making that are unique to humans? If so, why do you think only humans possess them?

A

We did find one case of a bias that we uniquely have as humans. It’s an effect known as the pricing bias. For example, I can change the way you taste a piece of chocolate or a glass of wine by lying to you and telling you the price. If I tell you that the wine is from a $100 bottle, you think it tastes better than if [I tell you] it’s from a $10 bottle, even though it is the same wine in both cases. Also, if I tell you a pain medicine costs more, you assume you’re actually experiencing less pain. We were interested to see if monkeys might show this [phenomenon] as well. What we found across a bunch of different studies is that monkeys did not show this bias. We knew from our earlier studies that we could teach them about price, so they understand price, but they could care less about the price of something. Although we share so many of the other biases with these monkeys, it seems as if the monkeys might not be making use of social information in the same way people are — we use information about what others think and prefer [to guide our decisions]. The idea is that maybe the monkeys might not be picking up on social preference information like humans.

Q

Why is it important to study animals?

A

I think it’s fascinating to understand humans and human nature better, and I think one of the best ways to do that is through studying animals because they can give us good clues as to what human cognition is like in the absence of language and other training. Some people think I am interested in animals. I am, sort of, but I am more interested in people. I just think the right way — a complementary way — would be [to learn about people] through animals. Studying animals allows you to be very specific about the phenomenon in question, and it really forces you to think about whether there are simpler ways that a strategy can be used. Often times you get some really exciting data about the underlying mechanisms. You definitely want to compare what you’re learning about through animals to what you find in humans because for most things, humans might be doing them in a different, slightly more complicated, way. But really the way you know if humans are unique is through studying [human strategies] in primates, too. We have the intuition that certain strategies are human only, and therefore very complicated. It’s only when we look to animals [that we realize] only this tiny part of a strategy is really human and special. We are less different than we think we are, but still different in important ways, and we are still trying to figure out what those important ways are.

Correction: Feb. 3

A previous version of this article neglected to mention that Laurie Santos serves as director of the Comparative Cognition Lab, in addition to directing the Canine Cognition Center. A photo that ran with the article was also miscredited to Santos. The credit belongs to Alexandra Rosati.