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Essential trust: The brain science of trust

A picture of a human brain taken by a positron emission tomography scanner, also called PET scan, is seen on a screen. (Fred Tanneau/AFP via Getty Images)
A picture of a human brain taken by a positron emission tomography scanner, also called PET scan, is seen on a screen. (Fred Tanneau/AFP via Getty Images)

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This is part II of our series Essential trust. Find part I here.

What happens in your brain when you decide to trust someone?

“When people make decisions to trust, it’s kind of the same as when they make decisions to gamble,” Jamil Zaki says.

“You see activities in the parts of the brain that are involved in its dopamine system that calculate on the fly, ‘Well, what does this gamble look like?’”

In episode two of our special series “Essential trust,” neuroscientists explain how our brains process trust, and why it’s worth the risk.

Today, On Point: The neuroscience of trust.

Play the trust game featured on the show here.


Jamil Zaki, associate professor of psychology at Stanford University. Director of the Stanford Social Neuroscience Lab. Author ofThe War for Kindness. (@zakijam)

Oriel FeldmanHall, associate professor of cognitive, linguistic and psychological sciences at Brown University. Director of the FeldmanHall Lab, which studies the neural basis of human social behavior. (@orielf)

Show Transcript

MEGHNA CHAKRABARTI: Welcome to episode two of our special series, Essential trust. We launched the series with episode one, where we learned about trust in the animal kingdom and how even guppies show that trust … an important factor in guppy society. Very, very cool.

Today, we’re going to refocus on human beings and what specifically happens in that spectacularly complex and beautiful network that is your brain when you trust someone, or what happens in your brain when someone trusts you. And we’re going to start by playing a game.

And joining me to do that is Oriel FeldmanHall. Oriel is the director of the FeldmanHall Lab at Brown University, which studies the neural basis of human social behavior. She’s also assistant professor of cognitive, linguistic and psychological sciences at Brown University. Professor FeldmanHall, welcome to you.

ORIEL FELDMANHALL: Thank you so much for having me here.

CHAKRABARTI: Great to have you. Jamil Zaki is also with us today. He’s the director of the Stanford Social Neuroscience Lab and associate professor of psychology at Stanford University. He’s author of the forthcoming book, The Hopeful Skeptic. Professor Zaki, welcome back to On Point.

JAMIL ZAKI: It’s great to be here. Thanks for having me, Meghna.

CHAKRABARTI: Okay. So, Jamil, I’m going to ask you to help get us started with this game that we’re going to play, called appropriately the trust game. What is it?

ZAKI: Perfect. So let me just put you in your roles first and foremost. So Meghna, you’re going to be the investor in this game. And Oriel. Hi, you’re going to be the trustee. So, Meghna, imagine that you have $10 sitting in front of you, all in $1 bills. And you can send as much of this money as you want to Oriel.

Now, whatever you send to her, I will multiply by four. So if you send $5 to her, it will turn into $20. If you send $10, it will turn into $40. Oriel, when you receive that money, you can then give whatever you want back to Meghna, but you don’t have to give anything back.

CHAKRABARTI: I could give her $5 and she’d get $20 and she could keep it all.

ZAKI: She could keep everything if she wants. She could betray you right in front of the entire nation right now.

ZAKI: So, Meghna, what would you like to invest in Oriel? How much of your money would you like to invest right now?

CHAKRABARTI: Okay. But she has the choice that she could give some of that back to me?

ZAKI: She could give as much of it as she wants back to you. So if you invest everything, $10, it will turn into $40. If Oriel then decides to do the fair thing and give you back half, you will end up with twice as much money as you have right now. And she will end up with $20, as well. So, you both win.

CHAKRABARTI: Okay. Or she could give me $40 back.

ZAKI: Technically, yes.

CHAKRABARTI: Okay. Oh, I’m only laughing because depending on the situation, my risk tolerance kind of fluctuates. Okay, Oriel, I’m going to give you $5.

ZAKI: Okay. Now I’m waving my magic wand. Oriel, you now have $20. How much of that would you like to give back to Meghna?

FELDMANHALL: Okay, so now I’m thinking through why I might reciprocate that initial trust for the actions. You didn’t give me the full $10, so maybe you’re a little dishonest.

CHAKRABARTI: I can’t even see your face. We’ve never met each other.

FELDMANHALL: I know. And then the question I’m, of course, considering is, are you going to have me back on air? And if you’re not, I just keep the $20 and go have a cup of coffee with some friends.

CHAKRABARTI: I just want people to know that we never pay our guests, okay? Like, just want to put this out there. Regardless of the outcome of this experiment, Oriel, I’m sure we would love to have you back some day.

FELDMANhALL: Because there’s a lot of people listening, I’m going to reciprocate that initial trustworthy action. You sent me $20 and I’m going to give you $10 back. So you’ve doubled your earnings and I’ve walked away with $10 as well.

ZAKI: … So the game is over. Congratulations to both of you. Meghna, you are left with $15. You kept $5 … you got $10 back. And Oriel, you have $10. You have $10 that you didn’t have before. And also I want to say congratulations because you all did what most people do in a trust game that is between two strangers.

So in a study of studies, a summary of lots and lots of different studies of the trust game, most investors send about 50% of the money that they have to a stranger, and most strangers send about 40 to 50% of the quadrupled amounts of money back to the investor that sent it to them in the first place.

CHAKRABARTI: So the stranger gets off with more money in the end. I will note that. But I have to say … Oriel, nothing personal, but the reason why I didn’t send all of the $10 to you with the hopes that we would both end up making more money. Is that I just thought, well, what if she keeps it? At least I walk away with $5, you know.

FELDMANHALL: That’s for sure. Although that initial trustworthy action, deciding how much to send, whether, let’s say you sent $1 or all $10, is actually a signal to myself the role that I was playing for, how much I usually send back. So the more you send, the data shows, the more you send to me, the more I’m willing to reciprocate that initial trustworthy action.

CHAKRABARTI: Okay, so demonstrating that you trust someone can build reciprocal levels of trust between people. That’s interesting. So, let’s talk about what was happening in our brains during the course of this experiment here. I mean, Oriel, what are some of the things going on neurologically that help determine how much I, you know, decided to trust you and vice versa?

Right. So when the brain is assessing whether to make a decision, just as you did with me, whether you want to trust me with a sum of money, and whether I’m actually going to reciprocate it back, there’s a region of the brain called the striatum. It’s sort of nestled deep in the middle of the brain, and it’s involved in reward learning.

So it uses rewards to learn about the world. And what happened when you made that decision is that the striatum became active and it was one of the key regions that was making the decision to actually trust that $5 with me.

CHAKRABARTI: Jamil, I have to say, money is kind of an interesting thing. I think I’ll just speaking for myself. My behavior when it comes to money is probably very different than my behavior if I just met a random person on the street and they were like, Hey, how’s it going? You know, do you know what I mean? Like, I think I’m more protective when it comes to money and less trusting.

ZAKI: Yeah, it’s really interesting. I mean, the trust game that you just played is probably the workhorse of social science, in understanding how people trust when they’re willing to be vulnerable to somebody else, on the hope that that person will reciprocate, as Oriel nicely said. But it’s interesting because using money to study trust is sort of like using money to study kindness or other parts of our social lives. I mean, when we are friends with somebody, we don’t just say, Oh, I can tell you’re having a hard day. Here’s $5.

We support each other. And when we trust somebody else, it’s not always by forking over a bunch of money and seeing if they pay us back. We trust people to watch our kids. We trust people when we confide in them. And there’s at least some evidence that when you move people, as you’re saying, into the domain of money, they actually start acting differently.

They start becoming more calculative, even sometimes more selfish. And so we might, at the same time, realize that there’s a lot of value in using games like the one that you just played. But there are limitations in understanding what social life is really like by just reducing it to money.

CHAKRABARTI: Okay, so that’s an important caveat here. … Now, Jamil, Oriel had talked about the striatum and reward learning. … Is it activated or is it active when we’re building or demonstrating trust? What is actually happening in that part of the brain that generates what we’re experiencing as trust?

ZAKI: Well, I think that as Oriel really put it very well, the striatum is a brain region or system in the brain that’s associated with evaluating options. And in particular, you can think of trust as a social gamble. You put resources, time, energy into another person and then if all goes well, you end up winning. In fact, you both end up winning. And it’s not just with money. Of course, trust builds all sorts of other good things in our lives.

But on the other side of that is the risk of betrayal, which both can be a monetary loss, but also can be an emotional loss if somebody hurts you or betrays your trust. And so the striatum is involved in assessing options that include risk. So maybe calculating, as you were doing, Meghna. Saying, well, I don’t know Oriel, but we are on the radio.

So there are some reasons maybe to think that she could just run away with the money. But there are other reasons to think that she wouldn’t. When you were evaluating the risk and likelihood, the ways that this gamble could turn out, that’s a calculation that was occurring likely in part in your striatum.

CHAKRABARTI: And Oriel, we’ve got about a minute before we have to take the first break. So are there other parts of the brain that are important here? I’ve been reading about the dorsomedial prefrontal cortex, which seems to come up a lot when it comes to human, particularly human behaviors.

FELDMANHALL: There’s a network of brain regions actually that are involved. So the dorsomedial prefrontal cortex is one of those, and that is a key region for mentalizing or thinking about what another person will do, what are their intentions? How might they reciprocate the action?

So when you are trying to assess the risk, just as Jamil said, part of what you needed to do was to figure out what I was thinking. So this part of the brain sits right above your nose, right in your forehead.

But there’s also another region, the amygdala, which is, you know, probably a very famous region. People know a lot about it because it’s almond shaped. It’s on two sides of the brain and it’s considered to be the threat or the fear center of the brain.

CHAKRABARTI: Jamil, Oriel was saying some really, really interesting things to me about the different parts of the brain that are associated or activated when we’re feeling trusted, or trusting someone. Because if I understand correctly, we’ve got parts of the prefrontal cortex which are important to assess the world around us. And make judgments. And then you also mentioned the amygdala, which is kind of a center of elemental fight or flight behavior.

And I know that I’m oversimplifying here, but for the sake of time, two really different parts of the brain here. But what’s actually happening when we say these parts of the brain are being activated, is there some sort of surge in a neurotransmitter? Or what’s going on?

ZAKI: Well, typically the studies that we’re talking about now are functional magnetic resonance imaging studies, which actually look at metabolism in the brain. So which parts of the brain are using oxygen in a particular moment, which neuroscientists then use as a proxy for which parts of the brain are active. Now, exactly which neurotransmitters are involved is usually mysterious. If we’re only using ephemera, there are other techniques that we would need to use to learn that, as well.

CHAKRABARTI: Oriel, when you mentioned reward being part of what’s going on here, in my layperson’s understanding, my mind first went to dopamine, right? Because the whole idea of like that little dopamine squirt that we get sometimes when we feel like we’re winning or get a reward. Is that part of the picture here at all?

FELDMANHALL: Yes. I mean, it is part of the picture. However, as Jamil said, it’s hard using just … imaging techniques to understand the role of dopamine, specifically. And so for the most part, what neuroscientists are doing is looking at broad brush strokes, which regions of the brain are involved, like the striatum or the amygdala or the prefrontal cortex, and whether there’s further engagement.

So that is just, as you said, surging activity, which we call the bold signal. And with more current and cutting edge techniques, we can also look at how patterns of neural activity encode the things that Jamil was speaking about earlier, like risk assessment. So not just about where and how strongly a region is involved, but what exactly that region is representing when it is engaged.

CHAKRABARTI: Okay. So what’s the significance of that? Help me understand more.

FELDMANHALL: Sure. So people have different risk profiles, just as you mentioned. And what we could do by using these different types of techniques with imaging is to assess what exactly your risk profile looks like in the brain. So if you have a pattern of activity in the striatum that is different than a pattern of activity in my striatum, it might link to a risk profile that is, let’s say, more risky than mine, which is more risk averse. And with these techniques, we can assess those differences at the neuro level.

CHAKRABARTI: Okay. So, you know, it occurs to me that I’m kind of interchanging when an individual decides to trust someone else or an institution, and when someone feels trustworthy. Now there are two different things, right?

So I mean, Jamil, I was reading about how there were some early studies maybe 15ish years ago that found that when someone feels trustworthy or someone else has demonstrated that they trust that person. So, like, if you showed me that you trusted me, that might generate a rise in me, in oxytocin. So even just feeling trusted has some sort of effect on a person and their brain.

ZAKI: That’s right. And I think this brings us to a really important point. … For instance, when we just played the trust game, you were focused on a couple of things, sort of, well, is this other person who I don’t know, going to walk away with my money? You were thinking about, as most people do, the vulnerability that trust gives us. The way that when we trust somebody, we’re putting something on the line. What I think we don’t often realize enough is how much we affect other people when we decide whether or not to trust them.

So when you decide to trust somebody, you’re sending a signal to them of who you think they are. And they’re also receiving a signal about who you are. That you’re a social person who’s willing to invest in them. And it turns out that receiving that trust teaches people a lot and also makes them feel good if they receive trust.

Which, as you said, is related to activity of oxytocin, but also when people don’t receive trust. If you had sent $1 to Oriel, or nothing, well, then she would have maybe felt alienated. And it turns out that that’s why people, or part of why people tend to reciprocate trust and retaliate against distrust. So in addition to seeing trust as a decision about whether we want to be vulnerable to somebody else, we should see it as a decision to affect somebody else, to be kind through our trust, or maybe to be unkind through our distrust.

CHAKRABARTI: So this is so fascinating to me because, I mean, the common wisdom is that maybe it’s hard to build trust, but really easy to lose it. I mean, your example about the game is spot on. Because Oriel, I can only imagine how you would feel if I just like, oh like I’ll give you $0.50. Right?

But it would be like the first real interaction between us. We don’t know anything about each other, you know? And that one little decision that I’d make would have a major impact on how much you would choose to trust me in the future. I mean, is it really that that quick in terms of how people are judging others’ trustworthiness?

FELDMANHALL: That’s correct. And one of the things, you know, the old saying, just as you mentioned, it takes a while to build trust and it’s easy to lose it. If you make that initial step to not trust, or actually to betray the trust. It makes it very difficult to come back and build that trust again. And that’s what we call the sticky prior. So you have this information that a person has betrayed you and it takes very a number of repeated experiences to push that aside and to start building trust again.

The other thing is that people make decisions about how trustworthy an individual is just by looking at their face. We don’t have that ability because we’re talking through the phone right now in a podcast, but if we were looking at one another in front of each other, you and I would both make snap second decisions or excuse me, judgments about how trustworthy we would look.

And those judgments about whether you look trustworthy or not are very predictive of important outcomes like elections, election success or prison sentencing. And so there’s a lot that goes into the equation of how we come to trust another person.

CHAKRABARTI: Hang on here for just a second. I mean, you’re referencing studies that you’ve done, work on this directly, that find that we are looking at other people and making decisions about trustworthiness in milliseconds, right?

FELDMANHALL: That’s right. That’s correct. Yes. And actually, not my work. This is Alex Todorov. … In milliseconds, essentially in 30 milliseconds, which is less than 1/10 of a second, people can make these decisions about how trustworthy an individual looks.

CHAKRABARTI: What are they looking at that that gives them the information that they’re processing in 1/10 of a second?

FELDMANHALL: Yeah. So the whole face is basically captured. But what they’re looking at is how far apart an individual’s eyes are, the luminance of their cheekbones, the widesetness of their jaw. This all gets taken in, in this very quick, automatic way in evaluated to render a decision of whether the person looks trustworthy, or whether they don’t.

CHAKRABARTI: What about those factors makes us evolutionarily programed, to use an awkward phrase, to think that certain things are more trustworthy than others?

FELDMANHALL: There’s inefficiency in making these decisions. So if I meet you in a dark alley and everything that my brain says is that Meghna’s not trustworthy. My amygdala is going to come online and tell me to turn around and run the other way. And that’s an important thing to have in your toolkit, because if you find yourself in a situation where you are approaching threat or there’s something that is threatening to you, you need to eliminate it immediately. And so our bodies and our brains are wired, so to speak, to figure out those threatening situations very quickly and very swiftly.

CHAKRABARTI: So, Jamil, I have to say, this is really, really compelling research that Oriel is referencing here, it also makes me feel more than a little concerned. Because … using her example, if there were some brief millisecond interaction between Oriel and I, and her brain determined that I was untrustworthy. And so, therefore, is it that people who look like me would also be untrustworthy? And if so, I mean, I’m of, you know, Indian heritage. So we’d be writing off more than a billion people here. I mean, I say that slightly tongue in cheek, but you see what I’m getting at? It’s worrisome, to say the least.

ZAKI: I do. I think Oriel just described this work beautifully, and it’s really important to understand that our brains might have these defaults. But just to say that we have these defaults is not the same as saying that they are correct in sometimes understanding when we might have instincts to trust or not trust people based on the way that they look. Actually, the way to use that information is to know that this might be a bias that you have that’s probably driving you in the wrong direction a bunch of the time. And being aware of those biases can help.

You know, another general bias that we have is just to not trust people enough, to be cynical about whether people in general, not just people who look a certain way, but all people will reciprocate. And one of the tricky things about that is that it’s very difficult to learn based on not trusting somebody. So if we trust someone and we’re betrayed, we learn, you know, it’s takes years to earn that trust and seconds to lose it.

And we’re always, you know, once burned, twice shy. But when we don’t trust somebody, we can miss an opportunity. But those missed opportunities don’t last in our memories the way that betrayal does. And as a result, we tend to be too risk averse in the social world. We tend to be too amygdala-driven in our trust decisions, and we lose out on opportunities. Not just to, you know, make partnerships or collaborations, but to form and build relationships as well.

CHAKRABARTI: You know, I understand how, let’s say in early human societies, this snap judgment literally of trustworthiness actually makes some sense. But of course, we’re living in a very, very different world now, and especially in places like the United States, where we are still working at trying to create, you know, this multi-ethnic democracy that relies fundamentally on trusting each other no matter who we are and what we look like, in order to make the country as a whole work. I mean, Jamil, just for a second here, take this to the national level. What you and Oriel have been researching for all this time, how does it work when it comes to, let’s say, how Americans trust each other?

ZAKI: Well, I think that we, I would argue, don’t trust each other enough. If you ask people to estimate how much a person will reciprocate in a trust game, they underestimated by about 30%. So we believe that people are less trustworthy than they really are. And I think that to your point, we especially loathe to trust people who are different from ourselves in any number of ways, whether people who look differently than us, people who identify differently, people who believe different things than we do.

And I think that those ancient instincts that, again, Oriel described really well might have served us in ancient times, but they’re not always serving us now. And that lack of trust across difference can be an enormous barrier to collaboration between people, to friendships, and I would argue to democracy. Because once we start to imagine that people we disagree with are just wholesale untrustworthy, that they’re bad people, well, then we foreclose on any possibility of finding common ground.

CHAKRABARTI: Now, Oriel, your lab, I think I have this part right, that your lab has found that because of these impulses that you talked about, we do tend to over generalize, like everybody does. Is that right?

FELDMANHALL: That’s right. So we did this study a number of years ago now where we had individuals play the game that you and I played. So essentially you could learn about individuals who were trustworthy and those who were untrustworthy. What we then did is we took the pictures of those individuals that they played with. So we were all looking at faces of the individuals, the partners, in the trust game.

But unbeknownst to our subjects, we morphed those people you learned that were trustworthy or untrustworthy, with different other people that they had never met before. And then we said, Let’s play the trust game again with these individuals who had some resemblance. They had some parts of their faces that resembled those initial people that they learned were trustworthy, or who are not trustworthy.

And what we found was that initial learning of, let’s say, someone being trustworthy or untrustworthy, very much biased their decision to trust a completely novel, unfamiliar stranger. So there’s this perceptual similarity that what you’re speaking about might not actually has a lot of import for, let’s say, stereotype bias. If you have one bad interaction or one good interaction with an individual who looks a certain way, you may then over generalize what that person looks like to people of the entire race and make judgments about whether they’re trustworthy or untrustworthy based on that initial learned response from a previous interaction.

FELDMANHALL: So just to be clear, I mean, we’re not providing any kind of excuse here for racism, right? We’re just trying to understand, you know, how the brain functions. But knowing what you know now, Oriel. We’ve just got a minute here before the next break, so I’ll let you start answering this. What should we do? How should we modify our own individual behaviors to prevent ourselves from over generalizing?

FELDMANHALL: That’s a tough but good question. I think there’s a lot of automaticity that happens with overgeneralization. And again, it’s an efficiency mechanism that allows us to come into a situation that might be threatening or aversive and walk away and thus preserve and begin to be adaptive to our well-being.

But the problem is, and Jamil mentioned this as well, is that our current society, the place that we live in the here and now, is not built in the same way. And so if we have these impulses that rise up. Part of what we need to be doing is checking those impulses, thinking about them, and then evaluating to see if they are actually correct or not.

This article was originally published on WBUR.org.

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