Steven Pinker: The mechanics of trust in money and relationships

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Become a member Login Full Interview Steven Pinker: The mechanics of trust in money and relationships "There is a big, powerful idea floating around linguistics and philosophy and economics and game theory." Philosophy Mind and Behavior Communication Psychology Social Media Steven Pinker: The mechanics of trust in money and relationships Steven Pinker Steven Pinker is an experimental psychologist who conducts research in visual cognition, psycholinguistics, and social relations. He grew up in Montreal and earned his BA from McGill and his Ph.D.[…] Overview Transcript Related Episodes Steven Pinker: The mechanics of trust in money and relationships Steven Pinker Steven Pinker is an experimental psychologist who conducts research in visual cognition, psycholinguistics, and social relations. He grew up in Montreal and earned his BA from McGill and his Ph.D.[…] Up Next 18mins The Big Think Interview Why modern fitness culture misunderstands human bodies "It's this modern idea of doing voluntary discretionary, physical activity for the sake of health and fitness." Daniel Lieberman 18mins The Big Think Interview Why modern fitness culture misunderstands human bodies Daniel Lieberman 53mins Members Full Interview The hard problem of consciousness, in 53 minutes Annaka Harris 7mins Members The Big Think Interview Our intuitions about consciousness may be deeply wrong  Annaka Harris 1hr 51mins Full Interview Why Stoicism treats self-control as a form of intelligence Massimo Pigliucci 25mins The Big Think Interview The real reason boys turn to extreme online role models Richard Reeves 25mins The Big Think Interview The real reason some people adapt faster than others George Bonanno 16mins The Big Think Interview The happiness shortcut that hides in plain sight Robert Waldinger 1hr 23mins Full Interview The terrifying ways that social media is altering teenage brains Clare Morell 7mins Members The Big Think Interview The psychological trap behind wanting your life to “make sense” Anne-Laure Le Cunff 47mins Full Interview Yuval Noah Harari: Why advanced societies fall for mass delusion Yuval Noah Harari

What happens when everyone knows the same thing, and knows that everyone else knows it?

Steven Pinker adventures into the subtle but powerful concept of common knowledge, revealing how it shapes money, power, and everyday life.

STEVEN PINKER: My name is Steven Pinker. I am a professor of psychology at Harvard University. I am a cognitive scientist and my new book is called "When Everyone Knows That Everyone Knows, Common Knowledge and the Mysteries of Money, Power and Everyday Life. "The Hidden Psychology Behind Common Knowledge." There is a big, powerful idea floating around linguistics and philosophy and economics and game theory. That's really a psychological phenomenon. I am a cognitive psychologist. And it drives many phenomena, explains many mysteries, and not enough people know about it. It is the concept called common knowledge. It has a bit of a technical meaning that is not the same as the way we use common knowledge in everyday conversation. We often say, "Well, it's common knowledge around here that you could bribe the police." Kind of an open secret. But common knowledge in a technical sense almost means the opposite. Namely, it's something that not only does everyone know it, but everyone knows that everyone knows it and everyone knows that and everyone knows that and so on ad infinitum. That is, I know something, you know it. I know that you know it. You know that I know it. I know that you know that I know that you know it and so on. And you might think, "Well, gee, didn't we all know that? What's the big deal with common knowledge?" I mean, everyone knows that there's a difference between something being private and something being public. But there's a big difference between broadcasting a message to everyone and making it common knowledge. So if everyone gets an email, but they have no idea who else has gotten the email because the recipient list is suppressed, that can be very different than if there's a big email list and you know that everyone's gotten the same email that you've gotten. Or if it is posted on Facebook or even more prominently if it's on broadcast media. Because in that case, if you know that other people are seeing it at the same time that you are and that they know that other people are seeing it, then people can feel that some norm is being challenged, which they then have to prop up. Or that some equilibrium is being upset, some social relationship is being upended. Probably the simplest illustration is the story of the Emperor's new clothes. Because when the little boy said the Emperor was naked, he wasn't telling anyone anything they didn't already know. They could see the Emperor was naked. But he was changing their knowledge. Now everyone knew that everyone else knew that everyone else knew that everyone else knew and crucially that changed their relationship to the Emperor from obsequious deference to ridicule and scorn. I give an example of what happened with Joe Biden in June of 2024. At the time, many people privately suspected that he was suffering cognitive decline. A majority of Americans did, but then he engaged in a disastrous televised debate with Donald Trump. Making sure that we're able to make every single solitary person eligible for what I've been able to do with the COVID, dealing with everything we have to do with, look, if we finally beat Medicare. Thank you, President Biden, President Trump. It was on broadcast TV, it was highly hyped, so that when people were watching it, they knew that everyone else was watching it and knew that everyone else knew. Now, of course, more people were exposed to Biden's fumbling performance. Indeed, it went up from, I think, 67% of Americans to 73%, I forget the exact numbers. It went up a little bit, but not that much. But still, it changed everything. What changed? Well, now that everyone knew that everyone knew that he was suffering cognitive decline, you could no longer respect him as the leader of a country, and he was forced to bow out of the race a few weeks later. Common knowledge is interesting because it's what ratifies our social relationships, our relationships of friendship or deference or love or transactional relationship. And that's because common knowledge, more generally, is necessary for any kind of coordination. When people agree to do something that benefits them both, but it only works if both of them do it. And for that to happen, you need common knowledge. Something as simple as a rendezvous. If you want to end up at the same place at the same time as someone else, it's not enough to know that that's where they like to go because they might think about where you want to go and end up someplace else. They might even think about what you know about what they know and you might think about what you know that they know that you know. And each of you could be at cross purposes that your plans could fall through the cracks, as we say, unless you could establish the common ground of common knowledge. Each of you knows the other one knows it. And this is also vital for social conventions like paper currency. Why do I accept a $50 bill in exchange for something of value? Well, because I know other people will accept it. But why will they accept it? Well, they know that still other people will accept it. How do they know that? Well, those other people also know that other people will accept it. That's what keeps currency of value, namely the common knowledge that it has value, which is also why currencies can become worthless if you get, if confidence in them falls, and then you can get hyperinflation or a run on a bank where the deposits in a bank no longer worth anything. If everyone is afraid that the bank might be unable to cover its deposits if people withdraw it, so they do withdraw their savings because they're afraid that other people withdraw their savings because those people are afraid that other people withdraw their savings. And so the self-reinforcing common expectation can create a new reality. In this case, the bank can fail. And in the case of the Great Depression, the entire economy can fail. You can get recessions, for example, when companies are squeamish about hiring a new employee because they're not sure whether they can sell enough of their product to have receipts that could pay the employees. And then consumers don't want to buy anything because they're afraid of losing their jobs. And why are they afraid of losing their jobs? Well, it's because employers are afraid that customers won't buy stuff. And so you can get a vicious circle of reverberant doubt or common fear. The nicest explanation of this came from Franklin Delano Roosevelt, where he said, "The only thing we have to fear is fear itself." That might sound kind of like a feel-good bromide, but it's actually literally true that what people had to fear was fear itself. And by tapping down on that fear by things like the bank holiday, where you couldn't take your money out of the bank, that was actually good because no one else could take their money out either. And so it stopped the headlong rush to everyone to withdraw their money before everyone else withdrew their money. A good example of common knowledge are channels for nonverbal communication that have often puzzled people, psychologists, philosophers, for centuries. Like, why do we laugh? So yeah, there's certain things, certain indignities, certain incongruities that we notice. But why should we be going, "Ha ha ha ha ha, why should it interrupt our breath? Why should it be so noisy?" Likewise, when something is, someone undergoes some loss or feeling of helplessness, why should tears trickle out of their eyes? Like, why should the body get involved the way it does? Why do we blush? Why does blood well up in our cheeks? What I suggest is that these are generators of common knowledge. When you laugh, you know you're laughing. It's interrupting your speech, interrupting your breathing. Other people know you're laughing. They can hear it. They know that you know because they can hear the interruptions in your breathing and you know they know that you know it. Likewise, when you blush, you feel the heat of the blush from inside your cheeks. At the same time as you know that other people can see the reddening from outside your cheeks and they know that you know that you're blushing. In fact, they might even make the blush worse by saying, "You're blushing." Studies have shown that you can even make someone blush if they're not blushing by falsely saying, "You're blushing." Blushing is also a matter of common knowledge. Tears, you see the world through a layer of your own tears at the same time that other people see your eyes glistening welling up and the trickle. Eye contact may be the most potent common knowledge generator of them all because you're looking at the part of the person that's looking at the part of you that's looking at the part of them and so on. It instantly generates common knowledge. Why do we do it? Why do we have these ways of generating common knowledge? Each one reinforces or establishes a certain kind of social reality. As I've been mentioning, social reality like any kind of social coordination is propped up by common knowledge. In the case of laughing, it's, we all recognize some infirmity, some weakness, some indignity in some target. It could be some fancy person who's been lording it over everyone, a teacher, a preacher, a president, a blowhard, the barroom gas bag that you want to take down a few pegs. Now, if you're the only one taking him down, then he could have you in his sights and you could be in trouble. But there's safety in numbers. How do you generate safety in numbers? Well, laughter, which is, of course, contagious, can signal to everyone we all recognize that person's weakness at the same time and we all recognize that we all recognize it. So none of us is in danger. We're standing up against this person. A well-known example was the White House Correspondents' Dinner in 2011 in which Barack Obama historically roasted Donald Trump. Trump had advanced the birther theory that Obama had not been born in the United States and was not eligible to be president. He was born in Kenya. Turned out to be false. So Obama said, "I've released my birth certificate and tonight for the first time, I am releasing my official birth video." This not only shows that he was willing to pretend as if he was actually taking the accusations seriously, but saying this is as absurd as a cartoon. But he followed it up by saying, "I want to make clear to the Fox News table that was a joke." Making fun of Fox News' credulity at anything that attacked anyone on the left. Well, then he went on to take aim at Trump himself and he said, "Well, there are rumors that Trump might be president and one thing that he could certainly accomplish is closing down the prison camp at Guantanamo because Trump has a history of showing he's been very good at running waterfront properties into the ground." Now, that was a dig at Trump's many bankruptcies, Atlantic City hotels and such. And amid the merriment, everyone was laughing, but Donald Trump, he was visibly seething. Partly because that indignity, that insult, struck to his proudest claim, namely that he was a successful businessman. It's often said that in laughter there is truth and that was a truth that Trump could not abide. There's even a theory that that was the last straw that tipped him over running for presidency as a kind of revenge and redemption. It could also be used, of course, convivially. Sometimes we often make fun of ourselves. We josh, we tease our friends. And there it's also a hierarchy-leveling signal, kind of egalitarian signal, because when you're friends, you want everyone to be equal. That's the whole point of friendship. You're not someone's boss. You're not someone's superior. And we constantly reinforce that signal in social gatherings by laughing when one of us makes fun of ourselves or our friends. In crying, in particular, the most common trigger for crying, some sort of loss or weakness or surrender, you often want to show that you're not willing to put up more of a fight for the same reason that an army might raise a white flag or a boxing coach might throw in the towel, as we say. Once it's clear who's won, who's lost, neither side wants the fight to continue. It's in both their interests to back off. And tears are a signal that someone has gone too far and the time is to back off. For eye contact, we can use it for all kinds of social relationships. It is, among many animals, not just humans, used as a threat signal, that the cold, hard stare can mean I am hereby asserting dominance, you better back off, or there could be a fight that both of us will suffer from. So it's in both of our interests if you back down and you know that I'm standing my ground. But in humans, we use eye contact to ratify all kinds of things. Basically means something which so far has been private knowledge. I'm going to, I am right now making common knowledge. As in, can you look me in the eye and say that? That is when you challenge someone who has been saying something that you know is false, that he knows is false, but that you haven't yet agreed is false. When you say that while looking me in the eye, you're saying I'm making that common knowledge and it's going to change the nature of our relationship. I got into the whole topic of common knowledge from my interest in language, because a well known phenomenon of language is that very often you can't figure out what someone really means just by looking at the grammatical structure of their sentences. Someone says, if you could pass the salt, that would be awesome. What does that mean? Well, it doesn't mean if you could pass the salt, that would be awesome. It means give me the salt. And polite requests are one of many examples where we don't just blurt out what we mean, but we expect our hearer to read between the lines, to catch our drift, and to figure out what is the intent behind the somewhat misleading wording. That's one of the reasons why I took so long to get computers to understand language. If you just programmed them with the rules of English grammar, they would come up with all kinds of wacky interpretations. Even though they're not wacky, if you look at the literal meaning of the nouns and verbs, but if you say to a computer question and answer system, could you tell me the shortest way to get to Times Square? The literally correct answer is, yes, I could tell you that. That's all you want. You want it to do it. You're not actually asking the question, are you capable of doing session to session? So it's a major phenomenon of language. It's one that I took on a number of years ago in a book that I believe I talked about on Big Think, the Stuff of Thought, Language as a Window into Human Nature, where I gave other examples where we clearly don't mean the literal content of our words. Want to come up for Netflix and chill? People kind of know that there's meaning. Want to come up for Netflix and chill? Or, gee, officer, is there someone we could paddle the ticket right here without like doing all that paperwork and going to court? People recognize that as a veiled bribe. I cited a dialogue from the Sopranos, where a member of the family approaches an old friend in a supermarket and says, "Listen, Danny, we just want you to know how glad we are, a guy like you was on that journey. Life in two kids, we know you'll do the right thing." Now, everyone knows that that is a veiled threat. So why don't we just cut the crap and say what we mean? That was the puzzle that I took up, and I think the answer depended on common knowledge. A threat is a criminal act, but a veiled threat may not pass the threshold of proof beyond reasonable doubt. Likewise, with a bribe, and with a polite request, like, "I was wondering if you could pass me the salt." Again, a very weird thing to say if you think about it literally. You're avoiding the impression of treating someone like a servant. You're not barking orders at them, but by indicating some prerequisite to what you want. Namely, a person couldn't pass the salt if they're incapable of passing the salt. So if you say, "Can you pass the salt?" You're cluing them that you want them to pass the salt, but you're treating them with greater respect. So the two different layers, two different levels of communication. The overt wording determines the nature of your relationship. The innuendo, the guess, the reading between the lines conveys the content of the proposition. So common knowledge is what allows people to be on the same page, to coordinate, to act together. That's generally a good thing because all of our conventions, like money, like technological standards, like recognizing who has authority, who's the boss, who can legitimately boss whom around, like our friendships, our romances, they are all cooperative endeavors that depend on common knowledge. But common knowledge, when people act together, can also be a fearsome thing as when you have a mob that engages in a riot because all of them feel that some attack on one of their members will result in them sinking in status or respect. So they lash out also in a public arena to make sure that everyone knows that they are formidable, they won't take it lying down. And so you can get riots and pogroms and revolutions. A good example of this is political polarization in the last 10 or 15 years. And it's been speculated, I think plausibly, that one driver of that could be social media. But not just social media, cable news channels, where people watching it are a subset of the population with common political assumptions. And there's not that much back and forth between different networks of either literal television networks like Fox News and informal networks within social media of people reposting something that other people repost that something that other people repost. Studies have shown that we are getting two different pools of people posting, reposting, seeing what each other sees. And that can lead to two different kind of pools of common knowledge, really common belief, because for something the common knowledge has to be true and contradictory things can't both be true. But you could have two, in fact, we are seeing two pools of common belief where people naturally assume something that they think is the truth, that their fellow coalition members also sees the truth, that people in the other coalition don't recognize as the truth. And some of the mutual incomprehension and scorn that we've been seeing could be that there are fewer channels that generate common knowledge compared to the era where there were a few nationwide TV stations and newspapers. Traditional media were excellent forums for generating common knowledge. When you watch Johnny Carson or Walton Cronkite or 60 Minutes or Saturday Night Live, you can come to work the next day and you could make some reference to them and you have high confidence that everyone else would have heard what you heard or seen what you've seen. That can sometimes generate bubbles and shortages and hoarding. There is, I described an episode in 1973. At the time, Johnny Carson, who hosted the Tonight Show for many decades, was the king of late night. He was so familiar that catchphrases from the show like, "Here's Johnny!" were kind of national models. Everyone knew where it came from. Anyway, one night, this was a time when there was an oil shortage because of the Arab oil embargo. There were also around that time shortages of meat and coffee and sugar and Carson quipped. "Do you know what else is disappearing from the supermarket shelves? Toilet paper. Ha ha ha, you can laugh now!" Now, it turned out there actually wasn't a shortage of toilet paper, but soon there was a shortage of toilet paper because everyone watching Johnny Carson knew that the rest of the country was watching Carson and therefore everyone else was going to run out and buy toilet paper. So they ran out because they wanted to get theirs before the shelves were bare. So it created a shortage over nothing because in the days of just three commercial networks, they were very good common knowledge generators. Likewise, everyone knows about the Super Bowl. Everyone knows that everyone watches the Super Bowl. That's why advertisers often use the Super Bowl to introduce products that depend on common knowledge. The most famous example is the Apple Macintosh. So it was introduced in 1984. It was a way better computer than the PCs that were available at the time, which had 80 rows of 24 characters and yet to memorize these convoluted strings of characters and text and one mistyped character and the whole thing would crash. So they introduced this insanely great new device that had windows and icons and a mouse, much easier to use. But the problem was that no one was going to go out and buy one if they thought they were going to be the only one because then the price wouldn't come down with a lot of demand. There would not necessarily be software. There wouldn't be peripherals. There wouldn't be support groups. There wouldn't be repair centers. You'd buy a Mac only if you thought enough other people were buying a Mac and why would they buy a Mac? Well, because they had to expect that a lot of people were buying a Mac. So Apple had this problem. They even introduced a predecessor called the Lisa, which flopped. So this time they hired Ridley Scott, one of Hollywood's most famous directors. He did Alien and Blade Runner. And he put together a striking ad that showed exactly once in all of TV history, and it was the most expensive ad ever made, it didn't say anything about the Macintosh. It had a grim corporate meeting where everything was in gray. And then there was a young woman in a tank top and red shorts who burst into the room, threw a hammer at the screen, which shattered into a fireball, revealing the rolling text. In January, Apple introduced the Macintosh and you'll see why 1984 will not be like 1984. Now, that's kind of a weird ad because it didn't say anything about this insanely great new product. But the whole point of the ad was not the product. The whole point of the ad was everyone else was watching the ad and that's what they had to generate. And that was true of other products that are heavily hyped on the Super Bowl. The Discover card, when it was first introduced, who wants a credit card that no merchant will accept, what merchant will accept a credit card that no consumers will have. More recently, the Super Bowl a couple of years ago was filled with high concept ads for cryptocurrency, which again said nothing about the benefits of cryptocurrency like the government can confiscate it and its hedge against inflation. All the ad said was everyone's buying crypto don't be left out. The idea would be to whip up a bubble where people would buy crypto because they think that other people were buying crypto. Why would they think that? Well, they just advertise on the Super Bowl and everyone knows that everyone watches the Super Bowl. The advertisers care a lot about the difference between the size of an audience and whether the audience knows that there is a big audience. Now, social media, of course, have a different mode of transmission. They are directed to you in a personal feed. One would think therefore that social media were incapable of generating common knowledge. Almost by definition, you get it. You have no idea who else is getting it. But people sometimes perceive social media to be common knowledge. Twitter now X was called the town square. And a lot of people stayed on X, even though they really hated it, just because the rivals that came up like Threads and Blue Sky just did not have as big an audience as X did. And so people stayed on X because other people were staying on X. Also, the thing about social media is that because of the like and repost buttons, anything could go viral. You could help make it go viral. And that's exacerbated by the feature of social media that have a trending column or a for you column that where you kind of think, geez, if I'm seeing a trending, a lot of other people are seeing a trending. And I can even make a comment that all those people who are seeing a trending will also see. So it opens up a whole new dynamic of common knowledge, or at least the perception of common knowledge. And I think it's one of the reasons we see social media shaming mobs, where someone might make some dad joke or an innocent remark, or a wisecrack. And someone decides it's racist, and they attack the person, then all of a sudden you could have hundreds of thousands of people attacking this person, even if it wasn't it wasn't intended to be racist in the first place. But it's, I think it's because we have a norm against racism, which is a really good thing. People don't tell ethnic jokes in public anymore, which they used to not so long ago. But what holds up that norm? It's not like the police that enforce it. It's held up like other social norms, because it's just something you don't do. And everyone knows that you don't do it, because everyone knows you don't do it. And so if someone seems to float that norm in public, and people perceive social media as potentially viral, they feel the need to punish it also in public to prop up the norm. It's not enough that the person who breaks the norm be punished, but everyone has to know that everyone knows that he's being punished. Otherwise, you have not propped up the norm. That's why we used to have punishments in public, like public hangings and public crucifixions and burnings at the stake, or even pillories and stocks. It isn't enough that the person suffer, everyone has to see and that other people are seeing the person suffer. And social media make it really easy to do that. It can spiral out of control. And it even brings out, since the cost in one person piling on is so low, and they can kind of gain a steam in their circle of like-minded people that they're on the side of the angels, it can be very tempted for people to pile on. Now, this wasn't original to social media, and many great novelists and dramatists have brought to life the phenomenon where once someone is being punished in public, you can get a kind of frenzied mob that all want to join in. I give examples from great works of literature like Arthur Miller's "The Crucible" about the Salem Witch Trials, Arthur Kessler's "Darkness at Noon" about the Stalinist show trials, and the famous short story by Shirley Jackson, "The Lottery," where the upsetting and puzzling thing about the story is you get a mob that attacks a woman for no reason at all. And readers are always wondering, "Well, why would they do that?" And I think Jackson was commentating on the fact that this phenomenon of stoning someone in a public place can take on a life of its own. It doesn't even need a reason.

Overview Transcript

What happens when everyone knows the same thing, and knows that everyone else knows it?

Steven Pinker adventures into the subtle but powerful concept of common knowledge, revealing how it shapes money, power, and everyday life.

STEVEN PINKER: My name is Steven Pinker. I am a professor of psychology at Harvard University. I am a cognitive scientist and my new book is called "When Everyone Knows That Everyone Knows, Common Knowledge and the Mysteries of Money, Power and Everyday Life. "The Hidden Psychology Behind Common Knowledge." There is a big, powerful idea floating around linguistics and philosophy and economics and game theory. That's really a psychological phenomenon. I am a cognitive psychologist. And it drives many phenomena, explains many mysteries, and not enough people know about it. It is the concept called common knowledge. It has a bit of a technical meaning that is not the same as the way we use common knowledge in everyday conversation. We often say, "Well, it's common knowledge around here that you could bribe the police." Kind of an open secret. But common knowledge in a technical sense almost means the opposite. Namely, it's something that not only does everyone know it, but everyone knows that everyone knows it and everyone knows that and everyone knows that and so on ad infinitum. That is, I know something, you know it. I know that you know it. You know that I know it. I know that you know that I know that you know it and so on. And you might think, "Well, gee, didn't we all know that? What's the big deal with common knowledge?" I mean, everyone knows that there's a difference between something being private and something being public. But there's a big difference between broadcasting a message to everyone and making it common knowledge. So if everyone gets an email, but they have no idea who else has gotten the email because the recipient list is suppressed, that can be very different than if there's a big email list and you know that everyone's gotten the same email that you've gotten. Or if it is posted on Facebook or even more prominently if it's on broadcast media. Because in that case, if you know that other people are seeing it at the same time that you are and that they know that other people are seeing it, then people can feel that some norm is being challenged, which they then have to prop up. Or that some equilibrium is being upset, some social relationship is being upended. Probably the simplest illustration is the story of the Emperor's new clothes. Because when the little boy said the Emperor was naked, he wasn't telling anyone anything they didn't already know. They could see the Emperor was naked. But he was changing their knowledge. Now everyone knew that everyone else knew that everyone else knew that everyone else knew and crucially that changed their relationship to the Emperor from obsequious deference to ridicule and scorn. I give an example of what happened with Joe Biden in June of 2024. At the time, many people privately suspected that he was suffering cognitive decline. A majority of Americans did, but then he engaged in a disastrous televised debate with Donald Trump. Making sure that we're able to make every single solitary person eligible for what I've been able to do with the COVID, dealing with everything we have to do with, look, if we finally beat Medicare. Thank you, President Biden, President Trump. It was on broadcast TV, it was highly hyped, so that when people were watching it, they knew that everyone else was watching it and knew that everyone else knew. Now, of course, more people were exposed to Biden's fumbling performance. Indeed, it went up from, I think, 67% of Americans to 73%, I forget the exact numbers. It went up a little bit, but not that much. But still, it changed everything. What changed? Well, now that everyone knew that everyone knew that he was suffering cognitive decline, you could no longer respect him as the leader of a country, and he was forced to bow out of the race a few weeks later. Common knowledge is interesting because it's what ratifies our social relationships, our relationships of friendship or deference or love or transactional relationship. And that's because common knowledge, more generally, is necessary for any kind of coordination. When people agree to do something that benefits them both, but it only works if both of them do it. And for that to happen, you need common knowledge. Something as simple as a rendezvous. If you want to end up at the same place at the same time as someone else, it's not enough to know that that's where they like to go because they might think about where you want to go and end up someplace else. They might even think about what you know about what they know and you might think about what you know that they know that you know. And each of you could be at cross purposes that your plans could fall through the cracks, as we say, unless you could establish the common ground of common knowledge. Each of you knows the other one knows it. And this is also vital for social conventions like paper currency. Why do I accept a $50 bill in exchange for something of value? Well, because I know other people will accept it. But why will they accept it? Well, they know that still other people will accept it. How do they know that? Well, those other people also know that other people will accept it. That's what keeps currency of value, namely the common knowledge that it has value, which is also why currencies can become worthless if you get, if confidence in them falls, and then you can get hyperinflation or a run on a bank where the deposits in a bank no longer worth anything. If everyone is afraid that the bank might be unable to cover its deposits if people withdraw it, so they do withdraw their savings because they're afraid that other people withdraw their savings because those people are afraid that other people withdraw their savings. And so the self-reinforcing common expectation can create a new reality. In this case, the bank can fail. And in the case of the Great Depression, the entire economy can fail. You can get recessions, for example, when companies are squeamish about hiring a new employee because they're not sure whether they can sell enough of their product to have receipts that could pay the employees. And then consumers don't want to buy anything because they're afraid of losing their jobs. And why are they afraid of losing their jobs? Well, it's because employers are afraid that customers won't buy stuff. And so you can get a vicious circle of reverberant doubt or common fear. The nicest explanation of this came from Franklin Delano Roosevelt, where he said, "The only thing we have to fear is fear itself." That might sound kind of like a feel-good bromide, but it's actually literally true that what people had to fear was fear itself. And by tapping down on that fear by things like the bank holiday, where you couldn't take your money out of the bank, that was actually good because no one else could take their money out either. And so it stopped the headlong rush to everyone to withdraw their money before everyone else withdrew their money. A good example of common knowledge are channels for nonverbal communication that have often puzzled people, psychologists, philosophers, for centuries. Like, why do we laugh? So yeah, there's certain things, certain indignities, certain incongruities that we notice. But why should we be going, "Ha ha ha ha ha, why should it interrupt our breath? Why should it be so noisy?" Likewise, when something is, someone undergoes some loss or feeling of helplessness, why should tears trickle out of their eyes? Like, why should the body get involved the way it does? Why do we blush? Why does blood well up in our cheeks? What I suggest is that these are generators of common knowledge. When you laugh, you know you're laughing. It's interrupting your speech, interrupting your breathing. Other people know you're laughing. They can hear it. They know that you know because they can hear the interruptions in your breathing and you know they know that you know it. Likewise, when you blush, you feel the heat of the blush from inside your cheeks. At the same time as you know that other people can see the reddening from outside your cheeks and they know that you know that you're blushing. In fact, they might even make the blush worse by saying, "You're blushing." Studies have shown that you can even make someone blush if they're not blushing by falsely saying, "You're blushing." Blushing is also a matter of common knowledge. Tears, you see the world through a layer of your own tears at the same time that other people see your eyes glistening welling up and the trickle. Eye contact may be the most potent common knowledge generator of them all because you're looking at the part of the person that's looking at the part of you that's looking at the part of them and so on. It instantly generates common knowledge. Why do we do it? Why do we have these ways of generating common knowledge? Each one reinforces or establishes a certain kind of social reality. As I've been mentioning, social reality like any kind of social coordination is propped up by common knowledge. In the case of laughing, it's, we all recognize some infirmity, some weakness, some indignity in some target. It could be some fancy person who's been lording it over everyone, a teacher, a preacher, a president, a blowhard, the barroom gas bag that you want to take down a few pegs. Now, if you're the only one taking him down, then he could have you in his sights and you could be in trouble. But there's safety in numbers. How do you generate safety in numbers? Well, laughter, which is, of course, contagious, can signal to everyone we all recognize that person's weakness at the same time and we all recognize that we all recognize it. So none of us is in danger. We're standing up against this person. A well-known example was the White House Correspondents' Dinner in 2011 in which Barack Obama historically roasted Donald Trump. Trump had advanced the birther theory that Obama had not been born in the United States and was not eligible to be president. He was born in Kenya. Turned out to be false. So Obama said, "I've released my birth certificate and tonight for the first time, I am releasing my official birth video." This not only shows that he was willing to pretend as if he was actually taking the accusations seriously, but saying this is as absurd as a cartoon. But he followed it up by saying, "I want to make clear to the Fox News table that was a joke." Making fun of Fox News' credulity at anything that attacked anyone on the left. Well, then he went on to take aim at Trump himself and he said, "Well, there are rumors that Trump might be president and one thing that he could certainly accomplish is closing down the prison camp at Guantanamo because Trump has a history of showing he's been very good at running waterfront properties into the ground." Now, that was a dig at Trump's many bankruptcies, Atlantic City hotels and such. And amid the merriment, everyone was laughing, but Donald Trump, he was visibly seething. Partly because that indignity, that insult, struck to his proudest claim, namely that he was a successful businessman. It's often said that in laughter there is truth and that was a truth that Trump could not abide. There's even a theory that that was the last straw that tipped him over running for presidency as a kind of revenge and redemption. It could also be used, of course, convivially. Sometimes we often make fun of ourselves. We josh, we tease our friends. And there it's also a hierarchy-leveling signal, kind of egalitarian signal, because when you're friends, you want everyone to be equal. That's the whole point of friendship. You're not someone's boss. You're not someone's superior. And we constantly reinforce that signal in social gatherings by laughing when one of us makes fun of ourselves or our friends. In crying, in particular, the most common trigger for crying, some sort of loss or weakness or surrender, you often want to show that you're not willing to put up more of a fight for the same reason that an army might raise a white flag or a boxing coach might throw in the towel, as we say. Once it's clear who's won, who's lost, neither side wants the fight to continue. It's in both their interests to back off. And tears are a signal that someone has gone too far and the time is to back off. For eye contact, we can use it for all kinds of social relationships. It is, among many animals, not just humans, used as a threat signal, that the cold, hard stare can mean I am hereby asserting dominance, you better back off, or there could be a fight that both of us will suffer from. So it's in both of our interests if you back down and you know that I'm standing my ground. But in humans, we use eye contact to ratify all kinds of things. Basically means something which so far has been private knowledge. I'm going to, I am right now making common knowledge. As in, can you look me in the eye and say that? That is when you challenge someone who has been saying something that you know is false, that he knows is false, but that you haven't yet agreed is false. When you say that while looking me in the eye, you're saying I'm making that common knowledge and it's going to change the nature of our relationship. I got into the whole topic of common knowledge from my interest in language, because a well known phenomenon of language is that very often you can't figure out what someone really means just by looking at the grammatical structure of their sentences. Someone says, if you could pass the salt, that would be awesome. What does that mean? Well, it doesn't mean if you could pass the salt, that would be awesome. It means give me the salt. And polite requests are one of many examples where we don't just blurt out what we mean, but we expect our hearer to read between the lines, to catch our drift, and to figure out what is the intent behind the somewhat misleading wording. That's one of the reasons why I took so long to get computers to understand language. If you just programmed them with the rules of English grammar, they would come up with all kinds of wacky interpretations. Even though they're not wacky, if you look at the literal meaning of the nouns and verbs, but if you say to a computer question and answer system, could you tell me the shortest way to get to Times Square? The literally correct answer is, yes, I could tell you that. That's all you want. You want it to do it. You're not actually asking the question, are you capable of doing session to session? So it's a major phenomenon of language. It's one that I took on a number of years ago in a book that I believe I talked about on Big Think, the Stuff of Thought, Language as a Window into Human Nature, where I gave other examples where we clearly don't mean the literal content of our words. Want to come up for Netflix and chill? People kind of know that there's meaning. Want to come up for Netflix and chill? Or, gee, officer, is there someone we could paddle the ticket right here without like doing all that paperwork and going to court? People recognize that as a veiled bribe. I cited a dialogue from the Sopranos, where a member of the family approaches an old friend in a supermarket and says, "Listen, Danny, we just want you to know how glad we are, a guy like you was on that journey. Life in two kids, we know you'll do the right thing." Now, everyone knows that that is a veiled threat. So why don't we just cut the crap and say what we mean? That was the puzzle that I took up, and I think the answer depended on common knowledge. A threat is a criminal act, but a veiled threat may not pass the threshold of proof beyond reasonable doubt. Likewise, with a bribe, and with a polite request, like, "I was wondering if you could pass me the salt." Again, a very weird thing to say if you think about it literally. You're avoiding the impression of treating someone like a servant. You're not barking orders at them, but by indicating some prerequisite to what you want. Namely, a person couldn't pass the salt if they're incapable of passing the salt. So if you say, "Can you pass the salt?" You're cluing them that you want them to pass the salt, but you're treating them with greater respect. So the two different layers, two different levels of communication. The overt wording determines the nature of your relationship. The innuendo, the guess, the reading between the lines conveys the content of the proposition. So common knowledge is what allows people to be on the same page, to coordinate, to act together. That's generally a good thing because all of our conventions, like money, like technological standards, like recognizing who has authority, who's the boss, who can legitimately boss whom around, like our friendships, our romances, they are all cooperative endeavors that depend on common knowledge. But common knowledge, when people act together, can also be a fearsome thing as when you have a mob that engages in a riot because all of them feel that some attack on one of their members will result in them sinking in status or respect. So they lash out also in a public arena to make sure that everyone knows that they are formidable, they won't take it lying down. And so you can get riots and pogroms and revolutions. A good example of this is political polarization in the last 10 or 15 years. And it's been speculated, I think plausibly, that one driver of that could be social media. But not just social media, cable news channels, where people watching it are a subset of the population with common political assumptions. And there's not that much back and forth between different networks of either literal television networks like Fox News and informal networks within social media of people reposting something that other people repost that something that other people repost. Studies have shown that we are getting two different pools of people posting, reposting, seeing what each other sees. And that can lead to two different kind of pools of common knowledge, really common belief, because for something the common knowledge has to be true and contradictory things can't both be true. But you could have two, in fact, we are seeing two pools of common belief where people naturally assume something that they think is the truth, that their fellow coalition members also sees the truth, that people in the other coalition don't recognize as the truth. And some of the mutual incomprehension and scorn that we've been seeing could be that there are fewer channels that generate common knowledge compared to the era where there were a few nationwide TV stations and newspapers. Traditional media were excellent forums for generating common knowledge. When you watch Johnny Carson or Walton Cronkite or 60 Minutes or Saturday Night Live, you can come to work the next day and you could make some reference to them and you have high confidence that everyone else would have heard what you heard or seen what you've seen. That can sometimes generate bubbles and shortages and hoarding. There is, I described an episode in 1973. At the time, Johnny Carson, who hosted the Tonight Show for many decades, was the king of late night. He was so familiar that catchphrases from the show like, "Here's Johnny!" were kind of national models. Everyone knew where it came from. Anyway, one night, this was a time when there was an oil shortage because of the Arab oil embargo. There were also around that time shortages of meat and coffee and sugar and Carson quipped. "Do you know what else is disappearing from the supermarket shelves? Toilet paper. Ha ha ha, you can laugh now!" Now, it turned out there actually wasn't a shortage of toilet paper, but soon there was a shortage of toilet paper because everyone watching Johnny Carson knew that the rest of the country was watching Carson and therefore everyone else was going to run out and buy toilet paper. So they ran out because they wanted to get theirs before the shelves were bare. So it created a shortage over nothing because in the days of just three commercial networks, they were very good common knowledge generators. Likewise, everyone knows about the Super Bowl. Everyone knows that everyone watches the Super Bowl. That's why advertisers often use the Super Bowl to introduce products that depend on common knowledge. The most famous example is the Apple Macintosh. So it was introduced in 1984. It was a way better computer than the PCs that were available at the time, which had 80 rows of 24 characters and yet to memorize these convoluted strings of characters and text and one mistyped character and the whole thing would crash. So they introduced this insanely great new device that had windows and icons and a mouse, much easier to use. But the problem was that no one was going to go out and buy one if they thought they were going to be the only one because then the price wouldn't come down with a lot of demand. There would not necessarily be software. There wouldn't be peripherals. There wouldn't be support groups. There wouldn't be repair centers. You'd buy a Mac only if you thought enough other people were buying a Mac and why would they buy a Mac? Well, because they had to expect that a lot of people were buying a Mac. So Apple had this problem. They even introduced a predecessor called the Lisa, which flopped. So this time they hired Ridley Scott, one of Hollywood's most famous directors. He did Alien and Blade Runner. And he put together a striking ad that showed exactly once in all of TV history, and it was the most expensive ad ever made, it didn't say anything about the Macintosh. It had a grim corporate meeting where everything was in gray. And then there was a young woman in a tank top and red shorts who burst into the room, threw a hammer at the screen, which shattered into a fireball, revealing the rolling text. In January, Apple introduced the Macintosh and you'll see why 1984 will not be like 1984. Now, that's kind of a weird ad because it didn't say anything about this insanely great new product. But the whole point of the ad was not the product. The whole point of the ad was everyone else was watching the ad and that's what they had to generate. And that was true of other products that are heavily hyped on the Super Bowl. The Discover card, when it was first introduced, who wants a credit card that no merchant will accept, what merchant will accept a credit card that no consumers will have. More recently, the Super Bowl a couple of years ago was filled with high concept ads for cryptocurrency, which again said nothing about the benefits of cryptocurrency like the government can confiscate it and its hedge against inflation. All the ad said was everyone's buying crypto don't be left out. The idea would be to whip up a bubble where people would buy crypto because they think that other people were buying crypto. Why would they think that? Well, they just advertise on the Super Bowl and everyone knows that everyone watches the Super Bowl. The advertisers care a lot about the difference between the size of an audience and whether the audience knows that there is a big audience. Now, social media, of course, have a different mode of transmission. They are directed to you in a personal feed. One would think therefore that social media were incapable of generating common knowledge. Almost by definition, you get it. You have no idea who else is getting it. But people sometimes perceive social media to be common knowledge. Twitter now X was called the town square. And a lot of people stayed on X, even though they really hated it, just because the rivals that came up like Threads and Blue Sky just did not have as big an audience as X did. And so people stayed on X because other people were staying on X. Also, the thing about social media is that because of the like and repost buttons, anything could go viral. You could help make it go viral. And that's exacerbated by the feature of social media that have a trending column or a for you column that where you kind of think, geez, if I'm seeing a trending, a lot of other people are seeing a trending. And I can even make a comment that all those people who are seeing a trending will also see. So it opens up a whole new dynamic of common knowledge, or at least the perception of common knowledge. And I think it's one of the reasons we see social media shaming mobs, where someone might make some dad joke or an innocent remark, or a wisecrack. And someone decides it's racist, and they attack the person, then all of a sudden you could have hundreds of thousands of people attacking this person, even if it wasn't it wasn't intended to be racist in the first place. But it's, I think it's because we have a norm against racism, which is a really good thing. People don't tell ethnic jokes in public anymore, which they used to not so long ago. But what holds up that norm? It's not like the police that enforce it. It's held up like other social norms, because it's just something you don't do. And everyone knows that you don't do it, because everyone knows you don't do it. And so if someone seems to float that norm in public, and people perceive social media as potentially viral, they feel the need to punish it also in public to prop up the norm. It's not enough that the person who breaks the norm be punished, but everyone has to know that everyone knows that he's being punished. Otherwise, you have not propped up the norm. That's why we used to have punishments in public, like public hangings and public crucifixions and burnings at the stake, or even pillories and stocks. It isn't enough that the person suffer, everyone has to see and that other people are seeing the person suffer. And social media make it really easy to do that. It can spiral out of control. And it even brings out, since the cost in one person piling on is so low, and they can kind of gain a steam in their circle of like-minded people that they're on the side of the angels, it can be very tempted for people to pile on. Now, this wasn't original to social media, and many great novelists and dramatists have brought to life the phenomenon where once someone is being punished in public, you can get a kind of frenzied mob that all want to join in. I give examples from great works of literature like Arthur Miller's "The Crucible" about the Salem Witch Trials, Arthur Kessler's "Darkness at Noon" about the Stalinist show trials, and the famous short story by Shirley Jackson, "The Lottery," where the upsetting and puzzling thing about the story is you get a mob that attacks a woman for no reason at all. And readers are always wondering, "Well, why would they do that?" And I think Jackson was commentating on the fact that this phenomenon of stoning someone in a public place can take on a life of its own. It doesn't even need a reason.

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