Steven Pinker on Common Knowledge

Steven Pinker: Thank you.

Steven Pinker: Common knowledge refers to the state where A knows something, B knows it. A knows that B knows it. B knows that A knows it. B knows that B knows that A knows it, ad infinitum.

Steven Pinker: So, as a cognitive psychologist–I’m not a logician, I’m not an economist, I’m not a game theorist, myself. So, having read about how vital common knowledge is to coordination–two people being on the same page, acting together in each of their interests, and how ubiquitous, how vital this is–my thoughts turn to, ‘Well, geez, what’s going on in someone’s head when they have it?’ Because clearly I can’t be thinking, ‘All I know is that she knows that I know that she knows.’ Your head starts to spin after one or two I-know-that-she-knows, let alone an infinite number which can’t fit into a finite skull.

And there has been some speculation in the various literatures on what’s going on in someone’s head if they really do have common knowledge.

And, one possibility is that we just think a bunch of I-know-that-she-knows and then think to ourselves, ‘Dot, dot, dot,’ or ‘And so, on,’ or ‘Ad infinitum.’ That introduces the problem of how do you know how to extend the series?

But, I think actually what happens more often is that we just have a sense that something is public, or salient, or self-evident, or out there. And, if two people are observing something as they observe each other, that is enough to guarantee common knowledge, because you’re seeing the other person seeing you seeing what it is that’s known.

And so that self-evidence, or salience, or conspicuity, is, I think–and the psychological intuition, ‘Uh-oh, it’s out there, I can’t take it back,’–that’s what corresponds to common knowledge in our minds.

Steven Pinker: And of course, that happens a lot.

Now, a lot of public events are designed so that they generate common knowledge. That’s why we have ceremonies like a wedding or an investiture. Everyone knows who is the leader, and everyone knows that everyone knows, and that’s what makes them the leader or that two people are committed to each other.

But, yeah: there are many possibilities for not just misunderstanding of the message of the event, but of whether other people are also aware of it, and more important, whether they’re aware that you’re aware of it.

Steven Pinker: Yeah. So, cooperation, at least as it’s often used in biology, and game theory, and economics, often refers to two people, each conferring favors on each other, each incurring a cost-to-benefit, a favor, a benefit on someone else. It was made famous 50 years ago in Richard Dawkins’s book, The Selfish Gene, because it poses a puzzle for evolutionary biology, namely how could altruism–that is suffering a cost to benefit someone else–ever have evolved if presumably natural selection favors those who act in their own interests?

And it’s also a puzzle in classical economics, because the simplest models assume that people act in their own interests. How do you explain people being generous, and giving to charity, and all the other kinds of cooperation?

And it is a genuine problem, and I think there are solutions. And, an enormous amount of writing, and attention, and bestselling books have dealt with the problem of cooperation, and they’ve introduced ideas like reciprocity, trading favors.

But, there’s another form of acting together that I think escaped a lot of people’s notice because it doesn’t have that air of paradox of, ‘What’s in it for me if I benefit someone else?’ And that is coordination, where two people do simultaneously act in a way that benefits them both without necessarily paying a cost.

In the animal kingdom, it’s sometimes called mutualism, when, for example, an oxpecker picks ticks off the back of a zebra. The oxpecker bird gets a meal, and the zebra is tormented by fewer pests, and everybody wins. No one pays a cost except for the ticks. But still–and there are lots of cases where humans both act in their own and in each other’s interest.

If two people bring complementary dishes to a potluck dinner, if they hold opposite ends of a heavy couch. Lots of cases. If they respect paper currency, one does something, and the other one pays him in greenbacks, and he, in turn, uses that to buy groceries. We drive on the right in the United States, or on the left in the United Kingdom, but all of us driving on the same side clearly benefits everyone; but no one’s paying any price.

But, what interests me as a psychologist, as with the problem of cooperation, where there is a puzzle as to how it’s possible–in the case of cooperation, is: why do someone else a favor? In this case, it’s a cognitive problem, an epistemological problem. Namely, how do I know that the other guy is going to make the same choice that I make? Not out of any fear of selfish motives, but a sheer ignorance or even unwillingness to take a chance?

And, the general answer is that public conspicuous signals, or if there isn’t any, focal points–namely solutions that occur to you and you think will occur to everyone else–that’s what cuts the knot and solves these coordination problems.

But the problem was introduced by the political scientist, Thomas Schelling, more than 60 years ago, where he imagined a couple that is separated in New York, and this was before the era of cell phones. How can they find each other? In both their interests, they really do both want to find each other. The problem being that he can’t think, ‘Well, she likes to go to a bookstore, so I’ll find you there,’ because she might think, ‘Well, he likes to go to a camera store, so I’ll go there.’ And then, he might think, ‘Oh, yeah. Well, she knows that I like going to a camera store, so she won’t go to the bookstore this time. She’ll look for me at the camera store.’ But then, she’ll anticipate that I know that she goes to the bookstore. Etc.

Nothing short of common knowledge. The most obvious case would be a cell phone call. That’s probably why we evolved language. Second best, and this is Schelling’s suggestion, is the idea of a focal point or common salience: namely, without the cell phones, they might think, ‘Well, what just pops out as an obvious solution to our problem?’ He gave the conjecture that it might be the clock in Grand Central Station, not because it necessarily was close to where they were separated, but simply because it’s so salient and obvious that each can guess that it’ll pop into the mind of the other as something would in turn pop into the mind of the other, and that they’re likely to coordinate by meeting at the clock.

Steven Pinker: Well, except it might be I’d like to think my wife would be smart enough to realize, ‘You know, now is not the time to pick what’s interesting. The rest of life is for what’s interesting. Now’s the time for what’s obvious.’

Steven Pinker: Well, this is what makes this whole topic so interesting–to me, especially, as a cognitive psychologist. That is, there are levels upon levels upon levels upon levels of mentalizing. So, she might think, ‘Well, yeah, he’ll pick what’s interesting.’ On the other hand, he knows that now is not the time to be interesting. Now is the time to be salient or obvious. Of course. And, these things can break down, and there can be misunderstandings, and we have a whole genre that’s designed to allow us to exercise this talent and to play with it.

And, these are mysteries where the whole plot hinges on who knows what about who knows what, and a skilled writer can keep the audience guessing.

Sometimes I find that mysteries have too many layers, and I just can’t keep track of it; or the author seems to know more than I could possibly know, and there’s no way that I could have figured out who stabbed the boyfriend because the number of layers of knowledge that the author has revealed isn’t enough to have anticipated the solution.

Steven Pinker: Quite right. But the–fiction in general, and especially fiction with complex plots and mysteries, as with any human talent, anything that we need for survival, we enjoy playing at, and playing is a practice for the real thing. So, sports are about aiming, chasing, and fighting, basically. Three very useful skills for our ancestors. But, we continue to exercise them for the sheer pleasure, ultimately probably to perfect them. And likewise, getting inside each other’s heads, and getting inside the head of someone who is trying to get inside the head of someone else, is a lot of what we exercise in fiction.

Steven Pinker: “The Gift of the Magi.”

Steven Pinker: Yes, that’s a good example. Yes.

Steven Pinker: We won’t give away–

Steven Pinker: No spoiler alert. But, it is about a couple, each of whom is getting in the head of the other couple, and the story is about what could go wrong.

Steven Pinker: It’s a great example, and I’m sorry it didn’t occur to me while I was writing the book because I could have used it instead of that couple who get separated at Grand–that old story from Thomas Schelling. But, it is an excellent example.

The thing about the Prisoner’s Dilemma is that even if the prisoners could communicate, it would not solve their problem because neither of them has grounds for trusting the other. Each would then be tempted to defect. Just as a reminder, I don’t know if every one of your listeners is familiar with the Prisoner’s Dilemma.

Steven Pinker: It’s a classic paradox in game theory, probably game theory’s best-known example, where two partners in crime are held in separate cells. The prosecutor doesn’t have enough evidence to convict either of them and is hoping that one of them will rat out the other, and so offers them the following devilish deal: If they both stay silent, they will get, say, a year sentence. If each one of them turns on the other, defects on the partnership, then they both get five years. If one of them turns in the other and stays silent himself, then he goes free and the partner gets ten years.

Now, what makes it devilish is that neither one wants to take the chance of going away for 10 years if he remains true to his partner, while the other partner defects and rats him out. That’s the worst possible outcome. So, each rats out the other. They mutually defect. They end up serving five years apiece, whereas if they had only stayed loyal to the coalition, they would only have served one year. But, neither of them can take the chance because he fears the other one will defect if only out of the fear that the other one fears that he will defect.

And, there’s no solution. What makes it a kind of tragedy is that they’re doomed to the second-worst outcome.

The interesting thing is, and I point this out in the book in a chapter on game theory, that Coordination Dilemmas don’t have that structure. They’re not Prisoner’s Dilemmas. And it’s easy to confuse them. One of the classic game-theoretic examples of a Coordination Dilemma was proposed by Jean-Jacques Rousseau more than 250 years ago–and to this day, it’s called a ‘Stag Hunt’ in honor of his example–which is: Imagine there are two hunters, and again, there are no telephones in those days. Every day they have to rendezvous, and they have a choice of either bringing heavy weaponry, which they can use to work together to fell a big, meaty stag, or, say, slingshots, where they can separately hunt for rabbits, much less satisfying a quarry. So, it’s in their interest that both of them bring their heavy weaponry.

But each one worries that the other one might think that the first one might just come with a slingshot, if only out of fear that the other one is worrying about the same thing. And so, they might both end up with slingshots, even if they could only exchange messages that they could coordinate and both bring heavy gear and fell the stag.

Now, it sounds a little bit like a Prisoner’s Dilemma, but the payoffs are different because in a Prisoner’s Dilemma, the worst possible outcome is if you cooperate and your partner defects.

In a stag hunt, the worst thing that can happen to you is if both of you defect. And the problem in a stag hunt is different, because if you could coordinate, neither of you really has an incentive to lie or to stay silent because it’s both in your interest to say, ‘Hey, let’s bring big bows and arrows today.’

Steven Pinker: Yes. So, I don’t think any of these undermine the interest of the game-theoretic analyses. On the contrary, they explain why we have these conventions and norms, and how they arose, and how they work.

And so in the case–and a norm basically is a matter of common knowledge–

Steven Pinker: We call it a norm as opposed to a law because the only reason that it works is that everyone knows that everyone else knows that it exists. And it can unravel if people start to flout it.

But, in the case of, say, Ostrom and others, like Larry[?Robert?] Ellickson, Order without Law, where the theory was that nothing short of enforcement by a third party can get people to stop overgrazing the commons. As you note, it is a multi-party Prisoner’s Dilemma. And in fact, Ostrom and others have noted that when you have communities of fishers, or ranchers, or farmers, and you don’t have Big Brother enforcing good behavior, they do often settle on norms that avoid the Tragedy of the Commons.

On the other hand, my understanding is that these are norms like: You don’t pull in baby lobsters and therefore deplete the stock for next year’s catch. Or: You don’t overgraze.

On the other hand, I think these norms are often enforced, certainly by gossip, if it’s known that someone flouts them, and that can hurt you in the community. And by the occasional anonymous threat, the occasional symbolic vandalism–a lobster fisherman who overfishes might see some of his trap floats cut loose to kind of send a message.

So, it is a genuine dilemma. And again, though the Tragedy of the Commons–the overfishing, and overgrazing, and so on–because it does have the structure of Prisoner’s Dilemma, is a little bit different from the coordination problems. Which are also problems; but, they’re problems that are less motivational. They’re less about greed, and generosity, and trust, and gratitude, and more about communication, being on the same page, and to some extent, a confidence versus nervousness.

Rousseau’s ‘Stag Hunt’ is sometimes called a security dilemma or an assurance dilemma, I should say, because the part of the problem is not just not getting the message, but fear of the worst that could happen if the other guy doesn’t get the message. Or, if the other guy gets the message but doesn’t know that you know that he knows that he got the message.

So, it’s a different set of issues, equally interesting. And generally, public things–public announcements, public beacons, smoke signals, bat signals, something that is out there that everyone can see and knows that everyone can see–it can be a way of solving these Coordination Dilemmas, which are somewhat different than the Tragedy of the Commons dilemmas.

So, I’ll give an example. It’s not mine. It comes from a political scientist named Michael Chwe, of the problem that Apple Computer faced when it introduced the Macintosh in 1984. The problem being a coordination problem. It was clear that compared to the old IBM PCs [International Business Machines Personal Computers], where you just had 24 lines of text and you had to type verbal commands, that a computer system with windows, and icons, and a mouse was way, way better, insanely great. The problem being you don’t want to be the only one who owns a Macintosh computer because then where will you get peripherals? Where will you get software? Where will you get tech support?

Everyone has got to–or at least a critical mass of people have to buy it at the same time. But, each one would be nervous that they may be the only one buying it.

So Apple cut that knot with a common knowledge generator, namely the American Super Bowl. This was the football game that everyone knows that everyone watches. And so, Apple paid for the most expensive commercial in the history of television. It ran only once, directed by Ridley Scott of Blade Runner and Alien fame. And, not coincidentally, it was 1984, and it played off the dystopia of 1984, showed a live athletic woman in red gym shorts and a singlet invading a corporate meeting filled with gray-clad drones and throwing a hammer at a screen which shatters in an explosion. And then, the message is, ‘On January 25th, Apple will introduce the Macintosh and you’ll see why 1984 won’t be like 1984.’

So, it was kind of meaningless. It didn’t say anything about the product, but what it was advertising was common knowledge. What it was saying is: as you’re sitting on your couch watching this, you know that tens of millions of other Americans are sitting on their couches watching it. So, if you buy this, you won’t be the only one.

And then, other examples that Chwe mentions were products that depend on coordination. Another example being the Discover credit card. Great credit card, but it doesn’t matter if no stores are going to accept it. No store is going to accept it if they think that no customers are going to carry it. How do you cut that knot? Again, Super Bowl ad.

Steven Pinker: It’s obviously not the only way to generate common knowledge. There are many ways as human ingenuity can come up with. But the point being that what has to be conveyed is not the qualities of the product, but the fact that somehow you’re not going to be the only one.

Steven Pinker: It is. Yeah. And then, Chwe did try to get independent empirical evidence for this hypothesis. So, for example, he found that products that are advertised on highly publicized events–at the time, there was the Super Bowl, but also the Barbara Walters’ famous interview with Monica Lewinsky at the height of the Clinton-Lewinsky scandal, which was avidly watched, especially during the three-network era when there wasn’t streaming video.

And, in highly public events, he counted up the products that were advertised, and he compared products that have network effects. Sometimes they would be technologically-dependent products like a new computer system, but also products that depend on brand image, and products where consuming them collectively is part of the pleasure. So, part of the pleasure of seeing a new movie is you talk to other people who’ve seen the movie. And so, if you’re the only one who sees it, it’s not as enjoyable as if it’s a blockbuster that everyone’s going to be talking about, or a best-selling book.

Anyway, what he found was that as opposed to, say, things like motor oil, or batteries, or underwear, where you consume it in private, or breakfast cereal, what he claimed was that products that are consumed in public or that otherwise have network effects are more likely to be advertised on highly public media, events like the Super Bowl–holding constant size of audience.

Likewise, what he found was that events like the Super Bowl commanded a higher premium, higher advertising price per minute, holding constant the size of the audience. That is, advertisers who play Super Bowl ads aren’t just paying for the number of eyeballs, but the rates reflect the fact that those eyeballs are aware that there are other eyeballs. At least that was the claim. So, it was clever.

And, a more recent example, another thing that depends on networks, is speculative investing. Going back to John Maynard Keynes with his metaphor of a beauty contest where the object is not to pick the prettiest face, but to pick the face that the most other contestants are picking, each one of them knowing that other contestants are anticipating what they will pick. And so, Keynes used this as a way of explaining speculative bubbles, and bank runs, and crashes, where everyone’s trying to get in the head of everyone else getting in the head of everyone else.

But, a more recent example of Chwe’s phenomenon is that two years ago, during the crypto craze, the Super Bowl was filled with high-concept ads for crypto exchanges–FTX [Futures Exchange] being the most famous–and Larry David’s appearance as himself in various periods of history resisting new innovations. But, the punchline being, ‘Don’t be like Larry. Don’t be left out.’

Again, nothing about the advantages of cryptocurrency, like their hedge against inflation, or you can buy drugs, or weapons, or hookers, or whatever. It’s nothing about what’s so great about crypto, but rather everyone else thinks that there’s something else great about crypto, so don’t miss out on the bubble.

Steven Pinker: Yes, that’s right. There is an idea that a number of anthropologists and psychologists of religion have proposed, that especially historically, as civilizations got bigger and more complex, and mere gossip networks, let alone the state, would not be adequate to police the laws, the ideology of an overseeing and moral God arose, a big God, the monotheistic God. A little bit like the song about Santa Claus: ‘He sees you when you’re sleeping. He knows when you’re awake. He knows if you’ve been bad or good. So, be good for goodness’ sake.’

The idea that there is a watcher and an enforcer can ensure large-scale cooperation in conditions where, as you note, you can’t have the transparency or the mechanisms of enforcement.

But, another thing, though, that religion, and in fact the law does, since neither God–let’s face it, there’s no guarantee that you will go to heaven or hell. There’s no guarantee that there is a heaven or hell.

Steven Pinker: There is uncertainty. Yes. Then people could take Pascal’s Wager, but still it’s not the same as if Big Brother was watching you through telescreens and the Stasi would whisk you away if you broke the law.

A lot of laws–and as we noted earlier, norms that aren’t enforceable by law work because they can be focal points. That, if everyone knows that everyone knows they exist–and that can happen with religious rituals–then that can ensure coordination even in the absence of outright enforcement. So, let me just say a couple of words about this.

Legal theorists have noted that the law is effective because there are courts, and police, and so on. But they couldn’t possibly detect every infraction of every law and send everyone to jail who breaks the law. But nonetheless, the law can work as a focal point. That is, whenever two people are in a confrontation, there’s a question of who will stand his ground and who will back down if they want to avoid the cost of a fight. And often some things aren’t worth fighting for. It’s better if someone gives way, but the question is, is it going to be me or him?

Well, what a law can do if it’s commonly known–if everyone knows that everyone knows–is that it can kind of tell you who should give way and who should prevail in these disputes.

I give an example of the time when smoking started to be outlawed. When I was a student, and since we’re the same age, I’m sure this is true of you, everyone smoked everywhere. In classrooms my professors would smoke. It was horrible, especially for a non-smoker such as myself. You could ask someone to put out their cigarette if they’re sitting next to you in a lecture hall. The problem is they could glare at you and say, ‘Who the hell are you to tell me what to do? I have the right to smoke if I want to smoke.’ It could create an ugly scene.

But then, when universities started to have policies against smoking–now, there were no smoking police that would haul you off in handcuffs if you lit up, but there were no-smoking signs in classrooms. And, what I could do is tap the guy on the shoulder, point to the no-smoking sign. And that would be, again, not enforceable. Well, it wasn’t enforceable, but no one would enforce it.

But still, in the confrontation, one of us had to give way. And, the idea was, ‘Okay, the convention around here is the smoker gives way, the non-smoker prevails.’

And so, a lot of laws and a lot of religious courts–like Sharia courts, rabbinical courts–had no police force to enforce their decisions. But, everyone accepted them simply because accepting some decision, any decision, was better than fighting over everything.

Steven Pinker: No, same–

Steven Pinker: Same, same. Yeah.

Steven Pinker: Well, in a tremendous irony–you know, I work at a university, and we have Bias-response Hotlines, where students are encouraged to anonymously snitch on their classmates for microaggressions–subtly racist comments like, ‘Where are you from?’ or, ‘You’re very articulate.’ And, helping, I think, explain the fantastic irony that college students at elite universities are kind of in adolescent heaven. Their meals are provided for, there’s constant cultural programming–

Steven Pinker: It’s a resort. They’re with their peers, and they claim to be lonely. What?

And, it’s because, I think, partly they can’t let loose and do what bonds people all over the world, namely: gossip, and off-color jokes, and politically incorrect sardonic remarks. Because we’ve got a modern equivalent of the Stasi. Namely, you don’t know which of your fellow students might rat on you in one of these anonymous Bias-response Hotlines. So, anyway, the irony is a little bit off topic.

Steven Pinker: Well, yes. And, I sometimes wonder if people who have not grown up with the shadow of Communism, the Soviet regime, and the Warsaw Pact, and reminders of what life is like under a dictatorship–not just the fear of speaking out and the way that it poisons intimate relationships, but things like the comical inefficiency of state-run enterprises.

A lot of jokes were told–if we had time, I could recount them–about the inefficiency of the Soviet system. Of which the Russian citizens themselves had mordant jokes about.

Now when you have the likely next mayor of New York, Zohran Mamdani, talk about government-run grocery stores, that’s almost like a punchline of a joke for the people who remember government-run grocery stores in the Soviet Union. And, there were a number of jokes commonly told about that. But we’re getting a little bit off-topic.

Steven Pinker: Rumors are spread throughout the neighborhood that the grocery store that day is going to have tomatoes. So, everyone is really excited, lines up around the block to get tomatoes. And, they wait there for about an hour and 20 minutes, and then the guy comes out of the supermarket, says, ‘I’m sorry, there are not going to be enough tomatoes. All Jews leave the line.’ And so they wait; another hour passes. The manager comes out and says, ‘I’m sorry, there are fewer tomatoes than we imagined. Everyone who is not a member of the Communist Party must leave.’

So, about two-thirds of the line leaves. Another 40 minutes, comes in again, ‘I’m sorry, the allotment was less than what we had originally been promised. Those who are not part of the members of the Central Committee of the Communist Party leave.’ So maybe a dozen people left. The day is getting late. The guy comes out and says, ‘I’m sorry, only members of the Central Committee who are veterans of World War II can get tomatoes.’

Then finally, it’s, you know, six o’clock. It’s about to close, and there’s only two people left in line. And, the manager comes down and says, ‘I’m sorry, there will be no tomatoes today.’ And, one says to the other, ‘Goddamn Jews always get special treatment.’

Steven Pinker: But, anyway, we’re–this is EconTalk–

Steven Pinker: So, it’s an economics joke, too.

Steven Pinker: And, what’s the point of a podcast if you can’t get spontaneous material that you won’t read about in–

Steven Pinker: Yeah. So, the first thing: Here is an experiment you can all try at home, listeners. Next time you’re in a social gathering, just pay attention to what triggers laughter. And, probably nine out of 10 times, it is not a joke, a witticism. It’s a minor indignity of someone, and it befalls the teller, or the hearer, or everyone. But, people laugh.

And, the puzzle of laughter is: unlike our facial expressions, which are pretty economical signals, communicators, when you laugh it takes over your respiratory system. You’ve got to at least momentarily stop talking. You can’t talk and laugh at the same time. It’s conspicuous. Laughter, it can be loud, it breaks into things. It’s unignorable. And it’s contagious, which is why TV sitcoms used to have laugh tracks. It genuinely does make people laugh more.

The common denominator in humor is that there usually is some indignity that is pointed out of some target, sometimes oneself, and that the laughter means that–the laughter generates common knowledge. That is, you hear it, you know everyone else is hearing it. If you are laughing, you can’t ignore it because you’ve stopped speaking, and it takes over your body at the same time that you know that others are hearing. So, it’s a common knowledge generator.

And it’s basically, you’re spreading common knowledge in some infirmity, indignity, weakness, embarrassment, sometimes aggressively. And, of course, humor can be used aggressively when you laugh at someone. And it’s very painful. Even children hate it when they’re the butt of a joke or the target of laughter.

Or it can be used convivially. A self-deprecating humor is a way that we bond, where we basically convey we’re all equals. Even though I might be smarter, or better looking, or richer, or better connected than you, I’m going to bring myself down to signal that the basis of our relationship is egalitarian friendship and not a hierarchy.

Steven Pinker: And, conversely, I have a chapter on emotional expressions that I propose are special because they’re common knowledge generators. So, laughter: you know you’re laughing, you know that other people are hearing you laughing. Crying is another one. You’re seeing the world through tears at the same time that other people are seeing the glistening or the trickling.

Blushing is another example. You feel the heat from the inside while other people see the reddening from the outside. And, when you blush, what makes a blush so painful is you know that other people see that you’re blushing.

In the case of tears, the most obvious occasion for crying is defeat, helplessness, surrender, when you’ve been hurt. And, it has probably evolved from signals of defeat in a conflict, just like there’s an interest when an army knows that it has lost, putting up a white flag of surrender, or in a boxing match, the manager throws in the towel. You want the fighting to stop. You’ve accepted defeat. Neither side wants there to be further fighting with a possibility of both of them getting hurt. And so, it calls the conflict to an end in a commonly known way.

But, although crying–a lot of occasions of tears isn’t necessarily of pain or surrender. We cry at weddings, we cry at people who win Miss America, or win the Super Bowl: We’ll have tears of joy. So, part of it might even be the opposite or antithesis of laughter.

And, Darwin proposed that facial expressions could sometimes evolve in opposite pairs, and that explains why they exist. Or bodily postures: That, if you display one posture to signal one state, to signal the opposite state there’s the opposite posture. And, in many ways, crying is the opposite of laughter. Laughter, you kind of slap your knees; and crying, you slump down. In crying, you have big inhalations. In laughter, you have loud exhalations.

And, it may be a way of appreciating–in contrast to laughter, which thrives on some indignity, some infirmity–that tears signal an appreciation of the best that life has. Of being moved by generosity, by joy, by good fortune. And so, some of the explanation for crying, I suggest, may be it’s the opposite of laughing.

Steven Pinker: Notwithstanding the texting abbreviation, LOL [laughing out loud], which is almost never true.

Steven Pinker: Yawning. Yawning is another example. Yeah.

So, in the case of laughter and tears, they’re absolutely contagious. It has been shown in studies. And there that’s consistent with the rules of common knowledge generators: namely, everyone knows that everyone knows that in the case of laughter, someone has a previously hidden infirmity or indignity. That is: What laughter does is it turns private knowledge into common knowledge.

Anyone who gets the joke–and usually a good joke–there’s some missing premise that the hearer has to fill in. When they do, what everyone knew privately, now everyone knows that everyone knows.

And, probably the same with crying.

With coughing and yawning, it’s much more mysterious why they should be contagious. One can tell a story. I didn’t in the book just because I didn’t think it was convincing enough. [More to come, 57:37]