Instant Genius - The hidden reasons common knowledge shapes our lives

Episode Date: September 6, 2026

When we know a certain fact about the world or hold a certain belief how do we know that others also know this, how do we base our thoughts and actions on our knowledge of what we assume others know, and what impact does this have on the way that society as a whole functions? In this episode, we’re joined by Steven Pinker, a professor of psychology at Harvard University, best-selling author and two-time Pulitzer Prize finalist, to talk about his latest book, When Everyone Knows That Everyone Knows – Common Knowledge and the Science of Harmony, Hypocrisy and Outrage. He tells us about the effects common knowledge has on social norms and the ways in which we all act, the profound effects that the Internet and social media has had on the way that common knowledge is spread and consumed, and how deepening our understanding of the implications common knowledge has on our lives can help us all get along more harmoniously. Take your curiosity further with a subscription to BBC Science Focus magazine. Every issue is packed with fascinating insights into the science behind everyday life, the latest breakthroughs and expert analysis, delivered straight to your door. Receive an extra £5 when you subscribe using the code SF5OFF, (minimum spend £20, see full terms and conditions on our website). https://www.ourmediashop.com/bbc-science-focus-magazine-pod30 Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Every Monday and Friday, you'll hear a world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Jason Goodyear, commissioning editor at BBC Science Focus. When we know a certain fact about the world or hold a certain belief, how do we know that others also know this? How do we base our thoughts and actions on our knowledge of what we are. assume others know. And what impact does this have on the way that society as a whole functions? In this episode, we're joined by Stephen Pinker, a professor of psychology at Harvard University,

bestselling author and two-time Pulitzer Prize finalist to talk about his latest book, When Everyone Knows That Everyone Knows, Common Knowledge and the Science of Harmony, Hypocracy and Outrage. He tells us about the effects common knowledge has on social norms and the way that we're in which we all act, the profound effects that the internet and social media has had on the way that common knowledge is spread and consumed, and how deepening our understanding of the implications common knowledge has on our lives can help us all to get along more harmoniously. So welcome to the podcast. Thanks so much for joining us. Thank you. So today we're talking about your latest book, when everyone knows that everyone knows. Common Knowledge and the Science of Harmony,

hypocrisy, and Outrage. So we're talking about common knowledge here, which is, you know, a term which people use quite a lot, but it's actually subtler than it might first seem, especially in this context. So I think the first place to start is what exactly do we mean when we say common knowledge? Yeah, so mathematicians and logic, co-opted the term from everyday use in a slightly different sense. When we talk about common knowledge ordinarily, we mean conventional wisdom, something that everyone knows. In this technical sense, it refers not to something that everyone knows, but something that everyone knows that that, hence the title of the book. And everyone knows that, and everyone knows that, and so on,

at infinitum. I know something, you know it, I know that you know it, you know that I know that you know it, and so on. So that's common knowledge in a technical sense. So early on in the book, you talk about one example, one story, which most people will have heard of, if the emperor's new clothes, and how this is a great example of how things, this idea can switch, you know, suddenly something becomes common knowledge. So what's your analysis of that story in this context? It's a story about common knowledge, 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 changed their state of knowledge nonetheless,

because by blurting it out with an earshot of everyone, now everyone knew that everyone else knew that the emperor was naked. So it first of all illustrates that there is a profound logical difference between everyone knowing something and everyone knowing that everyone knows it. But also, the punchline of the story, of course, is that it changed their relationship. with the emperor, from obsequious deference to ridicule and scorn. That's an illustration of how our social relationships of respect, of friendship, of love, of deference depend on common knowledge. More generally, the reason that common knowledge is of general interest, the reason that the economists and logicians and mathematicians and philosophers have been interested in it,

Is it common knowledge is necessary for coordination, for two people being on the same page? Even for something like a rendezvous, it's not enough to know that the other person is intends to be there. You have to know that they know that you intend to be there and that they know that they know. Otherwise, you could end up in different places. The twist that I add in the book is that our social relationships are coordination games, like a rendezvous. two people are friends or lovers or one defers to the other because of their mutual understanding. Each one knows that the other one knows that they know that they're in their relationship. So something I wanted to talk about there, which you've touched on, is so we use the sort of overarching term common knowledge.

But how about like the way that this exists between certain groups of people? you know, so a lot of people like belonging to certain groups and like being, you know, people say in the no. How does that fit into this idea? Yeah, indeed. Their common knowledge can be a pool of mutually understood facts within a particular culture or subculture. Yeah, so it's always relative to the group within which common knowledge exists. So if we start thinking about groups and common knowledge, So in order for it to be sort of common knowledge, we all have to believe it.

So then we sort of, if we start investigating that idea, we have to think, well, it's what we believe even true in the first place. And so this raises the question then, you know, what is the nature of truth as we believe it? And it's spread, which obviously in history has had some really devastating consequences. Indeed. And again, just to introduce some technical terms, but that are pretty intuitive. You call it common belief when it isn't necessarily true. You can't know something that isn't true. That's not what the word no means.

No presupposes that something is true. Philosopher is defined knowledge as justified true belief. Indeed, the sources of truth are often social. That is, even when you and I know something that is, you know, the scientific consensus, Most people can't actually trace through the whole chain of scientific discoveries and theories. You kind of trust the people in the white coats. And what we're often saying now is that big sectors of society don't trust the people with the white coats. And so a lot of our knowledge really depends on being plugged into a social network where the source of knowledge is credible.

If scientists are seen as just another tribe, just another priesthood, people will blow it off. Why should I believe those experts are supposed to my experts? Exactly. So you mentioned that's sort of the social aspect of this. And when we think about that in this context, I mean, certainly I think of social norms. So personally, I'm English and middle-aged now. But my parents had me when they were quite old. And I grew up in quite an old-fashioned traditional English way, in which in my upbringing, manners were very important.

There's very certain ways to act, which when you think about it is a bit strange. I still do it now. It's absolutely drummed into me. But how does that the idea of sort of the norm, social norms, fit into this? Yes. No, it's a great example. I'm very sensitive to it being Canadian, a culture that is famously polite. and I did grow up with norms of politeness and courtesy and chivalry.

So a norm is a good example of something that is held up by common knowledge. A norm is something that's not enforced by the police, but it's just what you do and what you don't do, and it exists to the extent that everyone knows that it exists, and it can cease to exist when people no longer know that other people know. And so, indeed, especially in the public sphere, one of the big surprises of the last 10 years is the way that norms of dignity and restraint and self-control, which used to be mandatory for people in positions of power, like presidents of the United States, CEOs of major corporations, you wouldn't shoot off your mouth, you wouldn't boast, you wouldn't insult your rivals. but Donald Trump, Elon Musk, many others have exploded that norm by saying things that you just would not, no one in that position of responsibility would have said 10 years ago.

And I suspect that the norm itself can come under threat when you can flout it and get away with it. And that's the outrage in the title, the science of, in my subtitle, I'm sorry, the science of harmony, hypocrisy, and outrage. Outrage is often the response to someone flouting a norm. It's what police is the norm, given that you can't call the real police, exists because you anticipate that people be outraged if you flout it. If you flout it and the outrage passes and you get away with it, it may no longer be a norm because people no longer know that everyone knows that that's something you don't do or you can't say.

Sometimes that's good, but sometimes that can coarsen our discourse. So yeah, let's stick with the idea of the outrage bit then. So, you know, and it seems like certain events throughout history, and recently there are several examples of which, where there is a sort of upwelling of public outrage in which people are, you know, joining protests and demonstrations, when, you know, perhaps they wouldn't have done that, were it not for this sort of idea of commonality. Is it, I mean, am I right there? Yes, there's a phenomenon of a communal outrage. I don't think it's recent, although just mass media make it easier. But when a member of your group is insulted, assaulted, attacked in a public setting, it's sometimes considered an intolerable

insult on the entire group. And then the group lashes out, often out of proportion to the original affront and has to do so in a public way. There are lots of examples. A recent one in the United States was the assassination of Charlie Kirk, the right-wing speaker and influencer, which led to a wave of cancellations, death threats, the feeling that the American conservative movement as a whole was under attack. But other examples were the killing of George Floyd in 2020. The Arab Spring was launched when a Tunisian fruit peddler was harassed by a government official in Tunisia, leading to a chain of regime changes and wars. 9-11, 25 years ago, led to invasions of Iraq and Afghanistan. Pearl Harbor led to American entry into World War II, the sinking of the Lusitania.

Many examples like that, where there seems to be a lashing out. And I think the logic is, just to get back to common knowledge, is that relationships of respect, authority, dominance, prestige, status are matters of common knowledge. They're like norms. They exist because people know they exist. If they are threatened in a public setting that is a setting where everything is common knowledge, everyone saw it happen, everyone knows that everyone saw it happen. If it's allowed to pass, then it creates a new reality, a new status hierarchy, in which, in this case,

the group itself feels that they've been taken down several notches in the world's respect, they must reestablish their preeminence, their status on the world stage by lashing out. Again, it has to be public. It has to be a matter of common knowledge that they have avenged this affront. So sort of coming off the back of that, and so we briefly talked about the existence of manners. So there's sort of, in a little way, the bigger brother of manners is moral. So how closely are morals related to this idea of common knowledge? Well, the often morals are implemented as norms.

Sometimes to the good, that is, you don't punch someone in public, although that norm itself can change culturally. There are cultures of honor in which a respectable gentleman must lash out with violence if his honor is compromised, if he's insulted or affronted. We've passed from a culture of honor to a culture of dignity in which it's actually considered now immature, coarse, boorish to threaten violence in response to an insult, although that itself might be changing with the coarseness of public figures

who are now quite belligerent in a way that would have been unseemly for a mature person until recently. But sometimes morals are well implemented by norms, but of course sometimes norms can lock in a situation which we decide is not moral, such as the repression, humiliation, denigration of gay people. I mean, that was a norm, keeping women in positions of political and economic subordination,

making casual jokes about dissey, flighty, women. There were norms that you could do that. The norms have changed, and that would be a case where a new moral awareness, say the equal dignity of all people, gets implemented in a new set of norms. Great news. The federal EV rebate is back. Eligible customers get up to $5,000 with the federal EVAP rebate on select 2027-volt and 2026 Equinox EVM models. Visit your local Chevrolet dealer today for more details. So I'd like to talk about one thing that I thought was really interesting, a really interesting example, a sort of logic example of the common logic effect.

And that's the, when people are at the dinner with spinach in their teeth. Oh, yes. So, yeah, could you talk us through that, please? Yes, this has been called the, someone's called the world's hardest logic problem. and it comes in different forms, some of them kind of not suitable for work by today's standards. But the version that I have is you have a conference dinner and there's at least one person with spinach in their teeth, but there are no mirrors around so no one can tell who it is. And it's considered unseemly to point to someone else and say that they have spinach in your teeth.

So in this particular scenario, there are three people with their spinach in their teeth. and the chairman says, at least one of you has spinach in your teeth. When I clink the glass, my glass, that will be an opportunity for those who have spinach to remove it. So she clinks the glass once and no one moves. She clings the glass a second time and no one moves. She clinks the glass a third time and the three people with spinach in their teeth remove it. How did they know? That's the puzzle.

Again, it comes in different guises. But it's a puzzle of common knowledge because the best way to understand it is to imagine the simpler case when there's one person with spinach in their teeth. So there, the chairman clinks her glass once. Everyone looks around. The person with spinach sees that no one else has spinach in their teeth. So they know it has to be them. That's the easy case.

Now imagine that there are two people. Well, the chairman clinks her glass, everyone looks around. Now, the two people with spinach in their teeth, each one sees that there's someone else with spinach in their teeth. So at that point, they don't know that they're the one. However, when they see after that clink that the other person has not cleaned his teeth, they deduce he must have seen someone else with spinach in his teeth. That's why he didn't remove it. I'm looking around.

No one else but him has spinach teeth. It must be me. And then on the second cling, and the other guy goes through the skinned. same logical deduction, and so on the second clink, both of them remove their spinach. And that logic can work for any number of people with the same number of clinks of the glass. Yeah, that's brilliant. I love that. By the way, I don't deserve credit for it. It goes back to the 1950s, but I might deserve credit for using spinach in your teeth as opposed to many violent and sexist versions from the

1950s when the norms said that you could do that in academia. Sure. Having said that, it's a little bit of a segue, but there's a whole chapter on something called Weasel Words. So first of, some people might not know exactly what that means. So first off, what are they? And how does this fit into this idea? It's like another layer of subtlety, isn't it? Well, that's how I got into this whole topic, because I'm a psycholinguist. I'm interested in human language. One thing that linguists have always, known is that people don't just say what they mean in so many words. If you say, if you could pass the salt, that would be awesome. That's kind of a very strange sentence. Everyone instantly understands

it as a polite request. So why don't we just kind of cut the crap and say what we mean? There are many examples. Veiled threats. A nice story you got there would be a real shame if something happened to it. Veiled bribes. Gee, if there's there any way that I could kind of maybe you had a cancellation and I could, you know, jump the queue while you slip the person a 20-pound note. Why can't we just say what we mean? And the analysis that I have in the chapter, weasel words is a, just an idiom for evasive language, like a weasel kind of furtively escaping. It's a way that you can try to implicate your meaning through innuendo without learning it out in so many words. What I say is that direct blunt speech, if I give you a 20, will you let

me jump the cue, generates common knowledge. Each one of you knows that the other one knows, that the other one knows what you mean. A failed bribe, an innuendo, a failed sexual come on. If you're a grown-up, you know what the speaker means. You don't know that they know that you know. There is a possibility that they think that you're naive or you're dense, and that maintains your social relationship. And so you can get the message across without challenging, say, the deference, the authority, the platonic relationship in the case of a sexual innuendo. Anyway, that's what opened up this whole world of common knowledge to me, looking for the solution to the puzzle. Why don't people just say what they mean literally?

Yeah. So sort of, having said all this, we mentioned, you know, how we can kind of figure out logically how common knowledge works. How about ways that the idea can be gameed? So we can second, sort of do second, third, fourth guesses. So if, you know, I think I know that you know that I know, or I know that you think that you know what I know, then we can sort of, it opens up the possibilities of our actions. And it can come into things sort of, on a personal level, also things like financial speculation and things like that. Oh, yes. So the reason that I thought there was a whole book to write about this is that, indeed, as you say, there are so many areas of human life in which thoughts about the other

person's thoughts about your thoughts about their thoughts play out. It plays out in murder mysteries, plays out in negotiation, plays out in threats, and indeed it plays out in financial speculation. This is an idea that goes back to the great economist John Maynard Keynes, who reported a kind of beauty contest in the British papers of the day. It's actually not clear whether they really had this contest, but here's the way he described it. The goal is not to pick the prettiest face. It's to pick the face that the most other contestants pick, knowing that they're choosing a face by the same criteria. And he said that the financial speculation that is buying a shares in a company, not just to enjoy the earnings of the company, but you think, because you think that the price of the share will be worth more tomorrow. Why do you think that?

Because you think other people will think that, so they'll bid up the price. Cryptocurrency speculation is like that. Right now, AI company speculation is like that. People are anticipating that other people will anticipate. the danger being that that can suddenly pop as people stop anticipating that other people are anticipating that the price will go up. Then you can have the opposite. You can have a run on the banks. You could have a burst bubble leading sometimes to a recession or even a depression.

And in fact, depressions used to be called panics precisely because they were triggered by the common expectation that things would get worse, which means. made things worse. Having said all of this, how do these things spread? Because things have changed the way that information spreads and is disseminated. It's changed immeasurably. We've gone from the sort of old-fashioned public gathering to printed media, to radio and TV, to the age of internet and social media that we now live in. So what effect has this had on the way that common knowledge functions within society? What you used to do. What you to be conveyed in public gatherings where there would be a crowd would assemble, everyone would see the event, everyone would hear the speech while seeing everyone else see the event.

That's why public rallies can be so potent. That's why they can topple a dictator when everyone turns out in the public square. Or in the case of Nikolai Chigetsu, the dictator of Romania gave a speech in front of his adoring citizens, and someone started to heckle. And the heckles then became contagious, and within a couple of days his body was hanging from a lamppost. The common knowledge is often generated by a public spectacle, a public gathering, a ritual, a ceremony, but of course now it can be done electronically. And sometimes the viral shaming mobs on X or other social media platforms are the electronic equivalent of the public hanging or the public pillory.

So, sort of near the end of the book, I think it's in the last chapter. You talk about something called radical honesty. So that's a new term for me. So what do we mean by that? And, you know, really interesting. But how does it fit into this wider idea? Well, so going back to the phenomenon

that got me interested in this whole concept, namely the fact that so often our speech is not literally true. We have genteel hypocrisy and politeness and weasel words, as I call them. And that inevitably leads to the idea, gee, why don't we just cut the crap, say what we mean, wouldn't life be so much more efficient, there'd be less room for misunderstanding? And there have been proposals, sometimes by business leaders, let's have a new kind of corporate culture where we all just say to each other how good or bad other other people's performances. You have psychotherapists who say the reason that your life is so messed up is that

you never say what you mean, but you never tell people what you want. You also have sometimes comedies where for comic effect, one or more characters is forced to always say what's on his mind. Liar, liar might be the most famous example. And we watch the hilarity that results. But I think those movements never go very far because we really do need a certain degree of genteel hypocrisy, simply because every act of communication does two things. It shares information, but it also establishes what kind of relationship you have with the other person. And sometimes, we want to preserve a relationship while conveying compromising information. And so we deliberately avoid talking about something. I mean, just a simple example, as a group of friends,

one of them might be obese, one might have a speech impediment. Everyone knows it. By But to actually say it would be mortifying because it would reduce the dignity of that person if it's public in a way that if it is just known but not known to be known, the dignity may be preserved. So one final question to sum up then. Do you have any thoughts on how we should be better approaching or thinking about common knowledge and its benefits and its dangers. Yeah, I think it's just good to be aware of it. There isn't just sort of one takeaway guideline, precisely because what we call tact,

savour fair, social skills, all of that is a matter of the nuance, the judgment, the balance, of being explicit enough so that other people know what you mean, what has to be done, but without constantly offending people, upending relationships, insulting them, corroding the very basis of the relationships that allow us to get along. And so we always already recognize there isn't some simple rule. That's what being a socially competent, mature, tactful person consists in. Thank you for listening to this episode of Instant Genius, brought you from the team behind BBC Science Focus. That was Stephen Pinker.

To discover more about the topics we've just discussed, check out his book, When Everyone Knows That Everyone Knows, Common Knowledge and the Science of Harmony, Hypocrisy and Outreach. If you liked what you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to see our guests and hosts in person,

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