How Common Knowledge Shapes the World | Steven Pinker | TED

Name: How Common Knowledge Shapes the World | Steven Pinker | TED
Duration: 13 min 48 s
Description: Common knowledge is the secret engine of social life, letting us coordinate everything from meet-ups to markets to international diplomacy. In this fascinati...

TED13:48Added Aug 31, 2026

Common knowledge is the secret engine of social life, letting us coordinate everything from meet-ups to markets to international diplomacy. In this fascinati...

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Contributed by 刘嘉琪

Transcript

00:04When the little boy said the emperor was naked,
00:07he wasn't telling them anything they didn't already know.
00:11But he added to their knowledge nonetheless.
00:13By blurting out what everyone could see
00:16within earshot of the others,
00:18he ensured that everyone knew that everyone else knew what they knew,
00:23that everyone knew that, and so on.
00:25And that changed their relationship with the emperor
00:28from obsequious deference
00:30to ridicule and scorn.
00:32Hans Christian Andersen's immortal story
00:35draws on a momentous logical distinction.
00:38With private knowledge,
00:40I know something, and you know it.
00:42With common knowledge,
00:44I know that fact, and you know it,
00:47but in addition, I know that you know it,
00:50and you know that I know it,
00:51and I know that you know that I know it,
00:54ad infinitum.
00:55Of course,
00:56the reason that common knowledge is significant
00:59is that it is essential for coordination.
01:02In a classic example from Thomas Schelling,
01:05a couple is separated in Manhattan, incommunicado,
01:08and somehow must find each other.
01:11Well he knows that she likes to browse the aisles of certain bookstore,
01:14so he heads there.
01:16But then he realizes that she knows
01:18that he likes to hang out in a certain camera store,
01:20so he changes course
01:22until he figures that she will anticipate
01:25that he will guess that she will opt for the bookstore.
01:28So he does another about face,
01:30only for it to dawn on him
01:32that it will occur to her
01:34that he knows that she is aware that he likes to haunt the bookstore,
01:37so he pirouettes once again.
01:40Meanwhile, she is whipsawed by the same futile empathy.
01:44Nothing short of common knowledge can guarantee that they'll end up
01:47at the same place and at the same time.
01:50Of course, no one can think an infinite Russian doll
01:55of “I know that she knows that I know that she knows” thoughts.
01:58Our heads start to spin with three or four layers,
02:01let alone an infinite number.
02:03In a well-known episode of "Friends,"
02:07Phoebe says to Rachel,
02:09“They don’t know we know they know we know.
02:12Joey, you can't say anything!"
02:14And he replies,
02:15"I couldn't even if I wanted to!"
02:17(Laughter)
02:19Instead, common knowledge can be captured
02:22in a simple mental intuition
02:25that something is public or conspicuous
02:28or out there,
02:29and that can be conveyed by direct speech.
02:32In the case of our separated couple, a cell phone call.
02:36Indeed, solving coordination dilemmas may be the reason that language evolved
02:41in our species in the first place.
02:44In the absence of a public event,
02:46the next best thing is conspicuous salience,
02:50or a focal point.
02:52Schelling suggests that our couple might gravitate toward the big clock
02:56in Grand Central Station,
02:58even if it wasn't particularly close
03:00to the point at which they'd been separated,
03:02simply because each might anticipate
03:05that it would pop into the mind of the other.
03:08A third solution is a convention.
03:11A tacit agreement to do something in a certain way
03:15for no other reason than they have agreed to do it that way,
03:19which is reason enough.
03:21Our separated couple might agree
03:23that should they ever be separated in the future,
03:25they will adopt the convention of chivalry
03:27and go to the bookstore.
03:29Or patriarchy, and go to the camera store.
03:32Or whimsy, and go to a lost-and-found in a department store.
03:36Or fairness, and take turns or flip coins.
03:41At larger scales, common salience
03:45or focal points and conventions,
03:47drive a lot of our legal and financial coordination.
03:51An everyday example is driving on the left, or on the right.
03:55It doesn't matter as long as everyone agrees to drive on the same side,
03:58as in the joke about the woman who calls her husband during his morning commute
04:02and says, "Be careful, honey.
04:04The radio says that there's a maniac driving on the wrong side of the freeway."
04:08And he says, "One maniac?
04:09There are hundreds of them."
04:11(Laughter)
04:13Another everyday example is money.
04:15I accept a piece of paper in exchange for an old chair,
04:19because I know that my grocer
04:22will accept it in exchange for some groceries
04:25because he knows his supplier will accept it, and so on.
04:29Now not all the examples are this obvious.
04:33Why are autocrats terrified of public protests?
04:37Well the basic reason was explained by Gandhi in the eponymous movie
04:42when he said, "100,000 Englishmen cannot control 350 million Indians
04:47if the Indians refuse to cooperate."
04:50The problem is that the Indians can't refuse to cooperate
04:54if each one fears that no one will join him.
04:57In a public demonstration,
05:00each protester can see the other protesters
05:03see the other protesters,
05:05and this common knowledge allows them to coordinate their resistance,
05:09whether by literally storming the palace
05:12or by bringing the apparatus of the state to a halt
05:15through boycotts and work stoppages.
05:18This is the basis for a joke from the old Soviet Union,
05:21in which a man in Red Square is handing out leaflets to passersby.
05:27Soon enough, the KGB arrest him,
05:29only to discover that the leaflets are blank sheets of paper.
05:33They say, "What is the meaning of this?"
05:35And the man says, "What's there to say?
05:37It's so obvious."
05:38(Laughter)
05:40The point of the joke is
05:41the man was generating subversive common knowledge.
05:45And in a case of life imitating a joke,
05:48in recent years,
05:49Russian police have arrested several people for holding, yes,
05:53blank signs.
05:55Another non-obvious example comes from the world of investing.
05:59John Maynard Keynes compared speculative investing
06:02to a newspaper beauty contest,
06:04in which the winner is not the woman with the prettiest face
06:09but the contestant who chooses the face
06:12that is chosen by the greatest number of other contestants,
06:15each of whom is anticipating the choices of other contestants, and so on.
06:21This can set off cycles of exuberant recursive mentalizing,
06:26as investors desperately search for focal points,
06:33each one buying an investment
06:34not because of its inherent productive value
06:38but because they hope to unload it at a profit on future investors,
06:43greater fools.
06:45This can set everyone off in search of conspicuous focal points
06:49such as Super Bowl ads.
06:52Everyone knows that everyone watches the Super Bowl.
06:55In 2022,
06:57cryptocurrency exchanges ran high-concept ads
07:01in which they tried to gin up a common expectation of rising prices
07:05not by touting any of the advantages of cryptocurrency
07:09but by warning,
07:10"Don't be like Larry.
07:11Don't miss out."
07:14Now, of course, it's only so long that an asset can levitate in midair,
07:19suspended by nothing but common expectation.
07:22Bubbles pop when the market runs out of greater fools
07:26who don’t want to miss out on the next best thing
07:29or when the doubt itself becomes common knowledge.
07:33This can send investors running for the exits,
07:36each desperate to sell a security
07:39out of fear that everyone else is desperate to sell it.
07:42The result can be a bank run or hyperinflation
07:45or a Great Depression.
07:47When Franklin Delano Roosevelt said,
07:49"The only thing we have to fear is fear itself,"
07:52it was not a feel-good bromide, but a theorem of common knowledge.
07:58With investors constantly on the lookout for focal points,
08:02financial leaders have to be wordsmiths,
08:05mystics and occasionally, comedians.
08:08Alan Greenspan once said,
08:10"Since I've become a central banker,
08:11I've learned to mumble with great incoherence."
08:14(Laughter)
08:15"If I seem unduly clear to you,
08:17you must have misunderstood what I said."
08:20(Laughter)
08:21After Jimmy Carter told his inflation czar, Alfred Kahn,
08:25never to use the self-fulfilling word "depression," he said,
08:28"OK, but we're in danger of having the biggest banana in 45 years."
08:33(Laughter)
08:34My own interest in common knowledge comes from its role
08:37in social relationships.
08:39A relationship is a coordination game.
08:42Two people are friends or lovers or allies,
08:46or dominant and subordinate or transaction partners
08:50because each one knows the other one knows that they are.
08:53And this common knowledge can be cemented
08:55by a number of common knowledge generating signals.
09:00Like eye contact,
09:01where you're looking at the part of the person
09:03that's looking at the part of you that's looking at that part of them.
09:07Or blushing, where you feel the heat of the cheeks,
09:11reddening of your cheeks from the inside,
09:14knowing that other people can see it from the outside.
09:18Or laughter, where an unignorable noise
09:21can expose the common assumptions among everyone who gets the joke.
09:27Conversely, when we worry about upending a relationship,
09:31we might go to great lengths to repress common knowledge.
09:35We might avoid looking someone in the eye,
09:37or pretend to ignore the elephant in the room
09:40or mumble with great incoherence.
09:44This is why, in everyday conversation,
09:46we often don't blurt out what we mean in so many words,
09:51but veil our intentions in innuendo and euphemism,
09:55counting on our listeners to catch our drift.
09:58A classic example is the sexual come on
10:02"Would you like to come up and see my etchings,"
10:04which by the 1930s was so familiar that James Thurber could draw a cartoon
10:08in which the hapless man says to his date,
10:10"You wait here and I'll bring the etchings down."
10:14A century later, etchings has become “Netflix and chill.”
10:18(Laughter)
10:20We also veil our bribes,
10:22as in, "Gee, officer,
10:24is there some way we might settle the ticket here?"
10:27And our threats, as in,
10:31"I'm so delighted to learn
10:32that you're on the jury of the Soprano trial.
10:35It's an important civic duty that we should all take part in.
10:38You've got a wife and kids,
10:40we know you'll do the right thing."
10:43The point of innuendo is not plausible deniability
10:49because these euphemisms don't pass the giggle test,
10:52but rather deniability of common knowledge.
10:55If Harry says to Sally, "Want to come up for Netflix and chill,"
10:59and Sally turns him down,
11:01well, Sally knows she's turned down a sexual overture,
11:03and Harry knows that Sally has turned down a sexual overture.
11:07But does Sally know that Harry knows?
11:09She could think, "Maybe he thinks I'm naive."
11:12And does Harry know that Sally knows that Harry knows?
11:15He could think, "Maybe she thinks I'm dense."
11:17Without the common knowledge,
11:18they can maintain the fiction of a platonic relationship.
11:22But if Harry were to have said, “Want to come up and have sex,”
11:25and Sally said no,
11:26well, now Harry knows that Sally knows that Harry knows that Sally knows.
11:30With this common knowledge,
11:31they can no longer maintain the fiction of a platonic friendship.
11:35And that's what lies behind the intuition that with bare-faced speech,
11:39you can't take it back.
11:40It's out there.
11:43The logic of common knowledge and social relationships
11:46explains why we all deplore the hypocrisy
11:50and white lies and role playing of everyday social interaction,
11:55but we wouldn't want to live without them.
11:57As dystopian comedies like "Liar Liar" play out.
12:02Our relationships are underpinned by a common understanding
12:07of limitless loyalty and generosity.
12:13Privately, we know they're fictions,
12:15but making these rude realities common knowledge
12:19would pollute the pool of common assumptions
12:21that allow us to get along.
12:25We also have to get along in less intimate relationships,
12:29and in those cases, we rely on norms:
12:33ways of living together that aren’t written down by lawgivers
12:38or enforced by the police,
12:40but exist because everyone knows they exist.
12:43In social life,
12:45basic civility depends on norms
12:47such as that you don't brazenly lie,
12:49you don't insult people to their faces,
12:52you don't prosecute personal vendettas.
12:56In the anarchic global arena,
12:58peace depends on norms such as that nation states are immortal,
13:04national borders are grandfathered in,
13:06conquest is unacceptable,
13:08and nuclear weapons are unthinkable.
13:12These norms are propped up by nothing but common acceptance,
13:16and so they're vulnerable to imploding if they are overtly flouted,
13:21or even if there is loose talk about flouting them.
13:25I hope that, having explained the logic behind human harmony,
13:30I don't have to explain how these norms are currently under threat
13:35or what is at stake if we lose them.
13:38As a man once said,
13:40"What's there to say?
13:41It's so obvious."
13:43Thank you.
13:44(Applause)