What naked emperors and Super Bowl ads have in common

It is the ultimate feel-good democratic fable. An emperor buys a new suit of clothes from two conmen, who tell him it is invisible to fools. Not wanting to seem foolish, the emperor and his courtiers all praise its exquisite tailoring. When the emperor parades in the street, the crowds applaud his elegance. Until a little boy points out the obvious: the monarch is naked.

Readers have long loved Hans Christian Andersen’s story. It reminds them of the vanity and fallibility of rulers. But there is another way of understanding it, argues Steven Pinker, a psychologist at Harvard University. The story “draws on a momentous logical distinction” between private and common knowledge. “By blurting out what every onlooker could see within earshot of the others, [the boy] ensured that they now knew that everyone else knew what they knew, [and] that everyone knew that everyone knew…And that changed their relationship to the emperor, from obsequious deference to ridicule.”

Common knowledge—knowing that others know something, and that they know we know—is powerful. Thinking about it “illuminates many enigmas of our public affairs and personal lives”, Professor Pinker argues, in this fizzing, erudite book.

It can topple dictatorships. On December 21st 1989 Nicolae Ceausescu, Romania’s despot, was addressing a large crowd, many of whom were state employees bused in to wave pro-regime flags. Suddenly, someone jeered. Then the jeers spread, and soon the whole crowd was jeering. Though the censors hastily cut the live television feed, the whole country realised that practically everyone hated the dictator. Four days later Ceausescu was executed by firing squad, and Romania was free.

Common knowledge can set off more mundane events, too. In 1973 Johnny Carson, a popular television host, warned that a toilet-paper shortage was looming. This was false when he said it, but quickly became true. Some people believed Carson. Others thought that others would believe him, and rushed to stock up on toilet paper before everyone else did.

The episode fixed in the public mind the idea that toilet paper runs out during emergencies, which may be why it happened again during the covid-19 pandemic. The virus did not noticeably disrupt supply chains of loo paper, but people panic-bought it because they assumed other people would. Shops ended the frenzy by putting up signs limiting people to three packages. The point was to “signal to customers that their fellow customers could not strip the shelves bare”.

Common knowledge allows co-ordination, for good or ill. Fiat currency relies on it: dollars have value because everyone knows that other people value them. And property rights “hinge not on a judgment of who ‘deserves’ a resource, but on common knowledge of who is recognised as having the right to keep and use it”.

On the bad side of the ledger, the single largest motive for young men murdering each other is a fight over something trivial, such as an insult or a jostle. Why? Because young toughs need a reputation for being someone you don’t mess with. If they publicly back down, they invite their peers to bully them. This is why, studies find, an argument is less likely to turn violent if no witnesses are present.

Something similar applies to countries. Surprisingly, “to the many people who think countries fight over resources”, most big wars between 1648 and 2008 were over “standing” or “revenge”. Donald Trump has failed to end the conflict in Ukraine partly because he imagines it is about real estate, rather than Vladimir Putin’s desire to “restore the greatness of Russian civilisation and rebuke its humiliation in the eyes of the West”.

Social-media pile-ons are vicious because they take place in public. What terrifies the speech police “is not that a dangerous idea might be thought, or even expressed, but that it might become common knowledge”. This is why heretics who break taboos publicly are punished publicly. It may also be why most American college faculty under 35 favour shutting down speakers they disapprove of, and a fifth support students who would use violence to this end. Professor Pinker is appalled. The murder of Charlie Kirk on September 10th showed why he is right to be.

Advertisers and spin doctors deftly exploit common knowledge. The value of a Super Bowl spot is not just that millions will see it; it is also that viewers know that millions of others have seen it. So the ads are seldom for goods consumed in private, like breakfast cereal. Rather, they are for brands people will buy if they think other people think them cool, such as cars or clothes, or products whose value depends on many people using them. The Super Bowl in 2022 was known as the “Crypto Bowl”, because so many cryptocurrency exchanges touted their wares. Many viewers then bought crypto because, after hearing Matt Damon, an actor, whisper “Fortune favours the brave” in a sexy, masculine voice, they thought other people would.

Autocrats have grown more skilled at disrupting the spread of common knowledge. In the second half of the 20th century non-violent protest movements were twice as likely to succeed as violent revolts. As a scriptwriter has Mahatma Gandhi say in a film from 1982, “100,000 Englishmen simply cannot control 350m Indians if the Indians refuse to co-operate.”

However, in the past two decades the success rate of non-violent protests has fallen (though they are still more effective than the violent sort). Oppressive governments have devised smarter forms of censorship. China, for example, not only has copious high-tech tools to shape the national conversation, but uses them judiciously. Instead of silencing complaints, which give the regime valuable information about which policy to fix and which official to purge, it stamps hardest on information that helps protesters co-ordinate their activities, such as notices of when and where rallies are to take place.

People are highly sensitive to common knowledge. Professor Pinker speculates that it is one reason why language evolved, since it generates it so cheaply. It also explains some odd human habits. We blush so others will know we are sorry, even if we can’t bring ourselves to apologise. We avoid eye contact to avoid embarrassment. Mormons joke: “Protestants don’t recognise the pope. Mormons don’t recognise each other in the liquor store.” If they pretend not to have seen each other, they can avoid acknowledging that they have been caught breaking the rules.

No other animal comes close to the human capacity for surmising what is going on in someone else’s head—a fact that once saved this reviewer’s life. He was walking through the bush in Zimbabwe when he chanced on two lions mating. The male was audibly annoyed at the interruption. The sensible guide ordered us not to run away, since lions see fleeing animals as food. The lions, concluding that we were not food, slunk moodily off. We survived because we know how lions think and they don’t know how we do. It’s good to be human—even if, from time to time, we have to remind some of our fellow humans that they are only human, too. ■