Why is your head not exploding? Steven Pinker can explain.

Steven Pinker.

Stephanie Mitchell/Harvard Staff Photographer

Science & Tech
Why is your head not exploding? Steven Pinker can explain.

Cognitive psychologist reveals uncommon depths of common knowledge in new book

Christy DeSmith

Harvard Staff Writer

September 24, 2025 7 min read

Common knowledge is something everyone knows that everyone knows. He saw it. She saw it. He knows that she saw it, and vice versa, ad infinitum.

According to Steven Pinker, Johnstone Family Professor of Psychology, humans are constantly occupied with whether information has achieved this potent status. It’s just that the deliberations usually happen below the level of awareness. In “When Everyone Knows That Everyone Knows …,” Pinker explains why common knowledge is nonetheless central to everything from planning a coffee date to driving on the correct side of the road.

“I also suggest it’s the basis of our social relationships,” he said. “Two people are friends not because they signed a contract but because each one knows that the other one knows that the first knows that the second knows that they’re friends. Or lovers. Or neighbors. Or transactional partners.”

In an interview with the Gazette, edited for length and clarity, the veteran author and professor identified the undercurrents of common knowledge in everyday language, laughter, and social media use.

You write that the concept of common knowledge is not exactly common knowledge. Yet it’s threaded through popular culture. Any favorites from the examples you collected?

There’s a “Friends” episode in which Phoebe says: “They don’t know that we know they know we know. Joey, you can’t say anything.” And the dull-witted Joey replies: “I couldn’t even if I wanted to.” It captures the humor in the very idea that we struggle to keep track of each other’s thoughts about our thoughts.

So if common knowledge is so essential to human affairs, how do we do it without our heads exploding? The answer is that we have a conceptual metaphor, or an image that captures the idea without going through the layers upon layers of mentalizing. “It’s out there” is the best everyday-language equivalent to common knowledge that I know of. Also, “It was in your face.” Or “the elephant in the room,” for common knowledge that we publicly deny.

Another allusion from pop culture is the “Seinfeld” episode in which George Costanza says he’s thinking of saying “I love you” to the woman he’s dating. Jerry asks, “Are you confident in the ‘I love you’ return? Because if you don’t get that return, that’s a pretty big matzo ball hanging out there.” He was referring to something that is publicly salient or conspicuous. Idioms like “the cat is out of the bag,” “spilling the beans,” and “the bell can’t be unrung” call attention to an un-ignorable public event as our tacit metaphor for common knowledge.

Was there an a-ha moment when you understood the importance of common knowledge?

I knew it was big when I was writing “The Stuff of Thought” (2007) and trying to explain a puzzle that had not been explored very well in the psycholinguistics literature. Namely, people very often don’t mean the literal content of their words, polite requests being the classic example. If I say, “If you could pass the salt, that would be awesome,” I’m not really expressing the hyperbolic statement that passing the salt is worthy of awe. We all effortlessly interpret that as a polite version of an imperative “give me the salt.”

Fundraising requests like “We’re counting on you to show leadership in our campaign for the future,” sexual come-ons like “You want to come up for Netflix and chill?,” and bribes like “Gee, officer, there’s got to be some way we can settle this ticket right here without a lot of paperwork” don’t so much allow for true plausible deniability. No grownup could be in doubt as to their intent.

What they allow for is deniability of common knowledge. That is, even if someone knows she’s been propositioned, she could still wonder whether he knows that she knows. She could think to herself, “Maybe he thinks I’m naïve.” He could think, “Maybe she thinks I’m dense.” And with the deniability of common knowledge, they can continue to maintain the fiction of a purely platonic friendship.

When did you know the subject was worthy of a full-length book?

As I started immersing myself in the literature, I realized common knowledge was much more than an explanation for euphemism. There were theorems about it in game theory. There was discussion in epistemology, the branch of philosophy about knowledge. It has been discussed by economists as an explanation for bank runs, speculative bubbles, currency attacks, hyperinflation, and recessions. In political science, there was discussion of how public protests, by generating common knowledge, can lead to regime change. Conversely, when people are punished for their opinions, you can have people falsifying their preferences, leading to spirals of silence.

There was a book by UCLA political scientist Michael Suk-Young Chwe called “Rational Ritual” (2001) that tried to make these ideas, you might say, common knowledge. But nothing had been written from the point of view of a cognitive psychologist, and it is essentially a phenomenon of cognitive psychology. I saw an unfilled niche.

What new ground does your book cover?

One thing I see as an original contribution is addressing emotional expressions that have long been a subject of puzzlement. Why do we laugh? Why do we cry? Why do we blush? I argue that they all function as common-knowledge generators.

When you laugh, you know that you’re laughing. You can’t miss it because it’s interrupting your speech. Because it’s loud and salient, other people can’t miss it, and they know you can’t miss it. When you cry, you’re seeing the world through a scrim of tears at the same time other people can see the liquid welling up in your eyes. I also address the puzzle that even though the prototypical occasion for crying is loss or defeat or helplessness, a lot of crying is witnessing the sublime, the moving, the tender, the empathic, the awesome.

Another of the book’s contributions concerns social media. How does the logic of common knowledge help make sense of people’s behavior there?

The basic phenomenon is that social and moral norms are a matter of common knowledge. A norm against insulting someone’s appearance to their face, or telling a racist joke, is not literally enforced by the police; it exists only because everyone knows that it exists. Therefore, the norm is threatened when flouted in public without the violator being publicly punished. That’s why, until there was an effective criminal justice system, we had public hangings and pillories and stocks. You had to see that everyone else was seeing a norm violation being punished for the norm to survive.

Social media posts are not literally common knowledge, because they are delivered in personalized feeds, but they can feel like common knowledge, because the “Like” and “Repost” buttons, together with the “Trending” column, are virality generators that can make the audience for any post increase exponentially.

And these buttons offer an opportunity for creating, not just perceiving, common knowledge. Since public norm enforcers gain esteem within their virtual communities, social media offers the temptation to millions of people to be norm police. This can snowball into shaming mobs, in which people act like kids who join a bully out of fear of becoming the one bullied. In this way the common-knowledge-generating power of social media helped give rise to cancel culture.