Herds, Focal Points, and the Fracturing of Common Knowledge
Last night I went to hear Steven Pinker in conversation with Tim Harford at the launch of Pinker’s new book, When Everyone Knows, That Everyone Knows. I’ll admit I came out both fascinated and slightly tangled up in the abstract theory he was deploying — recursive chains of “I know that you know that I know…” looping endlessly like an Escher staircase. At first glance it seems like the kind of mind-bending logic puzzle beloved by economists and philosophers, but Pinker’s point is that this isn’t academic trivia: it’s the hidden architecture behind everything from market crashes to internet pile-ons. And it struck me that if he’s right, we may have overlooked one of the biggest social transformations of our time: the breakdown of common knowledge itself.
Pinker opened with the so-called “spinach problem.” Imagine three people at a dinner table, each unknowingly with spinach in their teeth. The host taps her glass and announces: “At least one of you has spinach in your teeth.” She taps again. No one moves. She taps a third time, and suddenly all three discreetly clean their mouths.
How did they know? Because each diner could see the others’ teeth, reason about what the others saw, and reason again about what others knew they knew. It’s an abstract logic problem (and not one that I have the faculties to even think through to its conclusion myself, even after being told!) but it illustrates the principle of common knowledge: it’s not enough that something be true, or even that we all know it. What matters is that we all know that we all know that we all know it. That recursive certainty is what allows coordinated action.
In real life, these recursive chains don’t require superhuman mental gymnastics. They’re generated in a single stroke whenever something becomes publicly salient, witnessed, shared, or declared in a way that everyone can assume others have witnessed too. That’s the power of the announcement, the headline, the Super Bowl advert, the viral video.
Once you grasp this, you see it everywhere.
These examples show the double-edged nature of common knowledge. It is both stabilising (it makes markets, language, and norms possible) and destabilising (it fuels bubbles, mobs, and moral panics). The difference lies in what focal points we share.
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Book Online TourBook A VisitThomas Schelling, the game theorist, called these obvious shared cues focal points. If you and I agree to meet in London without specifying a place, we might independently pick Big Ben. Not because it’s objectively better than Nelson’s Column, but because it pops out as the most obvious shared reference.
Focal points matter because they provide shortcuts through the infinite regress of “what I know you know.” They anchor our coordination. And here’s the rub: for most of the twentieth century, focal points were easy to come by. Four TV channels (until Channel 5 came along, which literally blew my mind at the time). A handful of national newspapers. Mass rituals like the evening news or the cup final. You could assume that if something was on the BBC or the Times front page, “everyone” knew.
That doesn’t hold anymore. Social media has shattered the commons. Our attention is fractured across a thousand feeds, each algorithmically filtered, each personalised. What you see, I may never see. What trends in one corner of the internet may be invisible in another.
And yet the human brain hasn’t evolved past its pack instincts. We are still scanning for signs of what “everyone” knows, of what the herd is doing. Online, that leads us into traps: mistaking a viral hashtag for consensus, or assuming a pile-on reflects universal outrage when it may be the work of a vocal minority amplified by an algorithm.
Sitting in the lecture, what struck me most was how Pinker’s theory of common knowledge connects with our basic herding behaviour. We are, after all, pack animals. Much of our decision-making is positional: are we in the group, or outside it? Do we join the crowd, or stand apart? Common knowledge is the signal we use to gauge the pack’s mood.
In the age of mass media, that meant we could at least agree on what mattered, even if we disagreed bitterly about its meaning. Today, each of us is herding inside a different corral. The pack has splintered into countless micro-tribes, each with their own salience, their own sense of what “everyone knows.”
That fracturing may help explain our political malaise. Without shared focal points, we lose not just common narratives but the very conditions for disagreement. If you and I don’t even know what the other’s news diet looks like, we can’t argue productively. We’re not debating the same reality. We’re not even sitting at the same table.
Pinker also noted how common knowledge can generate awkwardness. A sexual proposition, for example, can be hinted at with innuendo in a way that allows for deniability. Once said bluntly, it becomes common knowledge, and changes the relationship irrevocably.
That points to a broader truth: common knowledge is powerful precisely because it removes the option of pretending otherwise. Once something is publicly acknowledged, you can’t go back to ambiguity. That’s why authoritarian regimes fear public protests: once everyone knows everyone knows the regime is vulnerable, collapse can snowball. But it’s also why reputations can be destroyed by a single viral clip. Visibility is irreversible.
Which brings me back to the question that nagged at me as I left the talk: do humans actually need common knowledge in order to cohere as societies? And if so, what happens when our focal points fracture beyond repair?
Part of me wonders whether the atomisation of salience, each of us trapped in personalised feeds, is not just a cultural annoyance but a structural threat. Democracy, after all, requires a demos. Not unanimity, but at least a shared sense of what’s up for debate. If our informational commons has dissolved, perhaps that’s why our politics feels like a Babel of disconnected tribes.
The irony is that social media both undermines and hyper-charges common knowledge. On the one hand, it splinters us into filter bubbles. On the other, it creates the illusion of universality, and sometimes the reality, through virality. A single clip can still become a modern focal point. But the process is unpredictable, arbitrary, and often destructive.
Steven Pinker’s thesis is that common knowledge, this recursive awareness of what we all know we all know, is not a quirky corner of game theory but the scaffolding of social life. It enables cooperation, markets, language, norms. But it also explains manias, mobs, and pile-ons.
Listening to him, I found myself wondering whether our present turbulence is less about disagreement over facts, and more about the erosion of shared salience. Perhaps we don’t just crave truth. Perhaps we crave the comfort of knowing that everyone else knows it too. Without that, we are left adrift, scanning endlessly for the pack, clutching at illusions of consensus conjured by algorithms.
If that’s right, then the task ahead isn’t only to fix our politics or regulate our platforms. It’s to find new ways of binding ourselves together through shared focal points that are healthier than mob outrage and more reliable than algorithmic mirages. Because without common knowledge, there may be no common ground.
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References for Further Reading
Pinker, Steven. When Everyone Knows That Everyone Knows: Common Knowledge and the Logic of Cooperation. (2025). HarperCollins Publishers.
(Primary inspiration for this essay; explores recursive awareness and its role in social order.)
Schelling, Thomas C. The Strategy of Conflict. Harvard University Press, 1960.
(Introduces the concept of “focal points” and coordination in game theory.)
Keynes, John Maynard. The General Theory of Employment, Interest and Money. Macmillan, 1936.
(Explains the famous “beauty contest” analogy that illustrates recursive expectations in markets.)
Harford, Tim. The Undercover Economist Strikes Back: How to Run or Ruin an Economy. Riverhead Books, 2014.
(Referenced during Pinker’s discussion; connects economic behaviour with collective perception.)
Bikhchandani, Sushil, Hirshleifer, David, and Welch, Ivo. “A Theory of Fads, Fashion, Custom, and Cultural Change as Informational Cascades.” Journal of Political Economy, Vol. 100, No. 5 (1992): 992–1026.
(Classic study on how herd behaviour emerges when individuals imitate perceived group consensus.)
Gladwell, Malcolm. The Tipping Point: How Little Things Can Make a Big Difference. Little, Brown and Company, 2000.
(Explores how social epidemics form once ideas or behaviours reach a threshold of common awareness.)
Sunstein, Cass R. #Republic: Divided Democracy in the Age of Social Media. Princeton University Press, 2017.
(Analyzes how digital filter bubbles fracture shared knowledge and undermine collective deliberation.)
Centola, Damon. How Behavior Spreads: The Science of Complex Contagions. Princeton University Press, 2018.
(Empirical exploration of how information, norms, and social influence propagate in networks.)
Banerjee, Abhijit V. “A Simple Model of Herd Behavior.” The Quarterly Journal of Economics, Vol. 107, No. 3 (1992): 797–817.
(Foundational paper explaining how herd dynamics emerge even among rational individuals.)
Habermas, Jürgen. The Structural Transformation of the Public Sphere. MIT Press, 1989.
(Seminal theory on how public discourse and common knowledge evolve in democratic societies.)