Common knowledge/shared knowledge

2020-02-01 — 2025-06-18

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Figure 1

A simple case of group theory of mind. This is one of those concepts that

  1. is super important but
  2. people seem not to know, and yet
  3. the moment you do learn it you decide it was obvious in hindsight, and
  4. nerds tend to be really excited by it but explain it badly

Here is a good explanation:

Common knowledge is necessary for coordination, for making arbitrary but complementary choices like driving on the right, using paper currency, and coalescing behind a political leader or movement. It’s also necessary for social coordination: everything from meeting up at a time and place or speaking the same language to forming enduring bonds of friendship, romance, or authority. Humans have a sixth sense for common knowledge, and we create it with signals like laughter, tears, blushing, eye contact, and blunt speech.

But people also may strive to avoid common knowledge—to ensure that even if everyone knows something, they can’t know that everyone else knows they know it. And so we get rituals like benign hypocrisy, veiled bribes and threats, sexual innuendo, and pretending not to see the elephant in the room.

A neat publication on this theme is De Freitas et al. ()

People often coordinate for mutual gain, such as keeping to opposite sides of a stairway, dubbing an object or place with a name, or assembling en masse to protest a regime. Because successful coordination requires complementary choices, these opportunities raise the puzzle of how people attain the common knowledge that facilitates coordination, in which a person knows X, knows that the other knows X, knows that the other knows that he knows, ad infinitum. We show that people are highly sensitive to the distinction between common knowledge and mere private or shared knowledge, and that they deploy this distinction strategically in diverse social situations that have the structure of coordination games, including market cooperation, innuendo, bystander intervention, attributions of charitability, self-conscious emotions, and moral condemnation.

Those two terms then, are

0.1 Voting in an Election

  • Shared Knowledge: You and your neighbour both dislike a new policy, and you’ve talked about voting against it. You know each other’s views, but you don’t know how the rest of the community feels.
  • Common Knowledge: A public poll is released showing that 70% of voters oppose the policy. Now, everyone knows that most people are against it, and this can motivate more people to vote, since they believe their vote will be part of a larger movement.

0.2 Harvey Weinstein

The revelation of the Harvey Weinstein sexual assault allegations is a textbook example of a transition from shared knowledge to common knowledge.

For decades, Harvey Weinstein’s behaviour was described as an “open secret” within the film industry. This means that many people in Hollywood, including actors, agents, journalists, and employees at his companies, Miramax and The Weinstein Company, were aware of the allegations. Stories and rumours of his sexual advances and abuse were widespread among those in his professional circle. However, this knowledge remained largely confined to “hushed conversations” and internal industry circles. This represents a state of shared knowledge, where a specific group of people knew about the issue, but it was not yet public information, and individuals were not sure who else knew or if anyone would speak up, a classic problem in the case of such abuse of power.

The turning point occurred on October 5, 2017, when The New York Times published an article detailing decades of sexual harassment allegations against Weinstein. This publication, followed by others, moved the information from the private sphere into the public domain.

Key factors that marked this transition include:

  • Public Revelation The news was published for everyone to see, not just those in the industry.
  • Mass Awareness Following the initial report, dozens more women came forward with their own stories of assault and harassment by Weinstein.
  • Public Confirmation The sheer number of accusers and the media coverage ensured that not only did everyone know, but everyone also knew that everyone else knew. This created a collective understanding and public pressure that could no longer be ignored.

This shift had significant consequences, leading to Weinstein’s arrest, trial, and convictions, as well as the bankruptcy of his company

1 Why is it important?

Conceptually satisfying is not Important in strategic collective action.

2 References

Chater, Zeitoun, and Melkonyan. 2022. “The Paradox of Social Interaction: Shared Intentionality, We-Reasoning, and Virtual Bargaining.” Psychological Review.
De Freitas, Thomas, DeScioli, et al. 2019. “Common Knowledge, Coordination, and Strategic Mentalizing in Human Social Life.” Proceedings of the National Academy of Sciences.
Kleiman-Weiner, Vientós, Rand, et al. 2025. “Evolving General Cooperation with a Bayesian Theory of Mind.” Proceedings of the National Academy of Sciences.
Petersen, Osmundsen, and Tooby. 2020. “The Evolutionary Psychology of Conflict and the Functions of Falsehood.” The Politics of Truth in Polarized America.
Shteynberg, Hirsh, Bentley, et al. 2020. “Shared Worlds and Shared Minds: A Theory of Collective Learning and a Psychology of Common Knowledge.” Psychological Review.
Shteynberg, Hirsh, Wolf, et al. 2023. “Theory of collective mind.” Trends in Cognitive Sciences.
Thomas, DeScioli, Haque, et al. 2014. “The Psychology of Coordination and Common Knowledge.” Journal of Personality and Social Psychology.