What is common knowledge?
Common knowledge is a term we’re all familiar with to mean:
“something that everyone [in our group] knows”
for which “group” can mean a team at work, a company, a friend group, or even society. Here’s a good example of a symbol for which we have common knowledge:
However, the psychological definition of common knowledge is different and its implications much more powerful. In fact, it influences many of our actions despite us even being aware of it.
Deconstructing Common Knowledge
It can be helpful to understand the psychological definition of common knowledge on a “continuum of knowledge”:
On the left side of our continuum is private knowledge. Private knowledge is when one person knows something but does not know if anyone else knows:
With the knowledge of imminent rain, the left stick person is prepared, while the right stick person, who is unaware, is left exposed.
Shared knowledge is when one person has knowledge and the other person has the same knowledge:
In this case, left stick person knows it’s going to rain and right stick person knows it’s going to rain — both are prepared because they know the same thing.
Common knowledge is when someone knows something, and knows that someone else knows it, and knows that someone else knows that they know it… and so on:
When there’s common knowledge both people know that both people know something.
To help illustrate the difference between private or shared knowledge and common knowledge, we can turn to a popular fairy tale.
If you’re not familiar with the emperor’s new clothes, here’s the short version… A pair of sneaky and deceitful weavers convince an emperor they will make him a new robe made from magical thread that is invisible to anyone who is “unfit for his position or ‘hopelessly stupid’”. The emperor proudly dons his “new robe” which nobody can see (because there is no robe) but nobody says anything or reacts for fear they are the only person who can’t see it and will suffer retribution from the emperor for being unfit or stupid. The emperor prances around town, and nobody says anything despite the shared knowledge that he is naked. Finally, a boy yells out “but he isn’t wearing any clothes!” at which point it becomes common knowledge and everyone starts laughing at the emperor.
Notice that nobody (re)acted until everyone knew that everyone knew the emperor had no clothes.
Here’s the point of the story and this article: Common knowledge, unlike private or shared knowledge, has a massive influence on coordination, often without us knowing it.
Why Does Common Knowledge Matter?
At the risk of massive reductionism, our default decision-making is based on personal cost / benefit. I choose option A because it has the greatest benefit with the lowest cost, based on what I know. This decision-making process is personal — what’s best for me. How much effort should I put in at work vs personal time, for example. What is the cost / benefit of whistleblowing? Or joining a revolution?
But the presence of common knowledge changes how we make decisions and ultimately act. Think about “political correctness”. We choose words and actions not based purely on cost/benefit, but based also on how people will react to our actions.
Common knowledge has shaped society in many ways:
- Drinking and driving
- Littering
- Online bullying (pile-on behavior)
- Me too
- Smoking
- Arab Spring (and every successful uprising)
Each of these required common knowledge to change thoughts, behavior, and most importantly, actions.
While there has been a lot of study on altruistic behavior (why people make take actions that benefit a group at personal sacrifice), there has been little study on the psychological influence of actions based on common knowledge.
In a brilliant paper “The psychology of coordination and common knowledge” Kyle Thomas provides evidence that common knowledge is actually a distinct cognitive function and asks the questions of whether coordination without common knowledge is irrational.
I believe that it is a separate cognitive function and that not only is it irrational for us to cooperate without common knowledge, but that most coordination requires common knowledge as a prerequisite when people have the option to participate.
This publication will explore those implications and consequences — on how we live, how we work, and how we address systemic, political, and social issues that matter.
Now you know :).
