Dunbar's Number for Ideas

What do we know about the limits of our own motivation?

Posted Apr 15, 2019

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One of the subtle joys of a psychologist's career is to describe the limits of cognition. Psychologists love to declare that the mind is capable of great things, but only up to a point. For any given psychologist, it's likely that one of these points—whichever one is of greatest interest to her—is the highlight of her research career, the thing she gestures at during the climax of her lecture and says, "This is what I discovered." For eminent psychologist Robin Dunbar, that limit is rather specific: one-hundred and fifty.

It's known as Dunbar's Number, and it signifies the cognitive limits on social groups. We can only keep track of all the relevant social information in a group of about 150 people. Any more than that and we start to lose key information. Our brains just aren't equipped to handle it. The reason for this is that humans are complex. Not only are they complex, but they interact with one another—an exponential increase in complexity. In order to grasp a social landscape, not only do we have to know the subtleties of an individual's personality, but we also have to know what that person thinks about the subtleties of everyone else's personality. We can do that well until a group's size exceeds 150 people. 

Dunbar's number is much celebrated in psychology. Partially this is because what it describes is important: the boundary conditions of human social life. But also because it provides such a clean explanation. A single number describes such a big phenomenon. Delineating other aspects of cognition isn't so clearcut. However, there is another cognitive milieu in which Dunbar's number might offer a useful template: ideas.

How many ideas can a person interact with? The question makes sense in the same way it does in a social setting. A person can only have so many friends. She may know a lot of people. But she can only know so much about them. Similarly, a person might know about a lot of ideas. But she'll only steer her way through life driven by a limited number of them.

One of the constraints that a "Dunbar's Number for Ideas" could refer to is memory. Human memory is limited in well-understood ways. There's a pipeline: memories start off as sensory input (for a few moments), then get stored in working memory (for a few seconds or minutes), then get consolidated into long-term memory (where they reside indefinitely). Working memory is one of the constraints on the number of ideas you can entertain at any one time. What it is to "think" about something (in the colloquial sense we use the term) is to bring it into working memory. And we can only hold a handful of ideas in working memory at one time. But the notion of Dunbar's Number here is much more broadly applicable. 

Instead of thinking of Dunbar's Number as one large circle around 150 people, think of it as a series of concentric circles. The inner circle contains your best friends. Those are the people you know the most about, who would be the first you call in the case of an emergency or a wedding. The next circles continue out from there with various levels of social relevance. Eventually you get to 150, which defines this larger limit that Dunbar noted. Not everyone within that largest circle has equal influence on you. So it is with ideas.

One way to think about this is that the philosophical schools with which people associate center around a single idea (or a constellation of related ideas). No one ever proclaimed to be an Existentialist-Stoic-Christian-Pragmatist. They might all apply in different ways to a person and the way she thinks about the world. But one of them is likely to win out as the driving force. 

Which isn't to say that we're only ever captivated by one idea at a time. There are tons of different ideas that drive our behavior. For example, we could choose to give change to a homeless person because of the idea that they're more in need of those coins than we are; or we could choose not give the change because it's not the most effective strategy for charitable giving. A person might resonate with either one of these notions. But it's unlikely to be the most important idea in their life. It's within the larger circle, but not the inner one.

Ideas here are like people: complex. And they, too, interact with one another. When you combine two ideas, the implications get complicated. If you view the world through the lens of Christianity, or that all people were created equal, or some other motivating philosophical impulse, then that is going to affect everything else you believe. It'll change the orbit of the satellite beliefs circling around the core idea.

And while not everyone can articulate this central mission or belief (perhaps no one can, really), that doesn't mean it isn't there. Just as one single person doesn't have to dominate our entire social lives, nor does one single idea. The point is not to put boundaries on what's possible, but to recognize the constraints of what drives us. And with Dunbar's number as an organizational guide, perhaps we can better understand what motivates us to do what it is we do.

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