The term “Dunbar’s Number” refers to a particular hypothesis by primatologist Robin Dunbar. It is a very simple idea with rather complex implications, and it is one of those simple ideas that gets more complicated than ideal as we look into it more and more. Eventually, the idea is required by many who contemplate it to do more work than was ever intended, and in this way seems to fail, though it really doesn’t. I personally think Dunbar’s number is useful if it is properly understood, so I want you to give it a chance, and to help you do that I’d like to use an analogy.

I’m thinking of a number called Carrier’s Number. Carrier, in this case, refers to the company that installs air conditioners and heaters. Carrier’s number is the temperature in degrees F at which you, sitting there in your chair, notice it is too warm so you get up and go turn on the air conditioner. It is best measured as a post hoc number…we watch and wait, flies on the wall, as the room heats up, and when a person gets up and flips on the air conditioner, the temperature at that point was carrier’s number for that person at that time.

One might argue that a post hoc measure like this isn’t much use in science because in science we like to predict things. But just because carrier’s number is best measured pot hoc does not mean that it only exists post hoc. It existed before the test subject got up, we just didn’t know what it was. For a large number of test subjects, we should be able to estimate carrier’s number (it is probably in the upper 70’s F). However, this will vary across cultures, across seasons, humidity, as clothing styles change (in the days of Polyester Leisure Suits, it is said that Carrier’s Number went down by about four degrees) and so on. The fact that it varies does not make it a bad number. In fact, its variation and reasons for it can make it an extra good number depending on what one is trying to do with it.

Dunbar’s number is the number of full blown social interactions you can manage. This number is lower or higher across species of social primates, as it tracks adaptive suites of sociality and the ability of brains to manage sociality. So, you can measure Dunbar’s number across primate groups by looking at how large effective primate groups get across species and figuring that the number is just about that maximum group size. Or, you could estimate Dunbar’s number (retrodict it, as it were) by looking at relative brain size, if we assume that brain size is linked to Dunbar’s number, all else being equal. In this way, Dunbar’s number is a way of linking primate sociality with brain evolution, which was the original idea.

In modern society, and in human historical contexts, we may see Dunbar’s number in a lot of places. This is the number at which, more or less, groups start to break down (in some societies) and villages split. Military units max out at about Dunbar’s number (companies are about 100 in size) and so on. This does not mean that Dunbar’s number and its associated dynamics explain everything. It might mean that the breakdown of social interactions can be more important than, say, resource limitations, on human group fission and fusion. That is exactly what many anthropologists have been suggesting for decades. Dunbar’s Number is simply this concept quantified somewhat and expanded to primates.

There are variations and adjustments. Some organisms have apparent smaller brain size because their diets cause a different body size, so that has to be adjusted for (leaf-eating monkeys may small relative brain size because their bodies are large, not because their brain is small, for example). What a fully blown social interaction is may vary. A group of primates may have a subgroup that hardly ever interacts with the others. Perhaps pre-adolescent monkeys don’t count for as much as sexually mature monkeys, so if there happens to be a baby boom a couple of years back, the group size if you count everyone is higher than Dunbar’s number. Or perhaps the group includes two or three social geniuses who temporarily facilitate an extra large group size, or temporarily force an extra small group size, for some reason.

It makes sense that there is a limit on effective sociality, and thus, on effective social group size. Dunbar’s number is nothing other than the number you end up with because when you are making the damn graph you need a damn number to put there on one of the axes. It has been over-interpreted or over-used as a number like many of those from Physics, like the freezing point or boiling point of water, which it is not.

Desiree Schell and I spoke about Dunbar’s Number on the Skeptically Speaking that just became ready for you to download. Check it out here.

This video includes, during the last third somewhere, a discussion by Dunbar of all this.

And, here’s a few items by Dunbar you might find interesting:

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How Many Friends Does One Person Need?: Dunbar’s Number and Other Evolutionary Quirks

Grooming, Gossip, and the Evolution of Language