With the entire country asked to socially distance ourselves it has never been clearer that humans are very much group animals. To be among others is natural for us and we don’t like being alone for extended periods of time. Our group animal mind-set is no recent development. Most other primates are group animals as well and being part of a group has several important advantages, like reducing the risk that you’ll be eaten. What is special among us humans is the size of our groups and their composition. Although my old high school teachers often compared my class to a band of monkeys, they would quickly discover what real chaos is, if it had been 30-ish real unrelated monkeys. In contrast to most primate groups, the majority of group members in most human groups, including hunter-gatherer bands, are unrelated. This difference in how humans and other primates deal with large groups has intrigued researchers for a long time and they wondered if it was perhaps related to another large difference between humans and our closest relatives: our brain size.

Although there are animals with larger brains than humans, such as dolphins and elephants, the relative size of our brain compared to our body is nearly unparalleled. The brains of our earliest ancestors were not much larger than those of modern chimpanzees and our brain growth only really kicked off some 2 million years ago with Homo erectus, culminating in the large brains of Neanderthals and ourselves.

A researcher called Robin Dunbar hypothesised that perhaps our large brains enable us to live in these large groups and that selection for larger groups drove our increased brain size. He called his hypothesis the Social brain hypothesis. In fact it is not even our the size of our entire brain that matters according to Dunbar, but just the neocortex, which is associated with our cognitive capabilities. Dunbar then calculated the supposed ‘natural’ group size of humans, based on the relation between neocortex size and group size in other primates. He arrived at a group size of roughly 150, which has since then been called Dunbar’s Number.

So does this group size of 150 hold up to reality? Dunbar tested his idea with the group sizes of nine hunter-gatherer groups and only one did not match his number. The theory has also been tested on social media, but the results are difficult to interpret. Today we have many online ‘friends’, with whom we actually rarely interact and thus probably do not count for this 150 limit.

At first glance it seems that Dunbar’s Number fits well with reality and it has become popular well beyond the fields of archaeology and anthropology. However, a closer look reveals flaws in the theory’s match with reality. A broader research on modern hunter-gatherer group sizes revealed that about there are nearly as many groups not matching with Dunbar’s Number, as there are groups that do. Furthermore there is large variation in group sizes, ranging from 42 to larger than 440.

Apart from an ambiguous footing with what we know of current hunter-gatherers, Dunbar’s Number also suffers from some deeper lying issues. First of all, when talking about group sizes, you’d expect that that means the amount of people living together. That is how group size is usually counted among primates. However, that is not the group that Dunbar meant for humans. Instead Dunbar meant the group of people that have regular direct contact. The problem here is that, as we have seen with the studies on Dunbar’s Number in social media, it is quite hard to clearly define what kind of group we are talking about. This has led some researchers to accuse others of defining group size in such a way that it produces the desired results. A second issue is that it is not the same definitions that was used for non-human primates, while the relation between neocortex and group size is for a large part based on non-human primate data.

The calculation of Dunbar’s Number itself is also only based on a single brain of 1250 mL. To put this into perspective the modern human average is 1400 mL. 1250 is just below the average brain size of Homo heidelbergensis, the likely ancestor of the Neanderthals. Not to mention that there is quite some variability in human brain sizes. A calculation based on only a single measurement is hardly confidence inspiring.

What guided the evolution of our larger brains remains a mystery. The Social brain hypothesis is one of many theories trying to explain what could have led to the brains that we have now. Unfortunately it seems that the available evidence does not strongly support it. Additionally the Social brain hypothesis suffers from inconsistent definitions of group size and its calculation is based on a unrepresentative sample. Even if we have a ‘natural’ limit to our number of friends, it seems it is not determined by the size of your brain.