I can’t stop thinking about Dunbar number. Brain, and inside it, neocórtex, was extremely expensive to build. And even more to maintain: it needs lots of energy. But we can write our evolution from being apes to cyborgs in terms of brain size and sophistication increments. There is no goal in evolution, but it is always triggered and maintained by something. What was the spark for this change?

A gradual iterative dietary change? The animal size was increased and the proportion must be preserved? Dunbar proved in his seminal article that brains evolved not because we could afford it, but because of evolutionary pressure. A reason because people animals with higher brain development had more chances to survive. In other words

‘Large brains will evolve only when the selection factor in their favour is sufficient to overcome the steep cost gradient’.

Finding the reason to evolve.

Dunbar tried two hypotheses.

1.- Ecological: You need to have a better brain if you want to find fruit instead of eating leaves (Leaves are easier to find but nutritively poorer)

2.- Social: a complex brain is a requirement to cope with the increasing complexness of social and cultural relationships and rules.

And tried to validate them trying to find a correlation with these factors with brain evolution… in several species of apes. He verified the correlation of these two factors (measured as the percentage of fruit in the diet or the average size of the group) with brain size in several kinds of simians and prosimians. He found no relationship with fruit but a clear correlation with the average social group. So the morale for this tale was that the brain evolved to allow a more sophisticated social interaction. (Funny fact is that it is widely accepted that there are several overlapping factors interacting in a mutual feedback scheme: dietary changes, phonation organs changes, apes becoming bipeds, habitat changes from jungle to savannah…)

Ok, Anything started to allow more social interaction. But what’s our limit?

We can calculate then the theoretical group size that would correspond to the homo sapiens following the correlation brain evolution vs average group size he found on apes. And this is Dunbar number: about 150. But this is more a picturesque side outcome than the key point of the article (that it exists a relationship among social complexity and brain development). But Dunbar’s work is known mainly because of this number ¯_(ツ)_/¯

Well, we have no limits… but in Facebook and Twitter.

There are several details the magic number evangelists insist on ignoring in their quest for an appealing recap/number. First, the huge simplification that using group size to evaluate social complexity means. Dunbar himself apologized for using this measurement, justifying it because it is the only available for all the species in the sample… What about the group size of sardines? Most of the times group sizes are more influenced by environmental pressures or behavior particularities than related to neocortex in any way. It doesn’t make sense to use that to extrapolate to an acceptable group size limit.

We have no limits. I have something to prove this? Well… it depends on how picky we are with proofs. As Always.

Sidenote: Believe it or not, the average number of friends in facebook is 155. Oh no! Does this mean the Dunbar number is not so far from the truth? Nope, relax. Dunbar number was supposed to be a max, not an average. Also in Twitter the average number seems to be higher than 200... just saying 😉