150 = Magic Social Network Number

August 12, 2008

That’s according to Professor Robin Dunbar, who writes a fascinating short article on the link between brain size and social grouping sizes in the British Academy’s latest Review of research. Here’s an extract from “Why Humans aren’t just Great Apes“:

This figure of ~150 appears frequently in many aspects of historical and contemporary human organisation. It was the mean village size recorded for almost all English counties in the Doomsday Book as well as during the eighteenth century, and is the typical size of the company in most modern armies, the number of recipients of a typical Christmas card distribution list in Britain, and the size of the social network in reverse ‘small world’ experiments, amongst others.

A few things are interesting about this. First is the fact that this 150 “limit” is structured differently in modern networks (both in the real world and virtual world). In modern society, social groupings are fragmented and diverse – if you and I are in the same social network, the 150 people in my network will overlap with, but won’t be identical to the 150 in your network. Whereas in traditional hunter-gatherer and pre-industrial society, the 150 in a given community tended to all know each other.

The second interesting facet is concerned with different hierarchical “levels” within this 150. According to Prof. Dunbar’s research, our most intimate grouping will contain roughly 5 people, the next level of social closeness has 15, then 50, 150, 500 and 1,500. But beyond 150 the social network degrades to the point where we typically have little to no “personal” knowledge about the people in the group.

Third, and central to Dunbar’s argument, is that the social grouping number of 150 correlates with brain size (specifically neocortex size). This, he argues, is related to “intentionality” or theory of mind, i.e. I think that he believes that she supposes… etc. Most humans are limited to five orders of intentionality, though some can hold six in mind at once (some autistic people, of course, never develop theory of mind).

Social Networks

Social Networks

I wonder whether our varying ability to hold five or six levels of intentionality in mind – and, relatedly, our brain size – might be reflected in the size of our online social networks. I suppose I’m fairly average, judging by the size of my Facebook network (180). Most of those I would say I know reasonably well (or have known at some stage in my life).

However, as predicted, there are clear levels of intimacy within that group. There are probably 4 or 5 people I message and interact with regularly on Facebook, and another larger group that I will send a message to once in a while. Beyond that, my network is definitely “fragmented and diverse”, composed of a range of old school and university friends, work colleagues, as well as friends from church.

I’m sure that there are endless competitions of who can add the most friends on Facebook and other social networks, but does this reveal anything about advanced theory of mind of the competitors or are they just trying to up their stats to compensate for something else? Hmm…

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