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Dunbar’s number: implications for internet marketing and SEO - Part 1

~63,100 is the number of hits you will likely get searching google for “Dunbar’s number”. If you are interested in SEO and search for “Dunbar’s number” +SEO you will get 221 results for the last year and 55 results from last month only. What is this number and why SEM/SEO people discuss it?

What is “Dunbar’s number”?

Social group size vs. neocortex volume

In 1992 Robin Dunbar, currently a Professor of Evolutionary Psychology at the University of Oxford and the director of the Social and Evolutionary Neuroscience Research Group, published a paper in which he proposed that social group size among primates is a function of the relative volume of the neocortex. This became known as the “social brain hypothesis” - the idea that a species that can maintain larger social groups have an evolutionary advantage. In order to be able to maintain a larger social group a species will need additional computing power, that is, a bigger brain.

So, what is the number? Let me illustrate.

In 2003, Russell Hill and Robin Dunbar published a paper in Human Nature examining social network size based on the exchange of Christmas cards. The authors found out that maximum network size averaged 153.5 individuals, with a mean network size of 124.9 for those individuals explicitly contacted; these values are remarkably close to the group size of 150 predicted for humans on the basis of the size of their neocortex. Thus, Dunbar’s number represents a theoretical limit in the size of the social network for humans and is situated around 150 individuals. This the number of people we can have meaningful social relationships. Below is a good graphical representation I found in Google Images (I could not trace the original) of Dunbar’s Social Brain Theory.

Social Brain Theory: Levels of social interaction, according to Dunbar

Constraints limiting social networks
Dunbar maintains that two factors impact how large a social network we can maintain.

One factor has to do with our cognitive limitations directly - we need to be able to compute complex social problems, such as this “I intend that you believe that Fred understands that we want him to be willing to [do something]... ”.

Emotional closeness as a function of time spent

This is an example of what Dunbar calls a 5th level intentionality problem and is about the limit of what adult humans seem to be able to do on a regular basis.

Above that, things start to become fiendishly complex. However, notice that closely-knit groups (~15 members) are doing 3rd and 4th level computations regularly. We just seem unable and unwilling to do it on a larger scale.

The other factor has to do with time. It time our memories decay and also our emotional attachment decays - two fundamental components that allow us to maintain our social network. On the right side you can see a graph showing the decay of emotional closeness with time since contact (taken from one of Dunbar’s presentations, see below).

Another way in which time affect our ability to maintain a social network has to do with the fact that there is a limited amount of time (after sleep, food, hygiene and work) that we devote to socializing.

Social time as a function of group size

For instance, based on primate data relationship between social time and group size, assuming all else is equal, one would expect humans to need to spend a whopping 43% of their time budget on socializing in order to maintain a social network of ~150 relationships. This is of course not the case, as we have (among other things) language to ease our social interactions, but it still shows that time is a critical component of social networks.

If you want entry point into this topic I would highly recommend this presentation or this slideshow by Robin Dunbar.

In the next post I will address the implications Dunbar’s number has for internet marketing and SEO/SEM.



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