log(N) = 0.093 + 3.389 log(CR) (1) (r2=0.764, t34=10.35, p<0.001)

December 10, 2006

Recognise it? Well of course, it’s the best-fit reduced major axis regression equation between neocortex ratio and mean group size for the sample of 36 primate genera taken from Dunbar’s 1992 paper which was popularised, and not unduly trivialised by Malcolm Gladwell into a natural limit on human group size of 150 (or 147.8 to be exact). The idea is a simple one. The human brain has co-evolved with social conditions and as a result there is a natural limit on the number of social relationships we can maintain. Dunbar linked the number to village, nomadic and military size over time. The number is exercising several people on the ever idea-stimulating value networks list serve. The argument there relates to if this is or is not a natural limit on a network or a virtual community.

150 is not the only natural number. There are two others, so I could have titled this post The rule of 5,15 & 150. All of those numbers, plus a need to think more about identity than about individuals, should influence either evolutionary or engineering approaches to community/network design.

What I plan to do is elaborate the numbers and their origins. I then want to look at the way in which the debate around Dunbar’s law is limited by atomistic ontology. This all too common assumption, found in the anglo-saxon world assumes self sufficiency and moral autonomy of the person, and sees communities as assemblies, voluntary or otherwise of individuals. Moving away from social atomism allows to take a different view on communities, their limitations and possibilities, but that will be tomorrow’s blog.

  1. Five is linked to the natural limits on the short term memory. This was first put forward by Miller’s 1956 paper and relates to time more than items (it is a common urban myth to see it as items). This means that it will vary a bit by language, different languages can compress more or less data into a defined time limit. If you have ever spoken through simultaneous translation then you will know that it takes 30% longer to say something in Spanish that it does in English. Given that the Welsh generally speak english 30% faster than the norm, this can present problems! Translation aside, the number is useful and it relates to common sense experience (always helpful). Think about how many directions you can remember, or how we organise telephone numbers. Another way to validate this is to think about models, or lists and see how many elements they have. More than five and you need a crib sheet. One of the reasons I restrict models in my own work to five elements is because of this. Less then five and they pass the paper napkin test which means they are sense making models as they can be drawn from memory, which means they can be used operationally without reference back to authority.
  2. Fifteen comes from anthropology and relates to natural levels of deep trust. I define deep trust here as the ability to tolerate a degree of betrayal. The number varies a bit based on the average size of the extended family in a society and is probably an habituated pattern of behaviour learnt during key periods of plasticity for the human brain. Now readers might be able to help be here. I got this number from two sources several years ago. The number was actually an upper limit of thirty but I reduced it to fifteen for alliterative purposes as well as accepting the realities of modern civilisation compared with the tribal systems from which the number originated. Unfortunately I have lost the reference and I am trying to re-discover it to reference in the book. All help appreciated! Again this manages a common sense test. Think about the social groups to which you belong and which pass the relaxation test. This test is a simple one, its who do you feel able to relax with, without worrying too much how your are seen. I realise that this does not always apply to families! However other than in pre or post divorce situations the ideas is that it should. The size there is definitely under fifteen, and more typically is a small number of groups of around eight or nine on average.
  3. One hundred and fifty is Dunbar’s law and in effect is the number if identities that you can maintain in your head with some degree of acquaints that an individual can maintain. It does not necessarily imply that you trust them, but it does mean that you can know something about them and their basic capabilities. In other words you can manage your expectations of their performance and abilities in different contexts and environments. For the moment lets consider this in terms of individuals (the switch to identity is for tomorrow’s blog). Consider your work groups and the size of your organisation. How many people do you know by name? How many people would you invite to a party? Again you can see the common sense experience coming though in the number. Now the assumption in Dunbar’s working and subsequent writing is that this level of knowledge requires physical proximity. However we now live in virtual as well as physical worlds so the nature of interactions change. The natural limit is probably in place, but its form, and the nature of its creation will have new variants for a new environment

Now these three numbers, 5, 15 & 150 have an alliterative quality which helps us remember and use them. They also have some fairly immediate and practical implications for communities and networks. That is what I want to look at in tomorrow’s blog which will come from Hong Kong. I am shortly leaving for the KMAP2006 conference at which I am keynoting for the second year, and I will also run workshop on uses of narrative in knowledge management. Hopefully I will meet up with some old friends and make some new ones, the conference has an interesting mix and looks less academic that last year when it was held in Wellington, New Zealand.

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