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Dunbar's Function

27 Eliezer_Yudkowsky 31 December 2008 02:26AM

The study of eudaimonic community sizes began with a seemingly silly method of calculation:  Robin Dunbar calculated the correlation between the (logs of the) relative volume of the neocortex and observed group size in primates, then extended the graph outward to get the group size for a primate with a human-sized neocortex.  You immediately ask, "How much of the variance in primate group size can you explain like that, anyway?" and the answer is 76% of the variance among 36 primate genera, which is respectable.  Dunbar came up with a group size of 148.  Rounded to 150, and with the confidence interval of 100 to 230 tossed out the window, this became known as "Dunbar's Number".

It's probably fair to say that a literal interpretation of this number is more or less bogus.

There was a bit more to it than that, of course.  Dunbar went looking for corroborative evidence from studies of corporations, hunter-gatherer tribes, and utopian communities.  Hutterite farming communities, for example, had a rule that they must split at 150—with the rationale explicitly given that it was impossible to control behavior through peer pressure beyond that point.

But 30-50 would be a typical size for a cohesive hunter-gatherer band; 150 is more the size of a cultural lineage of related bands.  Life With Alacrity has an excellent series on Dunbar's Number which exhibits e.g. a histogram of Ultima Online guild sizes—with the peak at 60, not 150.  LWA also cites further research by PARC's Yee and Ducheneaut showing that maximum internal cohesiveness, measured in the interconnectedness of group members, occurs at a World of Warcraft guild size of 50.  (Stop laughing; you can get much more detailed data on organizational dynamics if it all happens inside a computer server.)

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