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September 13, 2004

150 good men to help conquer the world!

In Grooming, Gossip and the Evolution of Language Robin Dunbar argues for the primacy of social groups ~150 individuals as basic atomic units. He points to evidence that our brain physiology itself sets the constraints that impose this number as an upper limit. Today, I was reading a bit of Genghis Khan and the Making of the Modern World, and I stumbled upon this (page 41):

He also created an elite bodyguard of 150 warriors: 70 day guards and 80 night guards to surround his camp at all hours....

Later on he established 100 men as basic units of his mobile army. 100 was also the number of soldiers in a Roman Century, the most important organizational level in the Legion (the Cohort be intermediate). Conventional wisdom was that an army was one or lost by a general by the loyalty and fidelity he could expect from the Centurions. Reading about the life of Genghis Khan, it seems that he intuitively understood a few truths about human nature, and stumbled upon others by happenstance. His reward was world conquest.

More later....

Addendum: As Jaime points out, the Mongol army had lower levels of organization than 100. I knew this, but I wanted to emphasize 100 as the crucial number because this is the last level at which human social intelligence can operate unaided by formalized rules & regulations. In the Roman army the contubernium was nested within the century, 8 men who would basically be social intimates. But, it was to the centurions, who commanded the century, that Roman generals looked to when attempting to shore up their support. Centurions, traditionally the highest noncommissioned officers, would be familiar with both the rankers below and the generals above on some personal level, ergo, their importance.

By analogy, there are obviously levels of organization below the individual, but operationally the individual organism is thought of as a unit because the interests of cells, organs, organ systems and the genes that encode them are tied up in the individual (at least to a high degree). Beyond the level of the individual new dynamics come into play, we go from the realm of genetics and physiology to ecology and ethology. Similarly, between the dozen who might be social intimates and the 120 who are part of your social network, similar processes are at play, but beyond 120 formal rules & regulations that are not so rooted in social intelligence become more important.

Addendum II: More illustrative analogy of what I'm talking about: throw 4 marbles on a table and your friend can tell you there are 4 marbles without "counting," that is, they have an accurate, precise and innate sense of numeracy. Throw 40 marbles on the table and both accuracy and precision drop, to get the exact number your friend has to count mentally and will use some sort of formal system (divide into groups of five and count them up). Somewhere around ~6 our innate numeracy fails us as an exact mental faculty and we need to supplement it with mental aids, formal rules. Somewhere around ~150 individuals our social intelligence reaches its limit and larger organizations must be scaffolded by artificial laws and regulations.

On second thought, I think my use of the term atomic or basic might have been imprecise, though to be fair, atoms are composed of protons, neutrons and electrons, and protons & neutrons are composed of quarks.

Related: (see Robin Dunbar and the Magic Number of 150)

Posted by razib at 06:43 PM