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Three (times fifty) is a magic number

June 24, 2009

Ok, ok—so the title of the post is a vague allusion to the classic School House Rock song, “Three is a magic number.” In social neuroscience, it seems that—yes—there is indeed a “magic” number: one-hundred and fifty.

ThinkingMonkeyWhen we look at the brains of chimpanzees and other primates, we find many similarities between our own human brains and those of our furrier friends. One of the striking differences, though, is the variation of thickness in the most recently evolved brain region: the neocortex. Just a few millimeters thick—the thickness of a few credit cards stacked onto one another—the neocortex wraps around the outside surface of the brain and envelopes the more evolutionarily ancient brain structures. For being so relatively small, the neocortex is involved in a dazzling complexity of higher cognitive processing, such as logical reasoning and personal identity. A question many biologists have wondered is this: why is the neocortex in humans thicker than it is in other primates? Intuitively, we might immediate suppose that neocortical thickness is related to our intelligence. Or in other words, the thicker our neocortex, the better we will be at math or philosophy or abstract reasoning.

Interestingly, the driving evolutionary force behind the growing thickness of our neocortex seems to not be our primative need to mathematically model the fluctuations of the seasons or to generate abstractions to represent the movements of herds, or anything of the sort. Rather, our growing neocortex seems to have everything to do with our growing capacity for sociality.

British Anthropologist Robin Dunbar conducted a study in which he examined the group size for 38 primates. He then determined an equation that linked the size of social groups for primates with the primates’ neocortical volume. Extrapolating from his regression equation, Dunbar predicted that—based upon the volume of the human neocortex—that human social groupings are evolutionarily primed for a group size of 148 individuals.

This “magical” number (rounded up to 150), now referred to as “Dunbar’s number,” holds out with almost startling detail. Dunbar conducted numerous surveys in tribes and villages in which this social group size of 150 persons was a consistently recurring number. The number 150 was even codified in numerous instances, such as the size of army units, both in ancient Rome as well as modern history. In his book The Tipping Point, author Malcolm Gladwell further explores how Dunbar’s number even manifests itself in the day to day of modern life, such as offices, successful production plants, and thriving businesses.

What to make of this? One take-home message that I derive from this observation touches on the immensely powerful force of community. Successful community dynamic is so vociferously poignant, that it is an entity that steered the course of biological evolution and reshaped the human brain. Indeed, functional community is such a powerful force, that it literally transcends time (millions of years of evolutionary time), directs history, and shapes our species—or at least our neocortex. We are exquisitely social beings, with a finely-tuned social organ: our brain.


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