My latest Mind and Matter column from the Wall Street Journal is on Dunbar's number.
As far as scientific accolades go, a Nobel Prize is rare, a law named after you is rarer, your own unit of measurement is more elusive still, but the most select club of all is those who have their own number, or constant. Two hundred years ago this year, the Italian physicist Amedeo Avogadro put forward the hypothesis that eventually resulted in the naming of Avogadro's number-the number of molecules in a mole, 6.022 times 10 to the 23.
Avogadro dines in this exclusive club with Max Planck and Ludwig Boltzmann (and perhaps, to make a foursome, the science-fiction writer Douglas Adams, for his "Hitchhiker's Guide to the Galaxy," where the number 42 is a supercomputer's mysterious answer for the meaning of it all). In recent years, however, the evolutionary anthropologist Robin Dunbar has acquired future rights to membership in this posthumous Pantheon. Mr. Dunbar's eponymous number is 147.8, plus or minus a lot, and it is the size of the average human being's social network of friends, as predicted by the size of the average human brain.
Many years ago Mr. Dunbar famously noticed that there is a tight correlation between the size of a primate's brain and the size of the social group its species generally forms. On this basis human beings should live in groups of around 150. The neat thing about this prediction was the way it seemed to fit the number of good friends most people have, as measured by the length of address books, the size of hunter-gatherer bands, the population of neolithic villages and the strength of army units. In recent years, Facebook has also seemed to confirm the hunch, with rosters of friends often settling around the Dunbar number.
Now Mr. Dunbar, who teaches at Oxford, has taken the argument a step further in work yet to be published, by correlating the size of a specific part of an individual's brain with the size of that individual's social network. He and his colleagues asked volunteers to list the initials of every person they had had social contact or communication with over the previous week, before stepping into a magnetic resonance scanner to measure the volume of their "orbitomedial prefrontal cortex." Sure enough, the size of this lobe of the brain correlates well with the size of a person's circle of friends. (It remains to be seen, of course, which causes which.)
The prefrontal cortex is the most peculiarly enlarged part of the brain in human beings, but whereas the part near the top of the head generally seems to be involved in conventional intelligence, the "orbital" region tends to handle the processing of social information-that is, assessing the moods and personalities of other people.
Mr. Dunbar's "social brain hypothesis" rests on another idea-the theory of mind-which argues that we use our brains to imagine what others are thinking. So, drilling down further into the physiology of the brain, Mr. Dunbar's team has now found that a rich social network also goes with the ability to reason about others' intentional states. That is to say, people with more friends are better able to understand sentences like: "Sam thought that Henry knew the post office was on Bold Street and hence that Henry must have intended to mislead Sam." And that both of these features are well predicted by the volume of gray matter in two specific regions of the prefrontal cortex, regions that are known to be important in "decoupling the perspectives of other people from one's own."
All this supports a once-radical idea that has been floating about in psychology since put into words by the consciousness expert Nick Humphrey in the 1970s-that human beings evolved big brains not to understand the world, but to understand each other. The more fellow apes you need to understand, the bigger the mental engine you need.
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