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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
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
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
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.