Meet the boss: Robin Dunbar, evolutionary psychologist

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  In the early 1990s,

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  a British anthropologist called Robin Dunbar was studying social groups among monkeys and apes.

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  He'd come up with a ratio.

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  If the brain size of a primate equals X,

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  the group size will equal Y. That's when an idea struck.

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  So it occurred to me that we ought to be able to predict the natural size for human groups off the back of this primate relationship.

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  Dunbar applied his ratio to his own species.

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  And when you plug in human brain size,

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  what you get is group size of about 150.

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  It turned out that 150 is a very stable number,

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  found in human social groups everywhere from hunter-gatherer tribes,

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  to English villages,

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  to Facebook friendship networks.

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  In essence,

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  150 is the number of people with whom a person can have a meaningful relationship.

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  My casual definition of it is they're all the people who,

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