Robin Dunbar

And then since we did those originally, there have been about 25 studies now showing this in humans, neuroimaging experiments have been done on three groups of monkeys.

The same effect being shown at the individual level.

So you'll like this little bit because it's mathematical.

And by the way, if you want any evidence of how actually functional that is, that it really exists, this number, somebody did an analysis of 61 million Facebook pages, counting all the friends on each of these 61 million Facebook pages.

The average was 149.

In general, they are.

Your 150 friends, as I mentioned earlier, are divided up into a series of layers.

Those layers have very, very specific numbers.

They're 5, 15, 50 and 150.

So the 5 is what we call your shoulders to cry on friends.

They're the ones that will... When your world falls apart, they will drop everything and come pick you up again.

Turns out...

that those numbers are optima for the efficiency with which information flows through networks.

And what's more, those are the numbers you find not only between species of primates, but also within primate groups.

The structure of primate groups has exactly those numbers as well.

So they don't run to the 150 because they don't have as big a brain as us.

But for species like baboons and macaques and chimpanzees that live in groups of 40 or 50 individuals, they're substructured in exactly the same way as human social networks are.

You've got what I call the line-dancing problem.

Imagine you're in a group...

foraging through the grasslands or the woodlands of the savannah or the forest or whatever it is, how many people can you have in a group and still have the people at either end in time on a line dance?