5. 达尔文和他的朋友

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Darwin and Friends - Oxford Abridged Short Talks

Professor Robin Dunbar explores if there is a limit to the number of friends we can keep track of and explains the origin of "Dunbar's Number".

Thank you very much

I'm going to talk about something that's near and dear to almost all of us

and that's really how many friends you've got

as the vice chancellor eluded to next door

Darwin of course is an incredibly prolific

these are probably his three best known books

but he wrote a number of other books during the course of his life

which in sort of intellectual terms uh equally good reads

I recommend them to you book on barnacles

Might remind you

that even if none of these books had ever been written

ah indeed he'd never done anything on evolution at all

he'd still be regarded as one of the founding fathers of geology

something that very often sort of people aren't aware of

He was one of the sort of great geologists of the period as well

Anyway clearly Darwin had a go at almost everything

quite a remarkable career

but two things really he didn't say a great deal about

in the course of this life's work

really of brains and sort of the emergent properties of social behavior

you heard from Tristin at the beginning

that he was very interested in sort of

as it if were the signaling components of behavior

He wrote this extraordinary book on the evolution of emotions

which was really one of the foundational books of psychology

it can be regarded as

But he in terms of the sort of societal aspects of behavior

and the emergent properties of our interactions with each other

as it were, ah he had very little to say

Simply because they didn't know very much about these kind of things

they couldn't do much a great deal with brains in those days

So one of the things I've been interested in is trying to understand

how society or social systems emerge in mammals in general

primates in particular

and we drifted into humans sort of about 10 or 15 years ago

and it was a consequence of an idea

which had been proposed in the late 80's by colleagues of mine

up at Saint Andrews

now known as the social brain hypothesis essentially

That one of the reasons primates have such big brains

compared to all other species of mammals is

that they live in incredibly complex social systems

and it turned out in fact that primate societies do seem to

differ from the social systems from most of the animals

not all but most of the species

in the sense that they are deeply bonded with each other

and in some sense it's been a surprise to us

coming from the animal, end of the spectrum

that they are kind of qualitatively different in this sense

But one of the sort of core bits of evidence

to support the social brain hypothesis was these data here

which we produced now 15 or so years ago

showing that if you take a measure of social complexity

like group sizes for a species

and plotted against a measure of brain size

and this is the sort of the simple version of the analyses

what you find is that

species with big brains tend to live in big social groups

These are sort of species averages these are apes here

these are various monkeys

and the monkeys turned out to be a series of grades

in which if you like to think about it

as you move across these grades for the same group size

you're having to use

a bigger and bigger computer to manage that social system

With apes sort of lying out here on the right

suggesting that they kind of live in the most complex social world

compared to monkeys and apes

but in relatively small groups

compared to some monkeys and apes

This is peculiar to primates

and perhaps to one or two other groups of mammals

but it's a very kind of different story

in mammals as a whole and birds as a whole

So when you look for this pattern in mammals and birds

generally you don't find it

what you find there is that the species that have the big brains

are the ones that live in monogamous mating systems

and there's something about pair bonded monogamy

particularly life-long pair bonded monogamy

The birds showed this really nicely

that is deeply complex and deeply challenging in a cognitive sense

they need a big brain to do it

and what primates essentially seem to have done is taken the machinery

The psychological machinery required

to support pair bonded relationships

and generalized those to all members of the group

and essentially create friendships

And indeed this whole approach of using cross species analyses

the so-called comparative method actually goes back to Darwin

It's the methodology he really developed in great style in his books

and used to a very great effect

Ok so if we plug humans into this

we know where humans lie on this relative brain size component

and it's the neo-cortex

the sort of outer sheath of our brains are the critical bit

because that's what's expanded in primates

you get a figure if you read across from the 8th line

a predictive figure of about 150

which is now known thanks to the modern technological age

its wonderful stuff as Dunbar's number

this is a consequence of a big debate on Facebook

about the number of friends you can have on Facebook

and out of that came the labeling of it

much to my amusement

It turns out that this group size of a 150 appears all over the place

I'll just give you two examples

This was our very first attempt to actually look at it in real life

we did it on Christmas card distribution lists

this is not the number of cards you send out

but the number of people in the recipient household

and there's a lot of variance

in that some people are incredibly mean

like me and (INAUDIBLE) hang Christmas cards

and apparently have no friends

And then as we heard from vice chancellor

the director of the museum is incredibly social

and has a very large number and this so

But there's a strong peak

and the peak and the sort of modal value is very close to about a 150

and you can come up with lots and lots of similar examples

and we have looked at this thing in considerable detail

What's interesting is you find exactly these numbers in the military

all modern military, particularly the armies

are structured around this value of 150

It's a size of a company

and after that everything is scaled very nicely

I'll come back and show you why in a minute

So this really appears to be

the foundational element of both our personal social worlds

the number of people you know

and kind of an informal definition of that is kind of all the people

If you bumped into them in the departure lounge

at Hong Kong airport at 3.00am or something like that

you wouldn't feel embarrassed about going up to them

and saying: hi how are you, haven't seen you for ages

You might have some catching up to do, but you know them

you know where they sit in your social world

and they know where you sit in their social world

So it seems to be the limit on the number of people

we know as persons that we got kind of personal history with

It turns out that that the size of those

that, that social circle you had

is indeed as in the original social brain hypothesis

even at the individual level

appears to be related to the size of your brain

and we're showing you this now with two separate Neuroimaging studies

One of which is a sort of growth analysis

looking at the large scale areas

and it turns out that this area here

in particular the orbital frontal cortex

correlates with the size of your social network

and in a very detailed voxel by voxel based analysis

again it's this area up in the orbital frontal cortex comes up

so the sort of relative power of computing

and the effect that you can manage up in this area

This area turns out to be associated with social reward

and reward functions in general

And if you blow out those areas people lose their social skills

One of the things they lose is inhibition

so they act before they think and things like that

and say things which are inappropriate or rude

but it is sort of heavily

although it's not the only thing that's involved

There's sort of distributed network of

neural components all over the cortex

that is involved in this what seems to be particularly important

the control exercise by this area up here

So quite literally in some sense

people with big brains have big social networks

and are very intensely social

and what's interesting from the evolutionary point of view of course

is that the brain has evolved from back to front

So here's the back of the brain here's the occipital end

the visual system is sort of dealt with up here

the brain has expanded

during the course of primate evolution particularly

and into human evolution as it were from back to front

so it's these frontal areas

which are particularly well developed in primates and especially

so in humans we have that enormous frontal lobe

The reality for your social world is that

actually it looks a bit like this

it turns out from very complex analyses that we ended up having to do

that your world of a 150 is actually composed of

a series of layers of relationships

which scale very nicely in a kind of scaling ratio of five

so each layer is five, ah sorry 3

each layer is 3 times the layer inside it

and indeed we know from our data that

these layers go on for at least two more layers at 500 and 1500

and remarkably and I think it really is remarkable

Aristotle got that number right

that was the number you said

you can count the number of friends you have

on the fingers you have on one hand

true friends let's say

and Plato got the next layer out

which is his ideal democracy size

which we might remember

as we are about to debate a new voting system here

Ah in Britain, ah at 5300

and I give him 300 you know error margin on that

but they didn't know about statistics 350 B. C.

but it's quite extraordinary

but these, what's really kind of remarkable about these numbers

is they are replicated exactly all the way through

right up to Plato's number and beyond by the military

and the way the military structure their units as it were

and embed them within these increasing circles of hierarchy

The difference here is these have to up to this point

you have to relate maintain these relationships

by personal knowledge and personally interacting

So if you look through these layers

the frequency of interaction or the emotional closeness

that people say they have

you'll find its sort of falling off in these step functions

as you go around these edges

But beyond that we need kind of language

and rules and discipline and punishment as it were

to maintain the structure behind that

that's the well beyond that

that's the kind of natural limit

We're able to build bigger and bigger units

by having these kind of social rules as it were

that allow us in much more simplified form

to maintain bigger structures

but those people outside that 150

we don't have personalized relationships with

we know each of these as individuals

It turns out that what's critical really is

maintaining those relationships by face to face interaction

It really is personalized knowledge that you have

and we think from the work we've done now that

although things like electronic media are quite good at

allowing you to keep a relationship ticking over

in the end if you don't get together and bang your heads together

that relationship will actually decay and despite all the claims

and in some sense this is sort of where all of this got started

despite all the claims you don't get more friends on Facebook

than you have in the real world

round February Facebook did a complete analysis

of all forty-four million Facebook pages prompted by our work

to see how many friends people actually have

because obviously some people

are claiming 500, a thousand, even five thousand

it turns out that the average is between 120 and 130

so very close to real life

Now one of the interesting, ah sort of observations

we noticed as it were is that

Intensively girls that have the 120 and the 130

so they're keeping their sort of Facebook pages

pretty close to their real life things

It's the boys that have the 500's and the 1000's

so just going back to Tristan's talk at the beginning

what we think that probably has to do is mate advertising

How popular you are

being popular with other girls turns out to be a very good cue that

women use in humans for mate quality in addition to the smell

It also turns out part of one of the things we were interested in is

how relationships decline and drop down over these layers

So we just finished last year a long-term 18-month study

looking at what happened over long periods of time

when people moved away

and because it was tied in with technology

and lot of this stuff got tied up with the mobile phone industry now

who have become really very interested

Indeed they are involved in four different projects

with various computer scientists and mobile phone providers

How technology helps slow down the rate of decline on relationships

Slightly to our surprise what came out of this

is actually is that a huge gender effect

So what we've got here is that

in this study we asked people two things at repeated intervals

and this is sort of over 18 months

how often did you do stuff with the people in your network

go shopping, go to parties, help them move house or

have you and how often did you contact them

and have a chat with them either on a phone or face to face

And it turns out that as one might expect

if you do let stuff, if over 18 months

you spend less time banging heads together with them

or talk to them as often

the quality of the relationship declines over time

so this is a measure of the change in emotional quality of

the relationship between month 0 and month 18

If you keep the rate of both contact doing stuff

and talking to them the same

It naturally the relationship stays the same

And if you increase the amount of activity

or the amount of talking to them

the relationship increases

But its gender-specific

For boys, relationships are maintained by doing stuff together

For girls by talking

Very very striking effect which explains things very nicely to you

Why boys phone calls are only 7.3 seconds long on average

I'll see you down the pub at 7:00

and girls spend massive amounts of time on the phone

so phone technology and particularly mobile phone technology

is perfectly designed to support the female social world

This goes back my little character of

what relationships are really like for girls

They are very personal, very one to one, very intense

So the sort of the typical thing would be what life was like

when you were about 8 or 10

So when Penelope didn't invite you to her party

this was the ultimate crisis of the universe

Now for boys

the relationship was standing on opposite sides of the world

kicking a football backwards and forwards

and that's a relationship

And actually I have a view that it doesn't really matter

whether it's a little boy on the other side of the road or the wall

as long as the football comes back

It's fine and that may help explain a great deal here

A great deal about your experience of life

Thank you very much.