when I was a hospice doctor at the University of Chicago.

And I was taking care of people who were dying and their families

in the South Side of Chicago.

And I was observing what happened to people and their families

over the course of their terminal illness.

And in my lab, I was studying the widower effect,

which is a very old idea in the social sciences,

going back 150 years,

known as "dying of a broken heart."

So, when I die, my wife's risk of death can double,

for instance, in the first year.

And I had gone to take care of one particular patient,

a woman who was dying of dementia.

And in this case, unlike this couple,

she was being cared for

by her daughter.

And the daughter was exhausted from caring for her mother.

And the daughter's husband,

he also was sick

from his wife's exhaustion.

And I was driving home one day,

and I get a phone call from the husband's friend,

calling me because he was depressed

about what was happening to his friend.

So here I get this call from this random guy

that's having an experience

that's being influenced by people

at some social distance.

And so I suddenly realized two very simple things:

First, the widowhood effect

was not restricted to husbands and wives.

And second, it was not restricted to pairs of people.

And I started to see the world

in a whole new way,

like pairs of people connected to each other.

And then I realized that these individuals

would be connected into foursomes with other pairs of people nearby.

And then, in fact, these people

were embedded in other sorts of relationships:

marriage and spousal

and friendship and other sorts of ties.

And that, in fact, these connections were vast

and that we were all embedded in this

broad set of connections with each other.

So I started to see the world in a completely new way

and I became obsessed with this.

I became obsessed with how it might be

that we're embedded in these social networks,

and how they affect our lives.

So, social networks are these intricate things of beauty,

and they're so elaborate and so complex

and so ubiquitous, in fact,

that one has to ask what purpose they serve.

Why are we embedded in social networks?

I mean, how do they form? How do they operate?

And how do they effect us?

So my first topic with respect to this,

was not death, but obesity.

It had become trendy

to speak about the "obesity epidemic."

And, along with my collaborator, James Fowler,

we began to wonder whether obesity really was epidemic

and could it spread from person to person

like the four people I discussed earlier.

So this is a slide of some of our initial results.

It's 2,200 people in the year 2000.

Every dot is a person. We make the dot size

proportional to people's body size;

so bigger dots are bigger people.

In addition, if your body size,

if your BMI, your body mass index, is above 30 --

if you're clinically obese --

we also colored the dots yellow.

So, if you look at this image, right away you might be able to see

that there are clusters of obese and

non-obese people in the image.

But the visual complexity is still very high.

It's not obvious exactly what's going on.

In addition, some questions are immediately raised:

How much clustering is there?

Is there more clustering than would be due to chance alone?

How big are the clusters? How far do they reach?

And, most importantly,

what causes the clusters?

So we did some mathematics to study the size of these clusters.

This here shows, on the Y-axis,

the increase in the probability that a person is obese

given that a social contact of theirs is obese

and, on the X-axis, the degrees of separation between the two people.

On the far left, you see the purple line.

It says that, if your friends are obese,

your risk of obesity is 45 percent higher.

And the next bar over, the [red] line,

says if your friend's friends are obese,

your risk of obesity is 25 percent higher.

And then the next line over says

if your friend's friend's friend, someone you probably don't even know, is obese,

your risk of obesity is 10 percent higher.

And it's only when you get to your friend's friend's friend's friends

that there's no longer a relationship

between that person's body size and your own body size.

Well, what might be causing this clustering?

There are at least three possibilities:

One possibility is that, as I gain weight,

it causes you to gain weight.

A kind of induction, a kind of spread from person to person.

Another possibility, very obvious, is homophily,

or, birds of a feather flock together;

here, I form my tie to you

because you and I share a similar body size.

And the last possibility is what is known as confounding,

because it confounds our ability to figure out what's going on.

And here, the idea is not that my weight gain

is causing your weight gain,

nor that I preferentially form a tie with you

because you and I share the same body size,

but rather that we share a common exposure

to something, like a health club

that makes us both lose weight at the same time.

When we studied these data, we found evidence for all of these things,

including for induction.

And we found that if your friend becomes obese,

it increases your risk of obesity by about 57 percent

in the same given time period.

There can be many mechanisms for this effect:

One possibility is that your friends say to you something like --

you know, they adopt a behavior that spreads to you --

like, they say, "Let's go have muffins and beer,"

which is a terrible combination. (Laughter)

But you adopt that combination,

and then you start gaining weight like them.

Another more subtle possibility

is that they start gaining weight, and it changes your ideas

of what an acceptable body size is.

Here, what's spreading from person to person

is not a behavior, but rather a norm:

An idea is spreading.

Now, headline writers

had a field day with our studies.

I think the headline in The New York Times was,

"Are you packing it on?

Blame your fat friends." (Laughter)

What was interesting to us is that the European headline writers

had a different take: They said,

"Are your friends gaining weight? Perhaps you are to blame."

(Laughter)

And we thought this was a very interesting comment on America,

and a kind of self-serving,

"not my responsibility" kind of phenomenon.

Now, I want to be very clear: We do not think our work

should or could justify prejudice

against people of one or another body size at all.

Our next questions was:

Could we actually visualize this spread?

Was weight gain in one person actually spreading

to weight gain in another person?

And this was complicated because

we needed to take into account the fact that the network structure,

the architecture of the ties, was changing across time.

In addition, because obesity is not a unicentric epidemic,

there's not a Patient Zero of the obesity epidemic --

if we find that guy, there was a spread of obesity out from him --

it's a multicentric epidemic.

Lots of people are doing things at the same time.

And I'm about to show you a 30 second video animation

that took me and James five years of our lives to do.

So, again, every dot is a person.

Every tie between them is a relationship.

We're going to put this into motion now,

taking daily cuts through the network for about 30 years.

The dot sizes are going to grow,

you're going to see a sea of yellow take over.

You're going to see people be born and die --

dots will appear and disappear --

ties will form and break, marriages and divorces,

friendings and defriendings.

A lot of complexity, a lot is happening

just in this 30-year period

that includes the obesity epidemic.

And, by the end, you're going to see clusters

of obese and non-obese individuals

within the network.

Now, when looked at this,

it changed the way I see things,

because this thing, this network

that's changing across time,

it has a memory, it moves,

things flow within it,

it has a kind of consistency --

people can die, but it doesn't die;

it still persists --

and it has a kind of resilience

that allows it to persist across time.

And so, I came to see these kinds of social networks

as living things,

as living things that we could put under a kind of microscope

to study and analyze and understand.

And we used a variety of techniques to do this.

And we started exploring all kinds of other phenomena.

We looked at smoking and drinking behavior,

and voting behavior,

and divorce -- which can spread --

and altruism.

And, eventually, we became interested in emotions.

Now, when we have emotions,

we show them.

Why do we show our emotions?

I mean, there would be an advantage to experiencing

our emotions inside, you know, anger or happiness.

But we don't just experience them, we show them.

And not only do we show them, but others can read them.

And, not only can they read them, but they copy them.

There's emotional contagion

that takes place in human populations.

And so this function of emotions

suggests that, in addition to any other purpose they serve,

they're a kind of primitive form of communication.

And that, in fact, if we really want to understand human emotions,

we need to think about them in this way.

Now, we're accustomed to thinking about emotions in this way,

in simple, sort of, brief periods of time.

So, for example,

I was giving this talk recently in New York City,

and I said, "You know when you're on the subway

and the other person across the subway car

smiles at you,

and you just instinctively smile back?"

And they looked at me and said, "We don't do that in New York City." (Laughter)

And I said, "Everywhere else in the world,

that's normal human behavior."

And so there's a very instinctive way

in which we briefly transmit emotions to each other.

And, in fact, emotional contagion can be broader still.

Like we could have punctuated expressions of anger,

as in riots.

The question that we wanted to ask was:

Could emotion spread,

in a more sustained way than riots, across time

and involve large numbers of people,

not just this pair of individuals smiling at each other in the subway car?

Maybe there's a kind of below the surface, quiet riot

that animates us all the time.

Maybe there are emotional stampedes

that ripple through social networks.

Maybe, in fact, emotions have a collective existence,

not just an individual existence.

And this is one of the first images we made to study this phenomenon.

Again, a social network,

but now we color the people yellow if they're happy

and blue if they're sad and green in between.

And if you look at this image, you can right away see

clusters of happy and unhappy people,

again, spreading to three degrees of separation.

And you might form the intuition

that the unhappy people