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netsci08 opening keynote

Sun Dec 21, 2008

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Nicholas Christakis, Harvard, "eat drink and be merry, the spread of

health phenomena in social networks".

This talk is looking at the spread of desies throgh social interactions,

rather than other types of interactins. The main study was looking at

obesity using the Framingham Heart Study Social Network. This seems like

a very famouse social network health related study, so I'm not going to

go into detail about that, but the bottom line is that they were able to

construct the social interactions from this study by digging through the

huge paper archive. They were able to look at friend, relative and

co-worker ties.

The main study was looking at about 5k individuals out of 12k, taken

from 1973 onwards.

Nice, node ssize is related to a person's weight!

There is clear clustering of obese nodes in the network, now is this

clustering random or structured?

Well, it's more clustered than random.

There are a couple of reasons why this might be the case. It could be

that obese people like each other, people might be susceptible to local

factors, or there might be some kind of peer pressure.

By looking at time evolution the hope is that they might be able to find

'patient 0' for the obesity epedmic. OK, video is coming up now ...!

OK, looking at people getting fatter all over america from 1972 onwards,

I'm going for a run later!

The effect is not centered in one location, but it seems that it's an

epidemic that had multiple starting points in the network.

Looking at the directionality of ties of friendship helps you make

inferences about causes. Wow, if you are friends with someone who is

friends with you, and they get obese, you have 300% greater chance to

gain weight. Stay friends with thin people!

It looks like much of this is driven by social norms.

They also have gwo data from the network, that is really cool.

They can convert location to wealth, and can take this into account when

looking at the evolution of the network.

This data is really really cool.

No drop off in effect with distance, it is really the social tie that is


They also looked at the effect of smoking, and were able to take this

into account.

So their working hyppothesis is that it might be the spread of behaviour

and habit, perticluarly shared behaviour, going runnning vs going for a


It might be the spread of an idea, the spread of what an acceptable body

size might be.

OK, that's pretty amazing, and you can tease a hugh amount of

information out of this study. Liklihood of quitting smoking, of how

that is effected by education, and friendship tie.

I have to say, there is not a lot of results that are amazingly

astonishing. They have food networks, like the bannana network and the

friend chicken network.

They are also looking at emotions. We know that emotions can spread

through groups, on diads. Could emotions spread hyper-diadically, and

over longer time frames?

There is strong clustering of happienss, and your happiness seems

coreelated with people who are outside of your direct social horizon.

Interestingly happiness does not spread in the workplace (I think that

was the point), but happy people have higher clustering and better

centrality in the network.

There seems to be a half life for catching happiness from your network,

this is about 6 months. There is also a strong local effect, you need

happy people to be within about two miles of you, and to be having happy

events happening to them every six months or so.

Ahh, you can look at smiling on facebook. Right, I gotta put up some

happy pictures on my profiles!

Ahh, thiness also spreads, but the reason they have been looking at

obesity is that this study is looking at the obesity epidemic. The

network shows you the magnification of the phenomena, not the cause or

origin of the phonomenon.

Interesting question, if you wanted to hire flight attendants who you

didn't want to gain weight, should you hire them based on the bmi of

their friends? Well, the answer is that in a workplace if a certain

behaviour begins to spread it is likely to have a network effect. The

flpiside is that you can use these network effects to more economic

effect by trying to promote certain behaviour through targeting core

groups in the workplace.

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