Obesity, imitation and equilibria

Judging by the cyclists on our roads and the new members at my gym, the Olympics has inspired Rutland's fattest women to take up exercise rather than the milfiest. If this is the case - and what could possibly be wrong with anecdotal evidence? - it raises an issue about the fragility of equilibria in networks.

You might think that cost-benefit thinking predicts that fatter folk would be more likely to take up exercise, because the benefits of moving from (say) a BMI of 30 to one of 25 are greater than those of going from 25 to 20 whilst the costs of doing so are smaller; the first few pounds fall off very easily.

Such thinking, however, fails to explain why folk get fat in the first place. One reason why they do, economists believe, lies in peer (pdf) effects. If our friends and neighbours are chubby, we are more likely to become so, because being surrounded by lardies increases our estimation of what represents a normal weight.

This is where the Olympics enters. Jess and Victoria have given women very salient role models. The comparator for body weight, for some people, has shifted from their fat friends to slimmer Olympians, which has inspired them to lose weight.

This is why I say equilibria are fragile. Our behaviour often imitates others. But which others? Changes in whom we imitiate can change behaviour very radically. Take just two examples:

- Riots. If one or two people follow the first person to cause trouble, a riot can spread - as subsequent people follow the group. But if they don't, the riot won't happen. It's thus possible that the choice of just one person can make the difference between there being a riot and not, as Mark Granovetter explained in his threshold model (pdf).  

- Excess volatility in stock markets. Sometimes, investors herd; they buy when others buy (pdf) and sell when others sell. When this happens, we get bubbles and crashes. The shift from one to the other can be triggered by a very small cue, which stops or reverses imitative behaviour. This was the story of the 1987 crash, and it might be story of Facebook now.

What we have in these cases is the same as we have for Rutland's women - a shift in equilibrium, from peaceful behaviour to rioting, from believing an asset is cheap to thinking it's dear, and from being fat to taking exercise.

But here's the key point. Although it's possible to explain such shifts after they have happened as the result of shifts in peer effects, information cascades and social learning, they are pretty much impossible to forecast in advance.As Jon Elster wrote:

Sometimes we can explain without being able to predict, and sometimes predict without being able to explain. True, in many cases one and the same theory will enable us to do both, but I believe that in the social sciences, this is the exception rather than rule. (Nuts and Bolts for the Social Sciences, p8)

The failure of the social sciences to make successful predictions, therefore, might reflect not a failure of science, but instead the fact that social phenomena are just so damned complex.

Sexist? I'm writing about fat women here simply because I've not noticed increased exercise among fat men.

Comments

[Alex]

Great post. I find it slightly sad though that you had to put your "sexist?" explanation at the end. The post is so clearly not sexist that it says something about the prevailing orthodoxy on gender issues that you felt you had to comment at all.

[Andrew]

Have you just discovered that human societies are non-linear chaotic systems or something?

Of course you can't predict them.

Anyway, fatness was not at an equilibrium in the first place.

[chris]

@ Andrew - I haven't just discovered it. But everyone who thinks economics (and the social sciences generally) can make forecasts clearly haven't discovered it. And that's a lot of people.

[Account Deleted]

How do you know they are Rutland's "fattest" women, as opposed to just ones who are fatter than those you usually see when you letch on the exercise bike? Have you conducted an exhaustive study, or at least one that made you break sweat?

Perhaps these new recruits are those who know they are fat (i.e. self-aware), acknowledge it's a problem (not deluded), and also believe (whether justified or not) that they can do something about it. As such, they are perhaps acting quite rationally.

The "milfiest" (I really can't see that neologism catching on) would only join your gym if a) they were also over-weight (which implies you are a chubby-chaser), or b) they were already fit (in both senses) and rationalised that the example of Louis Smith and Tom Daley would inspire more lush young dudes to visit said gym.

[Luke]

I am troubled that I find myself thinking "arse to elbow" before I get to the end of your posts. Having said that, I do share your concerns about chris's patronage of Rutland gyms. Even the word "Rutland" gets you thinking.

[Tom Addison]

MILFs don't go to the gym, they have personal trainers. However, you may be able to find some at health clubs and the like

[Anonymous]

I wonder whether the Olympics have inspired Eric Pickles to exercise.

[redpesto]

I'm writing about fat women here simply because I've not noticed increased exercise among fat men.

Plus the fact that neither Mo Farah nor Sir Chris Hoy were really being held up as role models, especially as men are rarely the subject of debates about body image, let alone obesity. How you get to economics from those examples would be interesting...

[BenSix]

The, er, mixed success of the Americans and Australians in imitating the physiques of Carl Lewis and Ian Thorpe in the 90s and the noughties doesn't bode well for the maintainance of these Rutland regimens.

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great post

[Milf Hunter]

Never thought I'd read the word "milfiest" on your blog, Chris! (Though I suppose it does have a certain charm). I shall do my bit to popularise the term...

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