Causality and obesity

The standard adage for complex systems as seen in biology and economics is that “correlation does not imply causation.”  The question then is how do you ever prove that something causes something. In the example of obesity, I stated in my New York Times interview that the obesity epidemic was caused by an increase in food availability.  What does that mean? If you strictly follow formal logic then this means that a) an increase in food supply will lead to an increase in obesity (i.e. modus ponens) and b) if there were no obesity epidemic then there would not have been an increase in food availability (i.e. modus tollens). It doesn’t mean that if there were not an increase in food availability then there would be no obesity epidemic.  This is where many people seem to be confused.  The obesity epidemic could have been caused by many things.  Some argue that it was a decline in physical activity. Some say that it is due to some unknown environmental agent. Some believe it is caused by an overconsumption of sugar and high fructose corn syrup. They could all be true and that still doesn’t mean that increased food supply was not a causal factor. Our validated model shows that if you feed the US population the extra food then there will be an  increase in body weight that more than compensates for the observed rise.  We have thus satisfied a) and thus I can claim that the obesity epidemic was caused by an increase in food supply.

Stating that obesity is a complex phenomenon that involves lots of different factors and that there cannot be a simple explanation is not an argument against my assertion. This is what I called hiding behind complexity. Yes, it is true that obesity is complex but that is not an argument for saying that food is not a causal factor. If you want to disprove my assertion then what you need to do is to find a country that does not have an obesity epidemic but did exhibit an increase in food supply that was sufficient to cause it. My plan is to do this by applying our model to other nations as soon as I am able to get ahold of data of body weights over time. This has proved more difficult than I expected. The US should be commended for having good easily accessible data. Another important point to consider is that even if increased food supply caused the obesity epidemic, this does not mean that reducing food supply will reverse it. There could be other effects that maintain it even in the absence of excess food.  As we all know, it’s complicated.

5 thoughts on “Causality and obesity

  1. Criticisms like those you are answering often have a poorly delineated mix of,

    (1) Causal explanations for the population statistics, and
    (2) Prescriptions for individuals already over weight

    (1) is about good policy while (2) is about remediating life habits that have gotten someone into a situation they want to change.

    If one’s concern is helping people lose weight, you probably don’t start with the explanation that there is on average too much food available. An overweight person won’t be able to do anything about “too much food everywhere” in time to positively change their own situation.

    Instead they should probably focus on how eating certain kinds of food makes them feel more satisfied or gives them a feeling of more energy, etc. They should probably worry about exercising rather than how much exercise the population gets on average–the kinds of things the “calorie is not a calorie” people know a lot about.

    I am for both conversations, but I think we are detracting from both conversations by mixing them. If we do, we will have both overweight people convinced society is responsible for their individual problems (helplessness), and policies based on a distorted understanding of the dynamics of the system.


  2. I respectfully disagree with Dr. Skippy’s assertion that, if you are interested in helping people lose weight, you shouldn’t start with explaining that there is too much food available. In my opinion, that is exactly what we need to do. Most people seriously underestimate the daily calories they expend and overestimate the daily calories they need. Raising awareness of food (calorie) requirements based on activity level is a critical first step in resetting someone’s internal sense of how much food is too much. I agree that most Americans (ie overweight Americans) can’t do anything about over availability of food – we can, however, do something about over eating what is available. To me, that is the crux of the matter – how do we convince people to make the difficult personal choice to eat less in a country that is actively encouraging us to eat more – especially when “fast food” is so cheap. While over abudance of cheap food may not be the only reason for the obesity epidemic it is most definitely a signficant causal factor. Now our challenge is to reset personal and cultural norms around eating – in this case, bigger isn’t better – which has never been the American dogma.


  3. I wonder if it is empirically possible to increase the food supply while keeping obesity rate constant. I guess ‘obviously’ if the increased food went to underweight people or those below the obesity threshold this could be done. (eg internationally it might be easy given malnutrition rates outside usa).

    the issue is distribution.


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