What Drives Obesity? An Economist Takedown of The Economist

(iStockphoto)

Is higher obesity due to the rise in driving? Perhaps. It’s an intriguing hypothesis. But our friends at The Economist should know better than to report nonsensical correlations. Here’s the evidence they cite (drawn from this entirely unconvincing research paper published in Transport Policy):

Looks impressive, right?  (Well, apart from putting the explanatory variable on the vertical axis.) But before concluding that there’s anything here, let’s try a different variable, instead—my age:

Unlike the authors of the original paper, I didn’t even need to fiddle with the lag structure to get such a good fit, nor test alternative definitions of the variable. In fact, my variable fits even better than vehicle miles traveled.

Okay, I’m not arguing that my aging is causing higher obesity. Rather, when you see a variable that follows a simple trend, almost any other trending variable will fit it: miles driven, my age, the Canadian population, total deaths, food prices, cumulative rainfall, whatever.

Sure, The Economist offered the usual caveat that “correlation does not equal causation.” But this is so completely unconvincing as to warrant a different warning: “Not persuasive enough that you should bother reading this article.” I’m not saying the relationship doesn’t exist, simply that it makes more sense to highlight more persuasive research on this question.

P.S.: How cool is Stata’s “Economist” scheme? It lets stat-nerds like me replicate magazine-quality art.

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  1. Researcher says:

    I also think it is important to consider indirect causal links between policies and health outcomes (such as politics and culture), not just the direct ones.

    The authors don’t get into this in their study, but it is important consider the factors of social pressures, advertising, funding priorities, redistribution of wealth, etc., and their impact on obesity, vis a vis driving’s direct relationship to those factors.

    For example, as people drive more, they may be less likely to support funding for mass transit even if the need for mass transit is dramatically rising.

    The authors of the study point out its limitations very clearly, and their point is broader than one correlation. I would suggest reading the original article more closely before tossing it aside just because The Economist didn’t do a good summary.

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  2. Amy Alkon says:

    Actually, Taubes lays out evidence as well that a calorie is not a calorie. Doctor Michael Eades also lays this out on his blog, proteinpower.com/drmike — about how the body uses carb calories very efficiently but blows off non-carb ones (he uses the term “calorie wasting”).

    Also, eating animal fat and protein is satiating. On my morning omelet and bacon, I can go till about 3 pm without eating and not feel uncomfortable.

    Taubes also wrote a piece, supported by evidence, for New York Magazine, which shows exercise does not make you thinner; it makes you hungrier. People who don’t know good scientific sources, and are simply posting scientific hearsay, always amaze me. Because you read it on CNN.com doesn’t make it so.

    I use science in my work, and have learned to read studies, and trust very few people to interpret them. There are maybe five people in the world I trust. Dr. Michael Eades and Gary Taubes are two of them. In fact, Taubes is the single most skeptical human being I have ever encountered, and the most nervous about seeing that he is putting out evidence-based science.

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    • James says:

      “Taubes also wrote a piece… which shows exercise does not make you thinner; it makes you hungrier.”

      Ignoring much contrary evidence, including decades of experience from probably millions of individuals who’ve gone through military boot camp, or the many thousands who’ve ever followed a serious body-building program, or got seriously into bike touring, or…

      Of course we have to adjust our definition of “thinner” a bit, so as to account for increased muscle mass/decreased body fat percentage, but it does work.

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      • thehumble1 says:

        I believe the conclusions are that exercise makes us hungrier initially but actually decreases overall food consumption by making us more generally sated. This is also why you can change someone’s weight just by putting them on antidepressants, which may or may not lead to changes in activity but definitely does decrease overall caloric consumption. Heavy workouts burn almost no calories compared to the additional calories needed to complete those workouts, but people who work out a lot tend to be thinner. With straight math, you find that they don’t burn enough calories during the workouts to make up for the loss of weight (10 lbs = 35,000 calories = 282 miles @124 kCal/mile). If you jogged a 5k, 3 times per week it would take you about 8 months to lose 10lbs and that’s if you ate NO additional food after all of those miles jogged. What we see is that people drop 10 lbs in 2-3 weeks after starting something like 5k 3x/week. Somebody please correct my math or logic though, because to me this seems to highlight an issue with exercise: it can’t be the calories we burn on the treadmill that is causing the change. My understanding is that it changes our metabolism so that we actually eat less, are more intensely hungry for shorter periods of time, and are more satisfied and less anxious in general which must help in some other ways.

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    • Cliff says:

      Amy,

      Aren’t you a relationship advice columnist? It’s not like you edit a scientific journal. There is copious evidence that exercise does lead to weight loss.

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      • Enter your name says:

        There is also plenty of research showing that if you burn 200 calories on the treadmill, and then “reward” yourself with 400 calories of treats because you were such a good boy to get on the treadmill, then you will gain weight.

        Exercise causes weight loss only if you control for diet. If you increase both exercise and calorie intake, then weight loss is not in your future.

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  3. AaronS says:

    Well, you could just as convincingly argue that obesity is linked to the rise in computer usage. Oh, and inversely related to the use of mule-drawn plows…and black and white televisions…and non-homogenized milk.

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    • thehumble1 says:

      I’d really like to think that the use of multi-colored televisions has been the culprit for weight gain, but I’m more likely to think that it’s caused by trending increases in the minimum salary (in 2011 US$) of NFL draftees which shows good regression as well.

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  4. Sigivald says:

    Or classics along these lines – Global warming vs. Pirate population.

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  5. mjay says:

    Clearly driving causes ageing.

    Can I have my economics/sociobiology/evolutionary psychology PhD now?

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    • Chris S. says:

      Not yet.

      Flesh it out to about 45 pages, add some cool charts, and then get your advisor to sign off. That should do it.

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  6. Daniel says:

    on the ps note: I did a presentation completely with the economist scheme, and afterwards one of the audience members asked if I was doing soft advertising for the economist by using all their graphs. It took me a bit to convince him that I was only using the color scheme and not graphs actually produced by the magazine.

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  7. Joe says:

    Stata is indeed fantastic, though the graphics take some serious learning to master.

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  8. anonymous says:

    It is far too easy for a trained applied econometrician to point out a spurious correlation reported in the media — even in a highly regarded publication such as the Economist. If we are going to criticize others’ research we ought to at least provide some recommendations for improvements. How would one go about better estimating this potential relationship, rather than just showing its spurious nature?

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