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paper-machine comments on Dunbar's Function - Less Wrong

27 Post author: Eliezer_Yudkowsky 31 December 2008 02:26AM

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Comment author: [deleted] 07 June 2012 12:40:53PM *  9 points [-]

BMI is a ridiculous measure of obesity as far as I'm concerned. I know this doesn't directly respond to your comment, but I'm listing my main arguments here for posterity.

For anecdotal context I'm about ten or fifteen kilograms above the weight I was when I was in secondary school (i.e., fully-grown), ate a (modulo American (hi John_Maxwell_IV!)) healthy diet, and led an active lifestyle. I consider this a reasonably good approximation to my "healthy weight", but it corresponds to a BMI of 27.3. For philosophical context I'm not a "fat apologist" either; it is painfully obvious that obesity causes higher morbidity. It should go without saying that higher morbidity is (ceteris paribus) bad.

In no particular order:

  • The units are absolutely meaningless. If humans are to a zeroth approximation cylinders with comparable aspect ratios and average density, then weight is proportional to volume, which is proportional to height cubed, but BMI is the ratio of weight with height squared. NB: This is only a zeroth approximation; tall people tend to have different aspect ratios, and the power law approximation is probably better served by an exponent somewhere between two and three. See MacKay for more details.

  • BMI gives less information than intuition on extreme cases: army1987's grandmother is indistinguishable from a 2000-era bodybuilder. The classical response to this objection is that BMI needs to be augmented with other statistics (typically given ad-hoc: waist-to-height ratio, waist circumference, body fat percentage, and etc.), but this is not how BMI is used "in the wild." I've observed several online cases (yes, this is weak evidence here) of both 25-30's becoming obsessed with getting down to 20-25 (even at the expense of muscle), and on the other end 17-20's becoming obsessed with not going over 25 (even through gaining muscle).

  • Even if BMI alone doesn't help us classify the ~35+ and the ~20- folks, it is possible to argue that it is useful for the people in between. At this point Goodhart's law kicks in: the "easiest" (for some value of ease) way to lower your BMI is to get a liposuction (or more absurdly, an amputation), but that has little effect on overall health. Less absurdly, I claim that body composition is just too varied and complicated to be reasonably treated with a single statistic.

  • There is a good amount of fake accuracy in reporting BMI to three significant figures, as mass in kilograms is usually only accurate to 1.2-1.5 places. Even healthy people gain and/or lose two or three kilograms over the course of a year.

  • For obesity diagnosis (i.e., in a medical setting), BMI is easily replaced with, in decreasing order of accuracy and cost, 1) displacement measurement of body fat percentage, 2) electrical measurement of body fat percentage (despite being horrifically lossy), 3) qualitative visual assessment of body fat percentage.

  • For overall health (i.e., in a non-medical setting), BMI is less easily replaced with body fat percentage. Setting aside akrasia, there's a great deal of psychological baggage that tends to mind-kill one's ability to judge one's own weight. To make matters worse, my own criteria (weight maintained after puberty in an active lifestyle) is depressingly ineffective in light of widespread childhood obesity. I admit I don't yet have a good solution to this problem, but I claim that BMI obsession is overall more harmful than whatever lay purpose it serves.

Comment author: [deleted] 07 June 2012 01:10:27PM 1 point [-]

At this point Goodhart's law kicks in

Do people have any incentive to be interested in the number on the scales itself, regardless of its effects on health and looks?

Comment author: [deleted] 07 June 2012 01:22:47PM *  4 points [-]

I don't know for certain, but it seems to me that a common failure mode for people struggling with weight is paying too much concern to various statistics, invented or not.

Disclaimer: I don't believe the following is good general advice. I don't have any theory backing it up.

At this point, in my own current work, the most useful thing I've implemented is applying the nameless virtue to this. If I want to look better, then I need to have goals that involve looking better; if I want more endurance, I need to have goals that involve running harder and longer, and etc. It seems trivial but I don't see very much of it in the fitness community.