An article today at Slate discussing Body Mass Index (BMI) and how it's actually a poor measure of obesity -- and why it's used so commonly to diagnose said obesity. Rather like many population measures -- time at menarche or onset of puberty, age at death, literacy, whatnot -- the overall averages won't tell you what's going on with any given individual. But alas, the almight BMI is convenient, so it's commonly used.
This reminds of the (related) phenomenon, back during the Atkins Diet craze, when I read an article talking about it and its actual relationship to healthy eating and longevity (questionable at best, negative at worst). It quoted some scientists as arguing that at least Atkins brought people's attention to dietary questions; in my experience, however, it was just another fad diet, and people adhered to it specifically. There was little spillover into more generalized health concerns, exercise, etc., just cutting down on carbs -- which as many scientists in that article and out pointed out, is not terribly related to long-term health, as the fats associated with much of the protein people ate instead did little for their cardiovascular health, and they missed out on the nutrients and micronutrients chiefly gleaned from eating complex carbs (i.e. the anti-Wonder Bread).
Anyway, like BMI or even worse our focus mainly on weight, the article expressed a point of view that I like to summarize as, "Things are so bad, we need a solution -- even if it's a wrong one that won't work."