BMI: the GDP of health

I've railed many times about the inadequacies, and worse, of using Gross Domestic Product as the de facto indicator of social well-being. Which it has become, in the absence of any serious thinking about the social goals we want to achieve and the priorities we give them.

A similar phenomenon seems to have occurred in the world of health, where the body-mass index (BMI) is a long-used measure of obesity. BMI is body mass in kilograms divided by the square of the body height in metres. Apparently, as measured by BMI, obesity in the US plateaued around the year 2004 at 35 percent of the population. But, just as GDP ignores such things as leisure time, state of the environment, income distribution; and counts useless or anti-social economic activities as positives, so too does BMI have its flaws:

It does not consider distribution of fat, type of fat, muscle tone, age, sex, or even big bones. In spite of these flaws, healthcare professionals continue to use BMI as a guideline. A BMI of 20-25 is considered ‘normal’, and anyone larger or smaller is automatically counselled to achieve a healthier weight. .... Obesity is generally understood as a risk factor for heart disease, stroke, cancer, and diabetes, as well as an increase in overall mortality. Excess body weight also increases stress on joints and internal organs. Given these concerns, it’s easy to understand why so many people have celebrated the plateau in BMI. Unfortunately...BMI may not be the best measure of obesity. Your percentage of body fat and waist or abdominal circumference are far more reliable personal indicators of health outcomes than BMI. For example, central obesity, measured by waist circumference, is a more accurate determinant of personal risk and shows an even stronger correlation with poor health outcomes. Caroline Weinberg, Fat but fit?,  'Aeon', 27 August 

There is no clarity about goals. If our intention is to improve the health of people, why not target indicators of people's health, instead of easy-to-calculate but flawed measures such as BMI? If our intention is to prevent disasters arising from adverse climatic events, why not target for reduction the negative impacts of such climatic events, instead of atmospheric carbon dioxide levels or temperatures recorded in weather stations? If our intention is to improve social well-being, why not target indicators of social well-being instead of those economic activities captured by GDP?

BMI, greenhouse gas emissions, GDP per capita: they might answer certain specific questions, but their use as policy instruments is inadequate at best, dangerous at worst. Policymakers need some humility here: neither they nor their advisors know the best ways of improving health, preventing climatic disasters or improving social well-being. Even if they did, at one particular point in space and time, circumstances vary with region and our knowledge grows with time. No government, no single organization can hope to use fossilised knowledge on a one-size-fits-all basis and achieve meaningful results. Instead, policymakers should set broad goals, and let a motivated coalition of interests explore diverse, adaptive approaches aimed at achieving society's broad social and environmental goals. Government can set these goals and, indeed, it is probably the organization best suited to doing so. And, if it concentrated on that, it would come up with a better array of target outcomes than the flawed indicators it now uses, whether explicitly or not.

Government can also raise the revenue to reward the people who achieve these outcomes. But it has no business dictating how they shall be achieved, nor who shall achieve them. We need diverse, adaptive approaches, and those are exactly the sort of approaches that government discourages.