Two-thirds of Americans are either obese or overweight. Why?
You probably think you know, because we hear a lot of explanations for the obesity crisis. It's to do with fast food sedentary lifestyles, food "deserts," not exercising enough, and a host of other reasons, from poverty to the suburban environments that discourage walking.
Roland Sturm, an economist at the Rand Corporation, thinks they're all wrong. Or, at least, that none of these explanations have much power beside an historical perspective. The most important factor, he argues, isn't how much we're sitting or pumping iron. It's how the price of food has fallen over time relative to other goods.
That may sound like an academic sort of statement, but, of course, understanding obesity trends is key to doing something about it. If access to healthy food isn't really a key reason for widening waistlines, we shouldn't be focusing on this as the solution.
In a paper written with Ruopeng An, a researcher at the University of Illinois, Sturm notes that Americans have been getting heftier since the 1950s (not just in the last decade or two) and that the increases have been consistent across demographic groups and regions. So, while black women now have higher BMI (body mass index) than Hispanic women, increases in BMI have increased together (see the chart). All women have been getting heavier, on average. Similarly, a resident of Colorado is likely to have a lower BMI than someone from Mississippi. But the rate of BMI increase has been similar across all states, irrespective of diet or exercise habits.
What this tells Sturm is that differences between people are less important than the changes that have happened to all of us. "The striking finding is the similarity of increases in BMI across groups," he says. "Targeting selected sociodemographic groups might help to reduce disparities, a laudable goal itself, but it would seem very unlikely to address the much bigger effects that have occurred over time."
The big cross-society change, Sturm finds, is the price of food. In the 1930s, we spent one quarter of disposable income on feeding ourselves. In the 1950s, it was about one fifth. Now, it's about a tenth. "The obesity epidemic has been fueled by historically low food prices relative to income," the paper says. "Americans are spending a smaller share of their income (or corresponding amount of effort) on food than any other society in history or anywhere else in the world, yet get more for it." (In Kenya or Pakistan, they still spend up to half of income, for example).
Compared with the earlier decades, we're actually exercising more and eating more fresh vegetables and fruit, Sturm says. Which should suggest that we'd be getting thinner—except we're not. We're getting fatter because we eat too much food because it's cheap—particularly "discretionary" food (cookies, sugary drinks, salty snacks) that we don't need.
Rather than promoting positive responses to obesity, like more fruit, Sturm suggests strategies to reduce caloric intake, though he doesn't think current public policy ideas will have much impact. Taxes, for example, might raise the price of bad foods. But, to be effective, they might have to reach very high levels (to correct for historic food price changes).
Ultimately, Sturm writes in an email, social pressure may be more effective. "The big drop [in tobacco use] would not have happened without changing social norms," he says. "Fifty years ago, when people came to visit, they were offered cigarettes, now that would seem to be rather inappropriate outside a few small groups."