By Charles L. Baum

Mayor Nutter's proposed soda tax didn't end up going anywhere, but it did contribute to an unfortunate myth: the notion that any class of food or beverage is particularly fattening.

That was the stated motivation for Nutter's proposal. A ranking city health official declared that "there's good evidence for sugary drinks' being a major culprit in the increase in obesity." And public-health activists claimed that soft drinks contribute disproportionately to the government's obesity-related costs.

However, a new study I conducted with Lehigh University's Shin-Yi Chou suggests that the dynamics of obesity are far more complex than the soda critics suggest.

Using government body mass index data spanning a 27-year period, we analyzed multiple potential factors in the recent rise in American obesity, including food prices, physical activity at work, restaurant prevalence, urbanization, employment, and cigarette smoking.

Based on the current conventional wisdom, you might guess that food prices or restaurant prevalence would affect obesity most. But among the variables we studied, the most significant factor in BMI increases was the decline in smoking. This makes sense: Cigarettes are an appetite inhibitor.

This single greatest factor, however, still accounted for a very small share of the rise in obesity, about 2 percent.

Obesity is also linked to urban sprawl, not surprisingly: More driving means less walking. Occupational fitness is another factor: More time sitting at a desk means fewer calories burned.

Neither of these plays a large role either, though. Urban sprawl accounted for only 0.7 percent of the rise in BMI, occupational fitness 0.5 percent. And fast-food prices, grocery prices, and restaurant prevalence were all statistically insignificant.

In other words, there's no one or two central causes of rising obesity.

Philadelphia officials aren't wrong to be concerned about obesity. The obesity rate citywide is 40 percent, and 17 percent among high school students, according to Temple University.

But our results indicate that antiobesity public policies are generally of limited utility. Besides soda, activists have accused corner stores and fast food of contributing to a "toxic food environment." Yet fast-food prices, grocery prices, and restaurant prevalence all registered statistically insignificant effects in our study.

Policies seeking to reconstruct the urban landscape by banning new fast-food restaurants (as has been done in south Los Angeles) or giving tax breaks to supermarkets therefore may not have much of an effect on weight gain.

Governments can of course plow forward with taxes, zoning, and other often untested ways of battling obesity. But public policy is an instrument that's too blunt for this task. There are simply too many factors and individual choices; after all, any food or drink with calories can fatten people if consumed in excess.

Instilling personal responsibility and expanding nutrition and fitness education are more likely to have an effect. People gain weight when they consume more calories than they burn. And many different lifestyle decisions can lead to weight gain, just as there are many different approaches to weight loss. Some people may have to eat less; others may have to run up and down the steps of the Art Museum.

No public-policy regimen can effectively target obesity unless it literally punishes people for being fat. And who wants to weigh in at the nearest post office?

Charles L. Baum is a professor of economics at Middle Tennessee State University. He can be reached at charliebaumphd@gmail.com.