It's long been evident that countries tend to gain weight as they gain wealth: The world's industrialized nations have much higher obesity rates than the developing world, and the problem is getting worse. It's also not surprising -- or new -- that obesity would also track with the popularity of fast-food restaurants, given the high calorie loads they tend to carry. But a new study published in the February bulletin of the World Health Organization adds another causal factor: deregulation. As in, the freer your market, the fatter your population.

The authors first solidify the relationship between fast-food consumption and obesity, controlling for other factors, such as general macroeconomic growth. The faster the number of annual fast-food transactions per capita grows, the faster the average body mass index levels rise:

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The researchers then turn to the question of rules. Rather than focusing narrowly on laws surrounding restaurants -- such as soda taxes, mandatory calorie counts on menus or bans on unhealthy ingredients like trans fats -- the authors use the Heritage Foundation's Index of Economic Freedom, which measures a broad swath of factors including trade, government spending, "labor freedom" and property rights. Again controlling for other confounding factors, the authors find a strong positive correlation.

Can that kind of generalized economic environment really have a discernible effect on weight and fast-food consumption? The authors say yes: "One possibility is that indiscriminate market deregulation favours global food chains at the expense of smaller farmers and local food systems," they hypothesize.

Of course, the vagueness of the indicator used in the study means it's difficult to proscribe that governments should impose economic controls with the goal of reducing obesity. But using specific policy tools might become more important as economies liberalize.

More and less free countries, according to the Heritage Foundation.