Here's an example: many teenagers in Mexico, a developing country with an average income of US$6,000, are bloated as never before, even as the ranks of the Mexican poor swell. Individual wealth doesn't explain why the children of some families are more obese than others: the crucial factor turns out not to be income, but proximity to the US border. The closer a Mexican family lives to its northern neighbours and to their sugar- and fat-rich processed food habits, the more overweight the family's children are likely to be. That geography matters so much rather overturns the idea that personal choice is the key to preventing obesity or, by the same token, preventing hunger.