To Megan McArdle's "thinking thin" post about the causes of obesity, the relevant question for policy-makers is not whether there is a mono-causal explanation for obesity, it is whether policy-makers can and should do something about it. If everyone responded to the pressures of (a) a corn diet (b) TV advertising (c) the ubiquity of fat and sugary foods (d) the information disseminated by the government and the diet industry (e) technological enabling of a sedentary lifestyle in the same way, it is relatively easy to answer the question. If you tend to blame individuals for their choices, then your answer will be no. But the crucial fact is that obesity does not treat everyone equally. It discriminates according to status, class and geography. And its negative externalities are absorbed by these vulnerable populations. And in children, being overweight is increasingly become the default. Unless someone intervenes, if you go with the flow, if you live in a vulnerable population, you're going to be quite vulnerable to an obesogenic lifestyle. This debate isn't about government dictating lifestyle choices to adults. It's about whether changing policy can reduce obesity among children.