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POPSWhen Moms Work, Kids Get Fat Despite all the concerns about childhood obesity, most of the fat people in the world are old enough to look after themselves. So, what's going on? Here, traditional economics seems to offer a perfectly straightforward pair of explanations. First, the cost of exercise has risen: Most of us used to be paid to burn off calories in physically demanding jobs, after all. It is hard to undercut a form of exercise that pays you, and modern gyms haven't tried.