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In recent decades, the prevalence of obesity in America has increased dramatically. Though it has attracted less attention, the demographic composition of the American population also changed during this period. We decompose the increase in the average body mass index of the American population over 30 years and show that demographic changes explain a statistically significant but economically marginal amount of the change. Instead, the rise in average obesity is best explained by increases in BMI within demographic groups. Furthermore, our results indicate that groups' experiences have been heterogeneous with younger women experiencing especially large gains in weight. We uncover some evidence consistent with the hypothesis that this can be at least partially attributed to increased labor force participation.