We explain the recent increases in obesity in the United States with a model involving falling food prices, endogenous social body weight norms, and heterogeneous human metabolism. Calibrating an analytical choice model to American women in the 30-to-60-year-old age bracket, we compare the predicted weight distributions to NHANES survey data spanning (intermittently) the years 1976 through 2000. The model, the first to describe explicitly complete weight distribution dynamics for this group, predicts average weights and obesity rates with considerable accuracy and captures a significant portion of the recent growth in upper-quantile weights.