Abstract
This paper presents a dynamic theory of body weight and develops its implications. We argue that technological change has induced weight growth by making home- and market-production more sedentary and by lowering food prices through agricultural innovation. In addition, we illustrate that, while exercise and food intake are complements, reductions in exercise will always raise optimal body weight, as will increases in food intake. We also characterize how body weight varies with income, both within a country, and across countries. Within a country, income may have an inverted U-shaped relationship with body weight, due to the offsetting effects of the demand for food, and the demand for an ideal body weight. This can have important implications for the body weight impacts of public transfer programs. Across countries, however, mean weight is likely to be higher in richer countries. Finally, we present descriptive empirical evidence that illustrates the inverted U-shaped relationship between body weight and income in US males, and suggests the importance of secular trends in weight gain, which are consistent with the impacts of broad-based technological changes.