This paper develops a simple model in which individuals rationally determine their body-mass by choosing food ingestion and time spent exercising. We show that multiple equilibria in body-mass might exist due to two forces with opposite effects. Firstly, an increase in body-mass has a negative impact on current utility and therefore, slows down body-mass accumulation. Secondly, an increase in body-mass has negative impact on the mortality rate, and thus on the individual’s discount factor. This effect is associated with an individual’s “myopic” behavior as more weight is given to current utility flow compared with future utility flows. As a result, the impact on the mortality rate accelerates the accumulation of body-mass throughout an increase in food ingestion and less time allocated to exercise. Thus, some individuals might be willing to ingest less food and spent more time exercising if they place more value on the negative impact of body-mass on their current utility, while others individuals might decide to accelerate body-mass if they face lower discounted future utility flows. A second conclusion relies on the stability of the different equilibria, which assures persistence in body-mass and explains why radical treatments might be required to modify an individual’s weight.
