Major depressive disorder (MDD) is one of the most prevalent mental illnesses worldwide. Existing evidence suggests that it has both economic causes and consequences, such as unemployment. However, depression has not received significant attention in the economics literature. In this paper, we present a simple model which predicts the core symptoms of depression from economic primitives, i.e. beliefs. Specifically, we show that when exogenous shocks cause an agent to have pessimistic beliefs about the returns to her effort, this agent will exhibit depressive symptoms such as undereating or overeating, insomnia or hypersomnia, and a decrease in labor supply. When these effects are strong enough, they can generate a poverty trap. We present descriptive evidence that illustrates the predicted relationships.