The neuromodulator dopamine has a well-established role in learning, but its role in decision making is less well characterized. By boosting dopamine levels as subjects made economic decisions, we show that subjects are increasingly likely to take risks when options include potential gains but not losses. Aging, which is associated with a gradual decline in the dopamine system, has the opposite effect on behaviour. We introduce a parametric decision model that explains these effects as a change in Pavlovian approach behaviour. These results support a specific role for dopamine in decision making that may explain some changes in economic preferences over the lifespan.