In this week's Letters to Nature, Max Wolf and colleagues present a plausible model for the evolution of 'personality' in animals. The mechanism they propose links the propensity of an animal to take risks (such as being aggressive or exploring new territory) to its future potential for reproduction. Specifically, the authors theorize that animals who have more to lose (i.e. have a higher probability of successful and/or greater reproduction in the future) will behave in a less risky manner than animals who have little, or nothing to lose.