We model happiness as a measurement tool used to rank alternative actions. Evolution favors a happiness function that measures the individual’s success in relative terms. The optimal function, in particular, is based on a time-varying reference point –or performance benchmark –that is updated over time in a statistically optimal way in order to match the individual’s potential. Habits and peer comparisons arise as special cases of such updating process. This updating also results in a volatile level of happiness that continuously (...) reverts to its long-term mean. Throughout, we draw a parallel with a problem of optimal incentives, which allows us to apply statistical insights from agency theory to the study of happiness. (shrink)