A joke about economists can also be applied to theorists: ‘A theorist is someone who cannot see something working in practice without asking whether it would work in theory’. Less strange but more problematic is the case in which something works in theory but not in practice. Unfortunately for the theorist, then, the old rule applies: ‘When theory conflicts with empirical research, the problem usually lies with the theory’ (Marwell and Oliver, 1993: 39). In the case of the theory of rational choice we can say that it is very popular, but that its large claims have been subject to severe criticism. Especially in the fields of political science and sociology, rational choice is often interpreted as a product of ‘economic imperialism', not as an external inspiration. The essence of the critique consists of the simple but crucial observation: human behaviour is not rational. At least not always.