Thecausal relationships in a two variable system are examined, using a simultaneous linear differential equation model. Emphasis is placed on the necessity for considering mutual interaction instead of looking for unidirectional causality. To demonstrate the usefulness of this approach an example is taken from the management science literature concerning the relationship between labour turnover and overtime. It is shown that, on certain assumptions, the data disconfirm a hypothesis previously thought to be confirmed. Certain informal observations made in the original paper were shown to receive confirmation from the data.