The analysis of migration flows is naturally concerned with the relationships between migration and such variables as incomes or income differentials, employment opportunities, and distance. But before this can proceed, a prior question needs to be answered: whether to study gross or net migration flows. It is commonplace to observe that gross migration flows are typically far greater than the corresponding net flows. Little explanation is provided for this observation, and not much guidance is therefore available in the choice of the dependent variable. The choice is generally based on convenience, for example, the availability of data, or on rather arbitrary inclinations of the investigator.' This paper examines at the theoretical level the distinction among three