While scientists from many disciplines have contributed to the understanding of specific instances of human migration, there is need for a more general theory of voluntary migration. This paper presents and tests (using data on Irish migration) an economic model which could serve as the core of a multidisciplinary general theory. The economic model postulates earnings differentials between countries and regions as the proximate cause of the generation of a stock of migrants. As a result of institutional barriers, personal inertia, incomplete knowledge of earnings differentials, etc., not all of the existing stock becomes a flow of migrants in any year. Application of this model to migration between different countries or between the same countries at different time periods can be of value to scientists in many disciplines in isolating the underlying causes for differing rates of awareness of and response to earnings differentials by potential migrants.