This suggests that migration flows acquire a measure of stability and structure over space and time, allowing for the identification of stable international migration systems, which are characterized by relatively intense exchanges of goods, capital, and people between certain countries and less intense exchanges between others. An international migration system generally includes a core receiving region, which may be a country or group of countries, and a set of specific sending countries linked to it by unusually large flows of immigrants (Massey et al. (1993) Pop. & Dev. Rev. 19, 3, 431).