The volume and nature of international migratory movements continue to undergo rapid changes. Illegal or undocumented migration and refugee movements have gained particular importance; labour migration of considerable magnitude occurs in all regions. The principal root cause of this large scale contemporary migration is to be found in the disparity in economic development, employment opportunities and living standards between countries at different stages of economic and social development. Natural disasters and political crises are among other principal causes of shifts in migration patterns and the increase in the volume of overall international population movements; though the motivation for most international migration is higher wages. Historically, some migration may have been directly related to population pressure, but today wage differences are the main driving force.