As for economic effects, research suggests that migration is beneficial both to the receiving and sending countries. Research, with few exceptions, finds that immigration on average has positive economic effects on the native population, but is mixed as to whether low-skilled immigration adversely affects low-skilled natives. Studies show that the elimination of barriers to migration would have profound effects on world GDP, with estimates of gains ranging between 67 and 147 percent. Development economists argue that reducing barriers to labor mobility between developing countries and developed countries would be one of the most efficient tools of poverty reduction.
The academic literature provides mixed findings for the relationship between immigration and crime worldwide, but finds for the United States that immigration either has no impact on the crime rate or that it reduces the crime rate. Research shows that country of origin matters for speed and depth of immigrant assimilation, but that there is considerable assimilation overall for both first- and second-generation immigrants.
Many animals have migrated across evolutionary history (not including seasonal bird migration), including pre-humans. Human migration started with the migration out of Africa into the Middle East, and then to Asia, Australia, Europe, Russia, and the Americas. This is discussed in the article pre-modern human migration.
When people cross national borders during their migration, they are called migrants or immigrants (from Latin: migrare, wanderer) from the perspective of the country which they enter. From the perspective of the country which they leave, they are called emigrant or outmigrant. Sociology designates immigration usually as migration (as well as emigration accordingly outward migration).