Mass migration refers to the migration of large groups of people from one geographical area to another. Mass migration is distinguished from individual or small scale migration; and also from seasonal migration, which may occur on a regular basis.
A specific mass migration that is seen as especially influential to the course of history may be referred to as a 'great migration'. For example, great migrations include the Barbarian Invasions during the Roman Empire, the Great Migration from England of the 1630s, the California Gold Rush from 1848–1850, the Great Migration of African Americans from the rural American south to the industrial north during 1920–1950, and the The Great Oromo Migrations of Oromo tribes during the 15th and 16th centuries in the Horn of Africa.
Historians often identify an 'age of mass migration' occurring from c. 1850 to 1914 (sometimes 1940), in which long distance migration occurred at an unprecedented and exceptionally high rate.
There were three factors that led to the 'age of mass migration'. First, cost of migration decreased dramatically. Second, benefits of migration rise (returns on migration was higher in the United States than in other countries). Third, open border regimes. 'Age of mass migration' usually refers to the voluntary transatlantic migration of European peasants and labourers to the Americas. Immigration from Europe accounted for about 40% of total United States population growth in the late 19th century. However, it has been argued that the term should include other mass migrations that occurred in the same period, since similar large numbers of people migrated long distances within the continent of Asia, most notably during the Pakistan Movement and subsequent partition of India in 1947. UNHCR estimates 14 million Hindus, Sikhs and Muslims were displaced during the partition, the largest mass migration in human history. During open border regimes, immigrants attracted by falling costs of migration and higher wages in the U.S. Migrant selection varied over time and across sending country, depending on relative wage premium in U.S. for high/low skilled and cost of migration. In the late 20th century, migrants converge upon native-born in labor market but never fully catch up.
Mass migration may also be forced migration, such as the Atlantic slave trade. Religious persecution mass migrations, such as the biblical Exodus and migration through the upper Himalayan route from the east by Purohits of Aryan and non- Aryan descent. Similarly, mass migrations may take place in the form of deportation. For example, Japanese internment in the United States and imprisonment in Nazi concentration camps during World War II, deportations to Gulag camps in the Soviet Union, and coolie-labour in Southeast Asia and the Caribbean.
On the 15th of August 1947, the partition of British India caused the movement of 18 million people. This caused both religious and civil tensions between Hindus and Muslims. This resulted in the highest casualty rate for one migration according to the Guinness Book of World Records 2014. One million people were killed and 12 million became homeless.