Several geographic theories describe the reasons for the migration of people on a local, regional, or continental level. Since 1885, when E. Ravenstein pointed out the importance of distance as a determinant of migration, geographers and immigration researchers have studied friction of distance, that is, the extent to which increasing amounts of distance acts as a barrier to migration. Also known as distance decay, the theory of friction of distance states that as the distance between two locations increases, there will be a progressively smaller number of people who migrate from one location to another. The friction is mitigated by several factors that increase distance tolerance, including improving travel conditions and communication. Ultimately, migration flows occur when imbalances between the incentives and deterrents to migration exist.
In 1946, George Zipf examined patterns of migration from one city to another. He theorized that the friction of distance pertained to the degree of cost, effort, and incurred hardship that a migrant suffers when relocating, which increases with increasing distance. For migration to occur, this trouble of migration must be outweighed by factors that drive the migrant from his current address or pull him toward the new location. Push factors may include religious or political persecution, lack of economic opportunities, or deleterious environmental conditions, such as drought. Pull factors may include expanded freedom, job availability, or opportunities.
Friction of distance may also include other components than absolute distance. Intervening obstacles contribute to the perception of distance in the mind of an immigrant. For example, restrictive immigration laws, negative societal views of immigrants, and hazardous travel conditions increased the perceived friction of distance. On the other hand, if an immigrant has relatives or friends who have relocated to the new location ahead of him, the perceived friction of distance lessens. The subsequent migration of family and friends that follows an initial movement by the first immigrant to a city is called chain migration, reflecting the reduction of obstacles for later immigrants by the flow of information back to the original location from the trail blazer.
Immigration researchers have also discovered other factors that drive immigration flows. For example, young people are more likely to migrate than older people, even when the push factors affect them equally. Males are more likely to undertake immigration than females. Most immigration occurs toward larger towns or cities with more developed economies in less developed countries, but urban-to-rural flows may occur in more developed countries. Immigrants are also more powerfully drawn to recipient locations that are well-known or believed to be quite large.