Human migration is a complex behavior with many causes, but at its root it usually occurs because people believe they are leaving a bad situation for a better one. War, poverty and lack of opportunity are common causes of migration.
Scholars debate about the primacy of individual, family or societal causes when explaining human migration. However, migration is probably due to a combination of all three factors. Researchers look at a combination of push factors, or things that drive migrants from their home countries, and pull factors, those factors that lead them to seek a new home in another country, to help understand migration.
Sometimes individuals migrate from economically depressed areas in order to help support their families. A single individual may leave for a better area and get a better job. These individuals often retain strong cultural ties to their home countries and send money back to their families.
Certain governmental policies can also be seen as a root cause of migration. Free trade agreements often work to drive down wages in developing nations, causing people to migrate to countries with higher wages.
Natural factors can also cause people to migrate. Drought or flooding can destroy farms and cause farmers to move if they can. Mining operations shut down, causing people who relied on them for work to leave.