What are the push and pull factors of immigration from 1800-1925, and how did american policies reflect those factors?

Push factor:

People think about emigrating from places that have few job opportunities. Because of economic restructuring, job prospects often vary from one country to another and within regions of the same country.

Pull factor:

People immigrate to places where the jobs seem to be available. An area that has valuable natural resources, such as petroleum or uranium, may attract miners and engineers. A new industry may lure factory workers, technicians, and scientists.

What research did you learn from?