In many cases, researchers may want to control for other factors on immigration.
The literature suggests that immigration is path dependent in that current
immigration flows are related to past immigration patterns. For example, Kahan
(1978), Murayama (1991), Rephann and Vencatasawmy (2000) find distinctive ethnic
concentrations of immigrants, and Zawodny (1997) find that family connections
are the most significant immigrant determination factor. Additional evidence
suggests that immigration flows are larger, ceteris paribus, when a common language
is spoken. Adding these considerations to the model above creates the augmented
immigration gravity equation: