One theory which has gained wide acceptance is Jerome's analysis in 1926 of the "push and pull" factors—the impulses to emigration generated by conditions in Europe and the U.S. respectively.
En teori, der har vundet stor udbredelse, er H. Jeromes analyse fra 1926 af "push and pull"- (skub og træk-)faktorer — skubbefaktorerne for emigrationen er i høj grad forholdene i henholdsvis Europa og USA.