In the 1920's, the number of Mexicans and Puerto Ricans migrating
to the continental United States increased. One reason for this was
that

The story of the Puerto Rican people is unique in the history of U.S. immigration, just as Puerto Rico occupies a distinctive—and sometimes confusing—position in the nation’s civic fabric. Puerto Rico has been a possession of the U.S. for more than a century, but it has never been a state. Its people have been U.S. citizens since 1917, but they have no vote in Congress. As citizens, the people of Puerto Rico can move throughout the 50 states just as any other Americans can—legally, this is considered internal migration, not immigration. However, in moving to the mainland, Puerto Ricans leave a homeland with its own distinct identity and culture, and the transition can involve many of the same cultural conflicts and emotional adjustments that most immigrants face. Some writers have suggested that the Puerto Rican migration experience can be seen as an internal immigration—as the experience of a people who move within their own country, but whose new home lies well outside of their emotional home territory.