Immigrant segregation declined in the first half of the 20th century, but has been rising over the past few decades. This has caused questioning of the correctness of describing the United States as a melting pot. One explanation is that groups with lower socioeconomic status concentrate in more densely populated area that have access to public transit while groups with higher socioeconomic status move to suburban areas. Another is that some recent immigrant groups are more culturally and linguistically different from earlier groups and prefer to live together due to factors such as communication costs. Another explanation for increased segregation is white flight.