There were declines in population and declines in population growth from 1900 to 1930, and from 1940 to 1970. These periods related to the migration of people from rural to urban areas for work, especially as mechanization reduced the need for agricultural laborers. In addition, these time periods related to the Great Migration of African Americans out of the Jim Crow South to northern and midwestern industrial cities for more opportunities. The African-American population became highly urbanized. Expansion of the railroads, auto and steel industries provided new work opportunities in the early 20th century.