They're moving closer to big cities. Metropolitan complexes contain about one-third of Iowa's population and together account for all of of Iowa's 1950 to 1960 population growth.
Doerflinger, Jon A.
"Look What's Happening to - Iowa's Population,"
Iowa Farm Science: Vol. 18
, Article 2.
Available at: https://lib.dr.iastate.edu/farmscience/vol18/iss10/2