The mean center of the United States population is determined by the United States Census Bureau from the results of each national census. The Bureau defines it as follows:
After moving roughly 600 miles (966 km) west by south during the 19th century, the shift in the mean center of population during the 20th century was less pronounced, moving 324 miles (521 km) west and 101 miles (163 km) south. Nearly 79% of the overall southerly movement happened between 1950 and 2000.
Clinton County is a county located in the U.S. state of Illinois. At the 2020 census, the population was 36,899. Its county seat is Carlyle. In 1960, the United States Census Bureau placed the mean center of U.S. population in Clinton County. Clinton County is part of the St. Louis, MO-IL Metropolitan Statistical Area.
St. Clair County is the ninth most populous county in Illinois. Located directly east of St. Louis, the county is part of the Metro East region of the Greater St. Louis metropolitan area in southern Illinois. As of the 2020 United States census, St. Clair County had a population of 257,400, making it the second most populous county in Illinois outside the Northern Third. Belleville is the county seat and largest city.
Along the Mississippi River, Cahokia Village was founded in 1697 by French settlers and served as a Jesuit mission to convert tribes of the Illinois Confederation to Christianity. The area became the center of the French Illinois Country. Prior to the establishment of Illinois as a state, the government of the Northwest Territory created St. Clair County in 1790. In 1809, the county became the administrative center of the Illinois Territory and one of the two original counties of Illinois, alongside Randolph County. In 1970, the United States Census Bureau placed the mean center of U.S. population, which generally has moved west every decennial census, in St. Clair County.
Jefferson County is located in the eastern portion of the U.S. state of Missouri. It is a part of the St. Louis Metropolitan Area. As of the 2020 census, the population was 226,739, making it the sixth-most populous county in Missouri. Its county seat is Hillsboro. The county was organized in 1818 and named in honor of former president Thomas Jefferson. In 1980, according to the U.S. census held that year, the county contained the mean center of U.S. population. Notably, this was the first census in which the center of population was west of the Mississippi River. Jefferson County is part of the St. Louis Metropolitan Statistical Area and encompasses many of the city's southern suburbs.
Phelps County is a county in the central portion of the U.S. state of Missouri. As of the 2020 United States census, its population was 44,638. The largest city and county seat is Rolla. The county was organized on November 13, 1857, and was named for U.S. Representative and Governor of Missouri John Smith Phelps.
According to the U.S. Census Bureau, the mean center of U.S. population in 2000 fell within Phelps County.
Monroe County is a county in the U.S. state of Indiana. In 1910 the US Census Bureau calculated the nation's mean population center to lie in Monroe County. The population was 139,718 at the 2020 United States census. The county seat is Bloomington. Monroe County is part of the Bloomington, Indiana, Metropolitan Statistical Area.