The mean center of U.S. population is determined by the United States Census Bureau after tabulating the results of each census. The Bureau defines it to be:
…the point at which an imaginary, flat, weightless, and rigid map of the United States would balance perfectly if weights of identical value were placed on it so that each weight represented the location of one person on the date of the census.
During the 20th century, the mean center of population has shifted 324 miles west and 101 miles south. The southerly movement was much stronger during the second half of the century; 79 miles of the 101 miles happened between 1950 and 2000.