The fast growth of suburban counties is well documented in the latest county population estimates issued by the U.S. Census Bureau.
Natural increase refers to the balance of births over deaths.
The net migration component, on the other hand, is determined by numbers of people moving in and out of a particular county. Between the two components of change, net migration is subject to wider fluctuations and therefore it is almost always the leading factor in rapid population growth or decline. Population growth was slower in Shelby County, where net migration was positive but trailed behind natural increase. Population remained essentially stable in Madison County and Marion County, which both experienced net out-migration. The map portrays net migration for counties in nine metropolitan areas in Indiana and surrounding states.