Unincorporated areas added nearly 148,000 residents between 2000 and 2006, a growth rate of 7 percent, said the study, which was conducted by the Indiana Business Research Center at Indiana University.
Incorporated areas grew by 85,000 people, or 2.1 percent.
The trend was strong in Boone, Hamilton, Hancock, Hendricks and Johnson counties. The number of people living in unincorporated areas in each county increased at least 10 percent.
The majority of Hoosiers, 64 percent, still live within city or town limits, the study said. But fully 2.3 million people, or 36 percent, live outside those boundaries.
What's more, in 50 of the state's 92 counties, more than half the population lives in rural areas. In Brown County, for example, just 5 percent of the population lives within incorporated limits.
The study, which appeared last week in inContext, a magazine published by the Business Research Center and the Indiana Department of Workforce Development, said the trend is often considered "a natural consequence of economic growth, rising real incomes and cheaper and better transportation over time."
However, the study added, the trend in the long run could have serious implications for the environment, local government tax policy and services.