People switched between races, moved from multiple races to a single race or back, or decided to add or drop Hispanic ethnicity from their identifiers on census forms.
Researchers said the information used — race, ethnicity, sex, age, location and how the information was gathered — is not particularly helpful for figuring out why people decided to make those changes. They noted that there has been a tendency toward multiple-race responses, and new census form designs may have caused some changes in how people respond to questions about Hispanic ethnicity.
Age may have something to do with the changes, the researchers suggested.
"Compared to adults, children and adolescents may be more likely to change their race/Hispanic responses for two reasons: childhood and adolescence are times of personal identity development and young people's information was probably reported by their parents in 2000 but may be self-reported in 2010," researchers said.
The report showed that 1 in 16 people — or approximately 9.8 million of 162 million — who responded to both the 2000 and 2010 censuses gave different answers when it came to race and ethnicity.
If extrapolated across the entire population, that would mean that 8.3 percent of people in the United States would have made a change in their racial or ethnic identity in that decade, according to the paper authored by Sonya Rastogi, Leticia E. Fernandez, James M. Noon and Sharon R. Ennis of the U.S. Census Bureau and Carolyn A. Liebler of the University of Minnesota.
The largest change was from Hispanic (some other race) to Hispanic white, with 2.38 million people making that change on their census forms. But the next greatest change was from Hispanic white to Hispanic (some other race), with 1.2 million people deciding that designation fit them better. Put together, these two changes make up more than a third — 37 percent — of the race/ethnicity changes in the report.
The two next biggest changes were also paired: 710,019 people decided to change from white to Hispanic white, and 417,855 moved from Hispanic white to white from the 2000 to the 2010 census.
The groups most likely to change were people who were children and/or living in the West in 2000. That region also had a higher rate of interracial marriage, and multiple race reporting, the report said. The census defines the West as being Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming.
The most stable groups were single-race, non-Hispanic whites, blacks and Asians, with those who checked those boxes staying with them in both censuses. People were also consistent with their Hispanic/non-Hispanic choices.
But "there is extensive population churning among American Indians, Pacific Islanders and multiple-race respondents in our data," the report said. "Many people left each of these populations between 2000 and 2010, and about as many joined each group."
The census identifies races as white; black or African American; American Indian or Alaska Native; Asian; Native Hawaiian or Other Pacific Islander; and other for those claiming more than one race. There is also a Hispanic ethnic category.
Follow Jesse J. Holland on Twitter at http://www.twitter.com/jessejholland
Census report: www.census.gov/srd/carra/Americas_Churning_Races.pdf