Maryland and Virginia residents who identify themselves as being more than one race increased at a rate nearly twice the national average over the last decade, a new census reports says, while the growth of multiracial residents in D.C. was slightly behind the national pace.

Virginia led the way with more than 233,000 people saying in the 2010 census questionnaire that they are more than one race. That's an increase of 63 percent over the last decade, while nationally, multirace respondents climbed by 32 percent. Marylanders identifying with more than one race increased by 59 percent to nearly 165,000, while the District's multiracial population of more than 17,000 represents a nearly 29 percent growth.

"Our idea of what racial categories were in the past are changing," said William Frey, a Brookings Institution demographer.

More than one race
  2010 Pop. 2000 Pop. Change
  census share census share over decade
Virginia 233,400 2.9% 143,069 2.0% +63.1%
Maryland 164,708 2.9% 103,587 2.0% +59.0%
D.C. 17,316 2.9% 13,446 2.4% +28.8%
United States 9m 2.9% 6.8m 2.4% +32.0%

Although the multiracial portion of the population is small -- less than 3 percent -- that number will continue to grow.

"I think it's especially going to be changing for younger people as ... there's been an increase in mixed-race marriages, and that will eventually change the classic racial [categories]," Frey said.

Demographers and the authors of the Census Bureau report warn of factors that may skew results toward more people in 2010 answering that they are multiracial. Some wording changes in the questions on Hispanic origin and race since census 2000 "could have influenced reporting patterns," the paper's authors say.

Lisa Sturtevant, assistant research professor at George Mason University's Center for Regional Analysis, noted that 2000 was the first time the census asked about more than one race. Over those 10 years, identifying with more than one race has become more common.

"So in some cases, it's not reflecting an actual diversification of the population, it's just people are more comfortable identifying with two," she said.

Still, the numbers point to a diversifying population. Regionally, the numbers are more extreme. The outer ring of Northern Virginia counties saw the biggest growth in multiracial residents -- that population doubled in Prince William County and tripled in Loudoun County. Prince William also saw the region's largest numerical increase of any county with an additional 10,322 multiracial residents.

Sturtevant said those increases follow surges in minority growth in those counties. Asians were the fastest-growing population in Loudoun over the last 10 years, while Hispanics drove the minority growth in Prince William, according to census data.

Meanwhile, the multiracial populations in Alexandria and Arlington fell over the last 10 years as those jurisdictions have seen drops in their minority populations.

Montgomery, Prince George's and Fairfax counties are still home to the largest contingent of multiracial residents, combining for nearly 110,000 of the region's 173,000 residents who identify with more than one race. But multiracial populations in those jurisdictions all grew by less than one-third, a rate slower than the national average.