The 2010 census forms go to 135 million households at a time when the country has grown more ethnically and racially diverse because of immigration and a growing number of interracial marriages, USA Today reported Wednesday.
"It's a historic opportunity to see how things have changed or how things have not changed," said Nicholas Jones, chief of the Census Bureau racial statistics branch. Jones called multiracial Americans one of the fastest-growing demographic groups.
The Census Bureau first allowed people to choose more than one race in the 2000 census, and this year's census will enable some of those who were children identified as one race by parents to choose more than one now that they're adults.
The bureau predicts by 2050, there will be no racial or ethnic majority as the proportion of non-Hispanic whites slides below 50 percent.
"Tomorrow's children and their children will, in fact, be multiracial, leading to a potential post-racial society," said William Frey, a demographer at the Brookings Institution.
"The issue isn't just multirace," said census historian Margo Anderson, a professor at the University of Wisconsin-Milwaukee. "It's the blurring of the very traditional black vs. white. … The clarity is breaking down."
One census question asks if anyone in the household is Hispanic, Latino or of Spanish origin, while another asks the race of each person, specifying, "Mark one or more boxes" for race or check "some other race."
The government relies on the information to check compliance with anti-discrimination laws, fair employment and affirmative action.
The Census Bureau said 2.3 percent of the population -- about 7 million people -- say they're of more than one race. Mixed-race marriages climbed 20 percent since 2000, to 4.5 million, or 8 percent of the total.