Multiracial Americans have become the fastest growing demographic group, wielding an impact on minority growth that challenges traditional notions of race.

The number of multiracial people rose 3.4 percent last year to about 5.2 million, according to the latest census estimates. First given the option in 2000, Americans who check more than one box for race on census surveys have jumped by 33 percent and now make up 5 percent of the minority population — with millions more believed to be uncounted.

Demographers attributed the recent population growth to more social acceptance and slowing immigration. They cited in particular the high public profiles of Tiger Woods and President Barack Obama, a self-described "mutt," who are having an effect on those who might self-identify as multiracial.

Population figures as of July 2008 show that California, Texas, New York and Florida had the most multiracial people, due partly to higher numbers of second- and later-generation immigrants who are more likely to "marry out." Measured by percentages, Hawaii ranked first with nearly 1 in 5 residents who were multiracial, followed by Alaska and Oklahoma, both at roughly 4 percent.

Utah had the highest growth rate of multiracial people in 2008 compared to the previous year, a reflection of loosening social morals in a mostly white state.

"Multiracial unions have been happening for a very long time, but we are only now really coming to terms with saying it's OK," said Carolyn Liebler, a sociology professor at the University of Minnesota who specializes in family, race and ethnicity.

"I don't think we've nearly tapped the potential. Millions are yet to come out," she said.

In Middletown, N.J., Kayci Baldwin, 17, said she remembers how her black father and white mother often worried whether she would fit in with the other kids. While she at first struggled with her identity, Baldwin now actively embraces it, sponsoring support groups and a nationwide multiracial teen club of 1,000 that includes both Democrats and Republicans.

"I went to my high school prom last week with my date who is Ecuadoran-Nigerian, a friend who is Chinese-white and another friend who is part Dominican," she said. "While we are a group that was previously ignored in many ways, we now have an opportunity to fully identify and express ourselves."

The latest demographic change comes amid a debate on the role of race in America, complicating conventional notions of minority rights.

Under new federal rules, many K-12 schools next year will allow students for the first time to indicate if they are "two or more races." The move is expected to cause shifts in how test scores are categorized, potentially altering race disparities and funding for education programs.

Five justices of the Supreme Court have signaled they would like to end racial preferences in voting rights and employment cases — a majority that may not change even if Sonia Sotomayor is confirmed as the first Hispanic justice. Blacks and Hispanics, meanwhile, are touting a growing minority population and past discrimination in pushing for continued legal protections.

Left out of the discussion are multiracial people, who are counted as minorities but can be hard to define politically and socioeconomically. Demographers say that while some multiracial Americans may feel burdened or isolated by their identity, others quickly learn to navigate it and can flourish from their access to more racial networks.

"The significance of race as we know it in today's legal and government categories will be obsolete in less than 20 years," said William H. Frey, a demographer at Brookings Institution.

"The rise of mixed-race voters will dilute the racial identity politics that have become prevalent in past elections," he said.

Liebler noted a potential dilemma where a white student who is one-eighth Cherokee applies to college and seeks an admissions preference based on race and disadvantaged status. Should the college give the multiracial student the boost, if one-eighth of his family suffered a past racial harm but seven-eighths of his family were the perpetrators?

"It's a huge question for our legal system and our policies," she said. "Tomorrow we could have a legal case that challenges whether a multiracial person is a minority."

Census data also show:

• More than half of the multiracial population was younger than 20 years old, a reflection of declining social stigma as interracial marriages became less taboo.

• Interracial marriages increased threefold to 4.3 million since 2000, when Alabama became the last state to lift its unenforceable ban on interracial marriages. (The Supreme Court barred race-based restrictions on marriage in 1967.) About 1 in 13 marriages are mixed race, with the most prevalent being white-Hispanic, white-American Indian and white-Asian.

• Due to declining immigration because of legal restrictions and the lackluster economy, the growth rates of the Hispanic and Asian populations slowed last year to 3.2 percent and 2.5 percent, respectively, compared to multiracial people's 3.4 percent. The black population rose at a rate of about 1 percent; the white population only marginally increased.

Currently, census forms allow U.S. residents to check more than one box for their race. But there is no multiracial category, and survey responses can vary widely depending on whether a person considers Hispanic a race or ethnicity.

"It's all about awareness," said Susan Graham, founder and executive director of California-based Project Race, which advocates for a multiracial classification on government forms. "We want a part of the pie chart."

The 2008 census estimates used local records of births and deaths and tax records of people moving within the U.S. The figures for "white" refer to those whites who are not of Hispanic ethnicity. For purposes of defining interracial marriages, Hispanic is counted as a race.