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.