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After a lengthy period of formal racial segregation in the former Confederacy following the Reconstruction Era, and bans on interracial marriage in various parts of the country, more people are openly forming interracial unions. In addition, social conditions have changed and many multiracial people do not believe it is socially advantageous to try to "pass" as white. Diverse immigration has brought more mixed-race people into the United States, such as the large population of Hispanics identifying as mestizos. Since the 1980s, the United States has had a growing multiracial identity movement (cf. Loving Day). Because more Americans have insisted on being allowed to acknowledge their mixed racial origins, the 2000 census for the first time allowed residents to check more than one ethno-racial identity and thereby identify as multiracial. In 2008 Barack Obama was elected as the first multiracial President of the United States; he acknowledges both sides of his family and identifies as African American.

When did the US census start to allow residents to mark more than one ethno-racial identity on the census form?
the 2000 census for the first time allowed residents to check more than one ethno-racial identity and thereby identify as multiracial