What, exactly, makes someone American Indian?
Even Indians themselves don't agree as they debate the case of Senate candidate Elizabeth Warren, whose disputed claim of Indian identity is shining a rare spotlight on the malleable nature of Indian heritage and the long history of murky claims to such ancestry.
Ms. Warren, a Harvard Law School professor and Democrat who is running in Massachusetts against Republican incumbent Sen. Scott P. Brown, was listed as "Native American" in several law school directories.
Ms. Warren has said that her "family lore" described Indian ancestors, and the New England Genealogy Association said it found indications — but no proof — that Ms. Warren may have had a Cherokee great-great-great-grandmother. If confirmed, this would have made her 1/32 Indian, though not a Cherokee under the tribe's rules.
"I'm proud of my heritage," Ms. Warren said Thursday. Asked how she knew it included Native Americans, she replied, "Because my mother told me so." Her previous answers had included references to high cheekbones in the family, which prompted howls of derision.
Her opponents question whether Ms. Warren chose this heritage to gain advantages available to Indians and other underrepresented groups in academia.
"Warren has zero evidence that she is at all Native American," said Mr. Brown's campaign manager, Jim Barnett. The genealogy association acknowledges that it found only secondary references to Cherokee family members, not primary sources such as marriage, birth or census records.
When David Eugene Wilkins first saw Ms. Warren interviewed during her nomination to a federal post, he was smitten by her intelligence and politics. But when he heard about her claims of Indian ancestry, "I shook my head and said, 'Oh no.' "
"It's where you place your political, cultural, emotional allegiance. She lived her entire life and never had any association whatsoever with any community," said Mr. Wilkins, an enrolled member of the Lumbee tribe and professor of American Indian studies at the University of Minnesota.
There are 566 federally recognized Indian tribes, each with its own rules for membership, according to the federal Bureau of Indian Affairs (BIA). Some tribes require a "blood quantum" measurement of as much as one-half or one-quarter Indian ancestry; others require a certain place of birth or residence.
Mr. Wilkins is married to a Navajo with many siblings.
"I've asked them what defines a Navajo," he said. "One said you have to speak the language. Another said you have to live within our sacred mountains. Another said no, you have to take part in ceremonial life. All this in one family."
According to census figures provided by the BIA, an estimated 4.5 million people identify themselves as American Indians or Alaska Natives, including those who say they are more than one race. But in a 2005 report, the most recent available, the BIA counted just 2 million enrolled tribal members — which means that fewer than half the people who claim Indian heritage are recognized by a tribe.
"There's an old joke in this corner of Indian Country that if you meet someone who doesn't know anything about tribal affairs but claims they're Indian, they'll say they're Cherokee," Lenzy Krehbiel-Burton, a spokesman for the Cherokee Nation, said by e-mail.