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 Native American identity is shining a rare spotlight on the malleable nature of Indian heritage and the long history of murky claims to such ancestry.

Warren, a Harvard Law School professor and Democrat who is running in Massachusetts against Republican incumbent Sen. Scott Brown, was listed as Native American in several law school directories. Warren has said that her "family lore" described Indian ancestors, and the New England Genealogy Association said it found indications – but not proof – that Warren had a Cherokee great-great-great grandmother, which would make her 1/32 Indian.

"I'm proud of my heritage," Warren said Thursday. Asked how she knew it included Native Americans, she replied, "Because my mother told me so."

Her opponents question whether 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 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.

Among Native Americans, the varying opinions demonstrate that Indian identity is subjective even among Indians themselves.

When David Eugene Wilkins first saw 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.'"

"For us it was always about allegiance rather than biology or ancestry," said Wilkins, an enrolled member of the Lumbee tribe and professor of American Indian studies at the University of Minnesota.

"It's where you place your political, cultural, emotional allegiance. She lived her entire life and never had any association whatsoever with any community. So something doesn't wash for me," Wilkins said.

But David Treuer, an award-winning writer and Ojibwe Indian from Leech Lake Reservation in Minnesota, said there is a difference between Indian identity and ancestry – you can have one without the other.

"An Indian identity is something someone claims for oneself; it is a matter of choice," Treuer wrote in a Washington Post essay titled, "Elizabeth Warren says she's Native American. So she is."

There are 566 federally recognized Native American tribes, each with its own rules for membership, according to the federal Bureau of Indian Affairs, or 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.

Wilkins, the professor, 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 of all people claiming 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 spokesperson for the Cherokee Nation, said by e-mail.

Warren grew up in Oklahoma, home of the 310,000-member Cherokee Nation, the largest Indian tribe. Warren does not claim official Cherokee membership, which is based on the "Dawes Rolls," a federal list of Cherokees in Oklahoma from the late 1800s and early 1900s. Many people have legitimate Cherokee ancestry but are not eligible for membership because their ancestors were not among those counted, Krehbiel-Burton said.

But "some people falsely claim Native heritage simply out of ignorance," Krehbiel-Burton said. "They've been told for years that they had a great-grandmother (or something similar) who was a Cherokee princess and assume that it's true."

Warren spoke of a similar oral tradition when she mentioned an heirloom photo of her grandfather: "My Aunt Bea has walked by that picture at least a thousand times (and) remarked that he – her father, my papaw – had high cheekbones like all of the Indians do."

Even President Barack Obama has an Indian story, about his maternal grandmother, who was nicknamed "Toot."

"If asked, Toot would turn her head in profile to show off her beaked nose, which, along with a pair of jet-black eyes, was offered as proof of Cherokee blood," Obama wrote in his memoir, "Dreams from My Father."

But eyes, noses and cheekbones are not the issue for Rhonda LeValdo, president of the Native American Journalists Association and an enrolled member of the Acoma Pueblo tribe.

"If you're going to claim it, you have to help your people out," says LeValdo. She had seen no evidence of such involvement by Warren, but said she didn't know enough details to judge Warren's claim.

LeValdo said there are many fakers: "A lot of people find some sort of romanticism in being Native American. They think of the warrior type, or the Pocahontas stereotype. They're just taken with the idea of it."

"But to a lot of our people who live this life, it's tough," she continued. "We deal with a lot of things. A lot of us feel like if you're going to claim it, you have to do something. Don't just use it when you want to use it."

Warren has been adamant that she did not seek any advantage from Native American heritage. Records show that she declined to apply for admission to Rutgers Law School under a minority student program and identified her race as "white" on an employment record at the University of Texas, where she worked from 1983 to 1987.

She left Texas for the University of Pennsylvania Law School, where a report on minority faculty listed Warren's name. Her ethnicity became a campaign issue when the Boston Herald reported that Harvard Law, which hired Warren in 1995, listed her as a minority when the school was under pressure to diversify the faculty.

Besides potentially influencing hiring or promotion, Indian identity can have other economic advantages. Some tribes share millions in casino earnings; health care, scholarships and housing are available to some tribal members.

Native Americans have a high rate of intermarriage with other groups. Many are not identifiable by appearance, which has made it possible for almost anyone to assume a Native persona – for various purposes.

Some of the American colonists who boarded British ships during the Boston Tea Party wore Mohawk costumes. During New York anti-rent conflicts of the 1840s, white people assumed Indian garb and pidgin "Injinspeak" as they harassed patrician estates, according to the book "Playing Indian," by Philip J. Deloria.

The actor Iron Eyes Cody starred as an Indian in films from the 1930s to the '70s, and championed many Native causes. He claimed to be Cherokee, but near the end of his life was revealed to be the son of Italian immigrants. In 1976, former Ku Klux Klansman Asa Earl Carter published a fabricated and best-selling memoir, "The Education of Little Tree," under the name Forrest Carter.

"When that kind of fraud takes place it damages our people," said Wilkins, the professor.

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"You have people on the outside claiming this and that to draw attention to themselves," he said, "and then people on the outside may wonder, do Native people really know who they are?"

Associated Press Writer Steve LeBlanc in Boston contributed to this report.