Forget about Martin Scorsese’s “Killers of the Flower Moon”: We’re pretty sure the most anticipated debut related to Native Americans this year is a much-delayed and much-less-snappily named release from the U.S. Census Bureau known as Detailed Demographic and Housing Characteristics File A.

The report provides the most detailed data we’ve ever had on America’s racial and ethnic origins, including stunningly exhaustive data on nearly 1,200 tribes, native villages and other entities. We hoped it would shed light on one of the biggest mysteries in the 2020 Census: Why did the Native American population skyrocket by 85 percent over the past decade?

The number of Americans claiming Indigenous heritage jumped from 5.2 million in 2010 to 9.6 million in 2020, a stark increase that probably was not the result of good old-fashioned procreation. Native Americans had the lowest fertility rate of any group measured last year, roughly tied with Asian Americans, according to the U.S. Centers for Disease Control and Prevention.

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We noticed that Indigenous groups across the board were much more likely to be multiracial than other groups. We called Brookings Institution researcher Robert Maxim, a citizen of the Mashpee Wampanoag Tribe who studies Native American data issues. This, he explained, was the legacy of centuries of forced assimilation.

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So, for the moment, let’s set aside the complicated question of mixed-origin Native Americans and take a look at the largest group of single-origin Native Americans in these United States: the Aztecs.

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In the 2020 Census, about 387,000 Americans claimed a full background as Aztec — nearly 20 times the number reported in 2010. This improbable increase is probably due in part to a quirk of data collection. The 2020 Census form listed Aztec and Maya prominently as suggested Native American origins.

But maybe it also provided a clue: Could immigration from other countries have boosted our Native American population?

Probably not. From 2010 to 2020, immigration from Mexico slowed markedly, and even went into reverse. And after quadrupling from 1980 to 2000, the number of U.S. residents who were born in Mexico fell.

Mesoamerican Indians like the Aztecs are hardly recent arrivals. In the earliest days of Spanish colonization of what is now the U.S.-Mexico border region, settlers speaking Nahuatl, the Aztec language, probably outnumbered Spaniards.

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The second-biggest Native American origin in the United States is the Navajo Nation. But while it’s one of America’s top tribes by land area and enrollment, the Navajo population did not grow as rapidly in the 2020 Census as Native America writ large.

So we looked at the third-biggest group — and it’s a humdinger and a half. About 215,000 Americans claim to be exclusively “Cherokee.” And these generic “Cherokees” outnumber Census counts for all three federally recognized Cherokee tribes, none of which are included in the generic “Cherokee” total.

But the bit that really braised our brains is this: While the number of people claiming to be single-race Cherokee fell slightly in the 2010 Census, the number claiming to be at least part Cherokee skyrocketed from 770,000 to about 1.5 million.

That doubling dwarfs the increases of other native groups. It’s like we discovered an entire Alaska or North Dakota populated solely by newfound part-Cherokees. If we can pinpoint where they came from, we’ve probably solved this mystery.

We tracked down University of Minnesota sociologist Carolyn Liebler, an Indigenous demography expert who has repeatedly delved deep into the data to count the surprising numbers of Americans who embracenewracial identities from census to census. She thought she could finger the culprit.

“It’s definitely the Census Bureau,” she told us via email. Her research has shown that, yes, people are adopting Indigenous identities, and Indigenous people are immigrating to the United States. But that’s been the case for decades. This time, something’s different. And it’s got to be “very important changes in the race question and especially in the way they coded the responses that they received.”

As we’ve discussed previously, Census changed how it measured race in 2020. Unlike in 2010, the form provided a free-response line for all races. If you marked White or Black you were prompted to write a specific origin, such as Russian, Alsatian or Haitian. The bureau counted up to six responses and matched them, by hand if necessary, with their official origin list.

Crucially, Census tabulated your race based on the origins you entered, not just the racial box you checked. If you marked only White on the form but wrote in “Scottish, Romanian, Italian and Cherokee,” you’d be marked as American Indian and Alaska Native as well as White. You’d also show up as part Cherokee.

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The adoption of a tenuous Indigenous heritage may be a sign that Americans are shying away from a White identity that has become an uncomfortable mark of privilege. {snip}

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