The Native American population exploded, the census shows. Here’s why.

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.

We could make some less-than-educated guesses, informed by the observation that the rise was largest among Native Americans claiming a mixed heritage. But The Post didn’t greenlight a column called the Department of Less-Than-Educated Guesses. So we dug deeper.

That heartbreaking history helps explain why Native Americans are more likely to have a mixed heritage. But it doesn’t explain the giant increase in numbers in recent years.

At this point, it’s worth noting that data about Native Americans is unusually hard to parse. Census relies on each person’s own assessment of tribal affiliation, rather than tribal enrollment, and counts many more tribes than have official federal recognition, resulting in a sometimes haphazard system that, as Maxim­­ says, “leads to all sorts of wacky results.”

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.

The Census Bureau definition of Native American is not limited to tribes originating on the turf of today’s United States. More than 20 percent of those Census calls single-race American Indian and Alaska Native have Indigenous roots reaching outside our current borders, including the Aztecs, a Mesoamerican people whose modern descendants are known in Mexico as the Nahua.

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.

“It is no exaggeration to say that Indigenous Mexicans conquered, colonized and evangelized the borderlands, sometimes under Spanish direction, other times not,” said Travis Jeffres, a cultural resource consultant and author of “The Forgotten Diaspora.”

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 2020 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 embrace new racial 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.

A broad definition of Native America isn’t necessarily bad. Many Americans with deep Indigenous backgrounds haven’t enrolled in federally recognized tribes. But it does mean the total of 9.7 million American Indians and Alaska Natives includes people who did not check that box on the census.

Still, the Census Bureau didn’t create this population of surprise Native Americans. It just revealed it with a change in methodology. And it raises another big question: Why are there so many Cherokees, out of all the possible American Indian identities?

The unsatisfying answer would be that a surprising number of White and Black Americans suffer from what has been uncharitably called “Cherokee Grandmother Syndrome,” the century-old proto-meme that a dimly recalled ancestor contributed “Cherokee blood.”

A more thoughtful answer requires a deeper understanding of Cherokee history. As a dominant tribe in the American Southeast, the matrilineal Cherokee used marriage as a tool to bring outsiders into their kinship system, said Virginia Commonwealth University’s Gregory Smithers. That spun a wide web of genetic ties, and may have led Whites to view the tribe as more similar to them in culture and appearance. As one of the so-called Five Civilized Tribes, they also thrived economically. Their elites often owned enslaved Africans, which created a basis for Black Americans to have Cherokee heritage as well.

The devastating relocations known as the Trail of Tears followed by a century-plus of disruptive federal policies spread them across the region. That history also led some White Southerners to embrace the Cherokee as fellow victims of federal overreach — though Smithers is quick to point out it was often those Southerners’ ancestors who led the calls for Cherokee removal in the first place.

Together, it all means that Cherokee origins were pervasive enough, and desirable enough, to be smoothly passed down in garbled family legends. In “Becoming Indian: The Struggle Over Cherokee Identity in the Twenty-first Century,” University of Texas anthropologist Circe Sturm finds people who reported Cherokee roots but actually came from a different Southeastern tribe — one without such high brand recognition that its name has been attached to a top selling, gas-hungry Jeep SUV.

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. But there’s a more charitable interpretation, too.

“Most race shifters see themselves not as White people who ‘play Indian,’” Sturm wrote in The Conversation, “but as long-unrecognized American Indians who have been forced by historical circumstances to ‘play White.’”

To be fair, simple genetic math implies that Cherokee and other genes have probably spread far and wide through the population. Your number of potential ancestors doubles every generation, causing exponential growth. In 10 generations, or about 250 years, it tops 1,000. In 20, it tops a million. It doesn’t take long for our fast-expanding family trees to interlock.

Which helps explain why American Indians often point to concepts like tribal membership and kinship bonds — not the genetic links implied by measures of race — as the best markers of Native American identity.

“It’s why so many people insist that Native American identity is a question of political citizenship,” Maxim told us, “rather than race.”

Howdy! The Department of Data craves quantitative questions. What are you curious about: A deep dive into Native Hawaiians, Chamorros, Samoans and other Indigenous Americans outside North America? Are government jobs in the United States really more scarce than they’ve been at any point since the Great Depression? Is college really getting cheaper? Just ask!

If your question inspires a column, we’ll send an official Department of Data button and ID card. This week we owe a button to Sadie Jo Smokey-Crews in Phoenix, a citizen of the Washoe Tribe of Nevada and California, who asked about tribal populations and enrollments.