A woman performs a traditional Native American dance during the North American Indian Days celebration on the Blackfeet Indian Reservation in Browning, Mont., on July 13, 2018. (AP Photo/David Goldman)

Last Tuesday, the Supreme Court allowed a lower court ruling to stand, allowing North Dakota to require voters to provide identification that shows a residential address rather than a post office box number. Many Native American advocacy groups have argued that this decision violates tribal sovereignty and systematically disenfranchises voters. Sen. Heidi Heitkamp’s (D-N.D.) reelection may be at stake.

The reason: Many Native Americans living on rural reservations do not have traditional street addresses, and receive mail at P.O. boxes rather than at home.

But that’s not the only issue that keeps Native Americans from voting. Native Americans on reservations have long voted at very low rates, facing barriers to voting that include long travel distances to the polls, high poverty rates, and low high school graduation rates. When native voters need to travel to reservation border towns to cast ballots, the well-documented and long-standing mistrust between Native American communities and non-native populations likely discourages voting in those areas.

But what, exactly, are the factors that limit turnout in rural areas? What policies might improve turnout for native voters living on reservations?

Here’s how we did our research

In the summer of 2016, a large team of researchers from Claremont Graduate University (CGU), funded by the Native American Voting Rights Council, conducted more than 1,500 field surveys on reservations in South Dakota and Nevada to understand native attitudes toward government and voting. We found that long travel distances, administrative difficulties, and distrust of the voting process kept voters home.

Our collaborative research with CGU PhD students Aaron Berg, Joseph Dietrich, and Savannah Johnston also examined whether enabling Native Americans to vote on their reservations might improve turnout. We did that by investigating a “natural experiment.”

On Oct. 3, 2016, Judge Miranda Du of the U.S. District Court of Nevada issued an emergency injunction establishing satellite centers for early voting on the Pyramid Lake Paiute and Walker River Paiute reservations. Du ruled that Nevada had violated Section 2 of the Voting Rights Act of 1965 by failing to provide the reservations with equal access to the vote. Before the injunction, anyone living on the reservation had to drive 96 miles round trip from Pyramid Lake or 68 miles round trip from Walker River to vote in person. The ruling argued that individuals on the reservations faced an “abridgement” of their voting rights given that travel distance, especially considering that 25 to 31 percent of residents lived below the poverty line.

As a result, that November, Pyramid Lake and Walker River voters could vote early, on their reservations, cutting their travel to less than 2 miles on average. But — and here’s the natural experiment — that ruling didn’t apply to two similar Nevada reservations, which hadn’t joined the lawsuit. Voters in the Duck Valley Shoshone-Paiute Reservation and Yerington Paiute Reservation faced nearly identical socioeconomic conditions and travel distances. Yerington Reservation had discussed joining the lawsuit, but was not able to submit the required documents by the deadline. Duck Valley Reservation residents stayed out of the litigation because the reservation includes parts of Idaho, and they feared that would complicate the decision.

The four reservations had roughly the same turnout rates before the ruling. Most voting studies have trouble assessing whether higher voting rates come from a specific policy change or from the citizens’ greater eagerness to vote, which triggered that policy change.

So would having a voting booth on reservations bring out more voters? To find out, we looked at 2016 voter turnout across these four reservations. Our analysis, after controlling for other factors, suggests that, yes, it did. In the two reservations that had the on-site early voting option, turnout went up roughly 11 to 24 percent over that at reservations where citizens still had to travel to border towns.

What do our findings mean for Nevada’s Native Americans?

This research suggests that adding administrative barriers — like requiring street addresses for people who don’t have street addresses — will likely depress voter turnout on reservations still more.

Nor would it help to allow Native Americans to vote by mail. Our survey evidence found that very few Native Americans vote by mail, both because they do not trust the process and because they do not have regular access to mail. By a wide margin, we found that native voters trust their vote is more likely to be counted when it is cast in person. With limited access to mail and without the ability to use post office boxes to vote, reservation turnout probably won’t go up if it’s possible to vote by mail.

The way Americans vote has changed dramatically in the last few decades, as more voters cast ballots by mail and in nontraditional locations such as shopping malls. Knowing how Native Americans vote — and why they don’t — may be useful to those who wish to ensure that the franchise extends to all citizens.

Jean Schroedel is a professor of American politics at Claremont Graduate University and an academic consultant to the Native American Voting Rights Coalition.

Melissa Rogers is an assistant professor of international studies at Claremont Graduate University and a Visiting Fellow at the Center for U.S. Mexican Studies at University of California, San Diego.