Voter ID has again taken center stage in Pennsylvania.

Republicans are pushing stricter voting rules against the objections of Democrats, led by Gov. Tom Wolf, who last week vetoed an election overhaul that, among other major changes, would have required that all voters show identification at the polls. Republicans are now turning their focus to a constitutional amendment that would go straight to voters for approval.

But understanding the effects of voter ID laws is complicated. While the political debate often devolves into shouting matches about security and suppression, research into ID requirements has yielded a contradictory array of results.

We waded through the research and interviewed experts to make sense of it. Voter ID laws potentially have small negative effects on net turnout — but the research is muddy, allowing politicians to cherry-pick data or make broad generalizations. Plus, some experts said, turnout might not be an appropriate way to think about something as fundamental as voting rights.

Here are some key things to know about voter ID.

Voter ID is meant to prevent fraud that doesn’t exist

The justification for tightening voter ID rules is to prevent fraud. But significant voter fraud, especially voter impersonation, is virtually nonexistent.

“We have tremendous evidence that voter ID laws do absolutely nothing to prevent voter fraud, because there is no systematic voter fraud to prevent,” said Bernard Fraga, an Emory University political scientist.

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State Rep. Seth Grove (R., York), who authored the bill Wolf vetoed, has even acknowledged the ID rules don’t address an existing problem. “It’s not to say that there has been any” fraud, he told The Inquirer, describing the current rules as “a loophole that can be exploited” in the future.

‘Voter ID’ means many things

Small differences in law and implementation can matter a lot.

The 2012 Pennsylvania voter ID law that courts struck down was one of the strictest in the nation. The version in Grove’s bill would have been more flexible, including giving every registered voter an ID and allowing voters without one to sign an affidavit affirming their identity.

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But the proposed constitutional amendment would be much stricter, requiring “any valid government-issued identification” and not providing for free IDs.

Voter ID can prompt a backlash that hides its true effects

New election laws don’t take effect in a vacuum. Restrictions can anger and energize voters, while civic groups and campaigns often mobilize to counteract them.

Pennsylvania experienced a version of this last year, said Dan Hopkins, a University of Pennsylvania political science professor, in response to the state Supreme Court’s decision that “naked ballots” missing their secrecy envelopes must be rejected.

“The Democratic Party then did extensive voter education,” Hopkins said. “It looks like the effect of that education was to … really reduce the number of naked ballots cast.”

The costs of voting don’t always show up in turnout

Whether an individual voted is a straightforward question, but it can hide the costs of voting. Think about long lines at polling places: If a Black voter waits for hours while a white voter waits only a few minutes, both voted, but the cost for the Black voter was much higher.

“People might ultimately manage it. That doesn’t mean that it’s costless,” said Ariel White, an MIT political scientist. “We are still imposing very serious costs on people who are trying to vote.”

Voter ID would most likely affect already marginalized groups

Black and Hispanic citizens are less likely than others to have the necessary identification these laws require.

So while the impact is hard to measure, it’s likely to fall more on those groups, said Hakeem Jefferson, a Stanford political science professor: “When it does matter, it matters disproportionately for people of color.”

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It’s also likely to disproportionately affect other already marginalized groups, including poor and low-income voters, voters with disabilities, and the youngest and oldest voters.

Voter ID affects low-turnout groups

That disparate impact is one reason turnout might not fall when ID requirements are implemented: The groups most likely to be affected already have lower turnout rates and face other barriers.

So instead of blocking existing voters, ID rules might raise additional barriers that deter new or future voters.

“A lot of the people who would be voting are the kind of people who already have ID,” said Emily Rong Zhang, a political science graduate student at Stanford studying voter ID. “But we also know there’s a large pool out there who don’t have ID, and … they’re disinclined to vote in the first place. And if you care about democracy, it’s not the number of votes cast that you need to be concerned about, it’s losing voters.”

Voter ID laws can create confusion

Many people won’t know the details of the rules, but will hear about new ID requirements, especially the most controversial ones.

“These narratives scare people away from voting, and they make people think maybe there’s going to be a problem when they get to the polls,” Fraga said. “For groups who already feel like their voice doesn’t matter in politics … they hear about a voter ID mandate and it reinforces their worldview, even if they have an ID.”

And research suggests that even attempts to cut through the confusion can be met with unequal response. In an experiment, White found that local elections officials were more likely to respond to emails sent from what appear to be non-Hispanic white names than Hispanic ones.

“There’s this extra layer of challenges to getting your questions answered and get that help that you need to vote,” White said.

Turnout might not be the best way to measure voter ID

Some experts said people should take a step back from thinking about turnout and partisan impact, and instead adopt a more fundamental framing of voting rights.

“In a democracy, the right to vote is sacred,” Jefferson said, adding that “we should simply be bothered by the burden placed on voters, period, irrespective of whether the thing works or not, whether the restrictive voting law works to suppress turnout.”

The challenge is figuring out what to do in negotiating legislation.

Chris Warshaw, a political scientist at George Washington University, got pushback from Jefferson and others when he tweeted last month that preventing partisan gerrymandering is worth the trade-off of new ID rules.

Ultimately, Warshaw said in an interview, he opposes ID requirements but worries about other issues even more.

“In isolation, I would never in a million years support voter ID. But as part of a comprehensive plan, you have to make trade-offs,” he said. “I just think there’s not clear evidence that voter ID rises to that level of being so bad for democracy that it should never be part of a deal.”

Like so much about voter ID, there’s no one clear answer, White said.

“There are a lot of potential threats to the functioning of U.S. democracy right now, so I can understand why some legislators would be looking to potentially deal to make some compromises,” she said. “In some ways, this is a moral and a philosophical question and not just an empirical one.”