There are a lot of factors that affect voter turnout in the United States — race, income, education, electoral competitiveness, the list goes on and on.

Many of those factors are outside policymakers' control. But there’s one big realm that they have a lot of influence over: voting access laws, which vary significantly from state to state. Is early voting allowed? How about no-excuse absentee voting? Are there strict voter ID laws, lax ones or none at all? Can convicted felons vote?

These laws generally affect how easy it is to cast a ballot in a given election. In a new report, political scientists at Northern Illinois University, Jacksonville University and China’s Wuhan University seek to quantify the net effect of a state’s election laws to determine the “time and effort” it takes to vote there. They call their project the Cost of Voting Index and have published it in the September issue of the Election Law Journal.

To create the index, the researchers collected data on 33 types of election laws that generally fell into seven different categories: voter-registration deadlines, restrictions on registrations and registration drives, preregistration laws that allow people under 18 to register in advance of their first elections, laws governing ease of voting (like early and absentee voting), voter ID requirements and polling hours.

They mashed each of these qualitative factors together in a statistical blender to create a top-line summary number of the ease of voting and registration in each state. For the 2016 election, states are ranked according to their position in this index in the map above.

The easiest state to vote in that year was Oregon, according to the index. Voters are registered automatically, and the state mails out ballots to every voter several weeks before the election. Oregon is followed by Colorado, California, North Dakota (which doesn’t even have voter registration) and Iowa.

Conversely, the index finds that voting is hardest in Mississippi, which comes in dead last in the ranking. The state requires a photo ID at the polls. It doesn’t allow early voting, or no-excuse absentee voting. It’s joined at the bottom of the list by Virginia, Tennessee, Indiana and Texas.

What sort of effect do these laws have on voter turnout? Some quick calculations suggest that the effect is potentially quite large: The five most restrictive states had turnouts in 2016 that were, on average, nearly nine percentage points lower than turnout in the five easiest states to vote in. If we plot the Cost of Voting Index rank against 2016 turnout, it looks like this.

Even a simple correlation, like the one above, suggests a relationship between ease of voting and turnout: Generally speaking, states with the least restrictive voting laws have higher turnout rates.

But it’s also clear that voting laws don’t explain everything. Hawaii, for instance, had the country’s lowest turnout in 2016 despite having fairly permissive voting laws. Virginia, on the other hand, had high turnout despite a restrictive voting environment.

To isolate the effect of the legal landscape from other factors, the researchers who created the Cost of Voting Index ran a more comprehensive analysis controlling for per capita income and high school graduation rates, as well as the competitiveness of the election at the top of the ballot, as measured by the size of the final vote margin (under the assumption that more competitive races will have smaller margins).

Controlling for those other factors, they found that turnout in 2016 was predicted to decline by about 3.3 percent for every one unit increase in voting difficulty as measured by the index. In other words, the difference between the electoral environment in Oregon (ranked No. 1 in 2016) and Iowa (ranked No. 5). Overall, the model predicts an 11 point turnout difference between the least restrictive and most restrictive states in 2016.

Those findings strongly suggest that high turnout in some states is at least partly a direct consequence of choices made by policymakers to expand access to the ballot box. The converse would also be true: The low turnout rates seen in places like Mississippi, Tennessee and Texas are in part a result of lawmakers' deliberate efforts to make voting harder.

To even the turnout playing field, the researchers believe that same-day registration laws would offer the most bang for the buck.

“We can safely argue that if states desire higher citizen participation rates in elections, a reasonable place to start would be a same-day voter registration policy,” the researchers conclude. “The burden of getting re-registered to vote a predetermined number of days before the general election makes voting costlier. Allowing people to register at the actual polling station would do still more to reduce the cost of voting.”

As of 2018, just 17 states plus the District have a same-day registration policy, according to the National Conference of State Legislatures.