Requiring a photo ID to vote can decrease voter turnout, particularly among African-Americans and people 18–23, according to a study released by the Government Accountability Office this week.

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The GAO analyzed voter turnout in Kansas and Tennesse, which changed its voter ID requirements from the 2008 to 2012 elections, and compared it to states that did not alter those requirements over the same time period.

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In controlling for other variables, the GAO found that the decreases in voter turnout in Kansas and Tennessee were statistically significant and were attributable to the voter ID requirements.

Turnout in Kansas was lower by 1.9 to 2.2 percentage points and in Tennessee, by 2.2 to 3.2 points.

Starting in 2012, Kansas voters had to show photo ID when casting a vote in person, and must have their signature verified and include a copy of acceptable photo ID, or provide a full Kansas driver’s license or non drive ID when voting by mail.

Tennessee also requires voters to show government-issued photo ID, which does not include student ID cards from state universities.

The rules in Kansas become more strenuous in 2013, which is outside the scope of the GAO study, when in order to register to vote, you must prove U.S. citizenship.

Voter ID requirements reduced turnout by larger amounts, the GAO found, among young people 18–23 than their counterparts 44–55; among African-Americans more than white, Asian-American and Hispanic people.

In North Carolina photo ID laws will take effect in the 2016 elections. Other changes to the voting law—eliminating same-day registration and counting ballots cast outside a voter’s precinct—were overturned by a federal appeals court last week, but then were reinstated by the U.S. Supreme Court this week.