Early voting is ostensibly designed to make casting a ballot easier and more convenient. But does it actually decrease overall voter turnout? According to new research by University of Wisconsin-Madison political science professors Barry Burden and Kenneth Mayer, the answer is yes. They documented their findings (based on statistical analysis of the 2008 election) and suggested changes to the process in Sunday's New York Times:
Turnout not only doesn’t increase with early voting, it actually falls. How can this be? The answer lies in the nature of voter registration laws, and the impact of early voting on mobilization efforts conducted by parties and other groups on Election Day.

In most states, registration and voting take place in two separate steps. A voter must first register, sometimes a month before the election, and then return another time to cast a ballot. ...

When Election Day is merely the end of a long voting period, it lacks the sort of civic stimulation that used to be provided by local news media coverage and discussion around the water cooler...With significant early voting, Election Day can become a kind of afterthought, simply the last day of a drawn-out slog.
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Our research shows that when early voting is combined with same-day registration — that is, you can register to vote and cast an early ballot on the same day — the depressive effect of early voting disappears...By removing barriers that require potential voters to register weeks before a campaign reaches its height, less-engaged citizens can enter the voting process late — and political campaigns can respond by maintaining the intensity of their efforts through Election Day.