A study by the Caltech/MIT Voting Technology Project examined California absentee votes over two decades. It found that absentee votes are twice as prone not to be counted as votes cast on more modern voting machines.
The study estimated that in 2008 as many as 21 percent of absentee ballots requested nationwide never made it back to election officials.
A co-author of the study, Charles Stewart III of the Massachusetts Institute of Technology, says the nation may have gotten far ahead of itself in encouraging people to vote by mail.
Mail-in absentee voting accounted for 16 percent of ballots in the 2008 election.