During the dictatorship in Chile, the authorities sought to discourage political participation and implemented voluntary and cumbersome voter registration procedures. Together with compulsory voting rules already in place, this created the unique combination of voluntary registration and compulsory voting.
I learned this from a newly published paper (gated) by my friend Paulo Cox and his coauthor Alejandro Corvalan. If one guessed, e.g. on the basis of rational voter theory, that this rule would cause registration and turnout to plummet, one would be right. There is more to the story in Chile, though, as voters registered en masse to vote Pinochet out of power in 1988. After that, however, the prediction has it right, with the result of a gap in turnout of around 60% (ca 90-30) between older and younger voters in the 2009 election.
Corvalan and Cox’ main concern is the representativeness of the electorate, and they show that income is highly correlated with registering and voting among the young (when controlling for education, etc). As the young become old, this “class bias” might come to hold for all voters.