Mandating voting has a clear effect: It raises participation rates. Before Australia adopted compulsory voting in 1924, for example, it had turnout rates similar to those of the U.S. After voting became mandatory, participation immediately jumped from 59 percent in the election of 1922 to 91 percent in the election of 1925.

The political scientists Lisa Hill and Jonathon Louth of the University of Adelaide note that “turnout rates among the voting age population in Australia have remained consistently high and against the trend of steadily declining voting participation in advanced democracies worldwide.”

For economists, the puzzle is not why voting participation rates are so low in voluntary systems, but why they’re so high. The so-called paradox of voting, highlighted in a 1957 book by the political scientist Anthony Downs, occurs because the probability that any individual voter can alter the outcome of an election is effectively zero. So if voting imposes any cost, in terms of time or hassle, a perfectly rational person would conclude it’s not worth doing. The problem is that if each person were to reach such a rational conclusion no one would vote, and the system would collapse…

Beyond simply raising participation, compulsory voting could alter the role of money in elections. Turn-out-the-vote efforts, often bankrolled by big-money groups, would become largely irrelevant. Negative advertising could be less effective, because a central aim of such ads is to discourage participation in the opponent’s camp.