In practice, what is compulsory is not casting a valid vote, but going to the polling place, having one's name checked off and putting a ballot paper in the box. The secrecy of the ballot makes it impossible to prevent people writing nonsense on their ballot papers or leaving them blank. While the percentage of invalid votes is a little higher where voting is compulsory, it comes nowhere near offsetting the difference in voter turnout. Compulsory voting is not unique to Australia. Belgium and Argentina introduced it earlier, and it is practised in many other countries, especially in Latin America, although both sanctions and enforcement vary.

Because I was in the United States at the time of the Australian election, I was under no compulsion to vote. I had many reasons to hope for the defeat of John Howard's conservative government, but that doesn't explain why I went to some trouble to vote, since the likelihood that my vote would make any difference was minuscule - and, predictably, it did not.

When voting is voluntary, and the chance that the result will be determined by any single person's vote is extremely low, even the smallest cost - for example, the time it takes to stroll down to the polling place, wait in line and cast a ballot - is sufficient to make voting seem irrational. Yet if many people followed this line of reasoning and did not vote, a minority of the population could determine a country's future, leaving a discontented majority.

Poland's recent electoral history provides an example. In the 2005 national election, barely 40 per cent of those eligible voted - the lowest total since the advent of free elections after the communist period. As a result, Jaroslaw Kaczynski was able to become prime minister with the support of a coalition of parties that gained a majority of seats in parliament, despite receiving only 6 million votes out of a total of 30 million eligible voters.

When Mr Kaczynski was forced to go to the polls again only two years later, it became evident that many of those who had not voted in 2005 were unhappy with the outcome. Turnout rose to nearly 54 per cent, with the increase especially marked among younger and better-educated voters. Mr Kaczynski's government suffered a heavy defeat.

If we don't want a small minority to determine our government, we will favour a high turnout. Yet, since our own vote makes such a tiny contribution to the outcome, each of us still faces the temptation to get a free ride, not bothering to vote while hoping that enough other people will vote to keep democracy robust.

But there are many possible reasons for voting. Some people vote because they enjoy it, and would have nothing better to do with the time saved if they did not. Others are motivated by a sense of civic duty that does not assess the rationality of voting in terms of the possible impact of one's own ballot.

Still others might vote not because they imagine that they will determine the outcome of the election, but because, like football fans, they want to cheer their team on. They may vote because, if they don't, they will be in no position to complain if they don't like the government that is elected.

If these considerations fail to get people to the polls, however, compulsory voting is one way of overcoming the free-rider problem. The small cost imposed on not voting makes it rational for everyone to vote and, at the same time, establishes a social norm of voting. Australians want to be coerced into voting. They are happy to vote, knowing that everyone else is voting, too.

Countries worried about low voter turnout would do well to consider the compulsory model.

Peter Singer is professor of bioethics at Princeton University.

Copyright: Project Syndicate