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Paul Fairie is a political scientist in Calgary.

If the Liberals follow through on their promise to reform Canada's electoral system, it will change not only how Canadians mark their ballots, but also how parties compete with one another and even what choices voters make.

One common proposal is to move to a system called instant-runoff voting (IRV). Just like our current situation, IRV sends a single member to Ottawa from each constituency. The difference is that instead of simply marking off a first choice, voters rank the candidates in order.

If after counting everybody's first choice, no candidate has a majority of the votes, the candidate with the fewest votes is eliminated. After this, the second choices of voters who supported the eliminated candidate get redistributed to the remaining candidates. This process is repeated until a candidate has a majority of the votes. This is how many political parties in Canada choose their leaders, how Australia elects its House of Representatives, and how Ireland chooses its president.

In the 2015 election, it is likely that the Liberals would have won even more seats than they did on election night. Using second-choice data from pollsters Ipsos and Nanos, and applying the rules of instant-runoff voting, the Liberals would have gained around 20 seats and the New Democrats would have picked up one or two, with the losses coming from both the Conservatives and Bloc Québécois.

The gains by the Liberals in this scenario would have come about because not only did they do well as voters' first choice, the electorate was also very polarized about the Conservatives, who had relatively meagre second-choice support. Liberal voters preferred the NDP to the Conservatives as their second choice at a ratio of better than two to one, while New Democrats preferred the Liberals to the Conservatives as their second choice at a ratio of greater than five to one.

The kind of riding that would have likely seen a different result under IRV is exemplified by the British Columbia constituency of Central Okanagan–Similkameen–Nicola. In the recent election, the Conservatives won with almost 40 per cent of the vote, with the Liberals a close second, earning 37 per cent. The remaining 23 per cent was won by the New Democrats and Greens. Ipsos data suggests that the second choice of New Democrats in B.C. was overwhelmingly the Liberal Party, the split being 59 per cent Liberal and just 3 per cent Conservative. If a similar division occurred in this riding, the Liberal, and not the Conservative, would have been sent to Ottawa.

However, this shouldn't be seen as evidence that the Liberals would always do better under IRV than they do under our current system. In 1984, when the Liberals were swept from power by the Mulroney-led PCs, they would have almost certainly done even worse under IVR because they scored poorly on questions of second choice. According to the Canadian Election Study carried out by academics across the country, 51 per cent of Progressive Conservative voters said the NDP was their second choice in 1984, while 52 per cent of NDP voters said the same was true for the PCs.

The old Progressive Conservatives might have even been able to survive longer after their 1993 election disaster where they only won two seats. In 1997, they were strongly the second choice of both Liberal and Bloc voters, and they performed similarly well in 2000.

History also warns against the wisdom of seeing electoral-system reform as a good strategic move. British Columbia moved to IRV for their 1952 and 1953 elections, partly because the Liberals and Conservatives both wanted to ensure that the more left-wing Co-operative Commonwealth Federation (later known as the NDP) remained out of power. This backfired when many Liberal, Conservative and CCF voters unexpectedly supported the upstart Social Credit Party as their second choice, ultimately giving them the most seats in a minority Parliament.

Barring a repeat of that unusual situation, a move to IRV in Canadian elections would most normally result in parliaments similar to what we have had in recent years. The only visible changes would be in ridings where two or three similar parties are currently placing second, third and fourth, as well as in ridings that elect members who win with less than 35 per cent of the vote. It would also result in majority and minority parliaments at roughly the same frequency as we have now, and smaller parties would still be shut out of Parliament.

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