If you follow international news, you may be aware that, as part of the agreement for a new government, the Liberal Democrats have argued for a national referendum on the alternative vote, or what we would call in the United States (and what I will call in this post) instant runoff voting. Now, I'm no fan of IRV. But I think that, if the UK went this way, they might actually come out a bit better for it.
At first, I will be ignoring any non-outcome-based effects (i.e., costs) and holding everything else equal. And under these conditions, IRV would provide slightly better outcomes than plurality, given the assumption that a significant number of voters vote honestly rather than tactically. And, examining the election data for the UK, it seems that more voters are honest. There are two reasons that this may be the case.
One, the UK is a parliamentary system rather than a presidential one, so there is an extra layer of dis-connectedness between voters and the executive branch. This means there's less incentive to rally around or against the prime minister, and less direct control over them, meaning there is (or at least there is perceived to be) less to gain by voting tactically (this is, I think, a large reason for the LibDems success in the last several years).
Two, compared to the US, there is practically no polling done in the UK, except on the national level. And knowing that, say, 23% of people plan to vote for the LibDems, tells you practically nothing about what percentage of people will be voting for them in your local election, fancy new attempts to model such things not withstanding. Without that information, it's difficult to know how to vote tactically. Election results bear this out: there are a great many constituencies where tactical voters could have easily swayed the outcome in their benefit. So, given that there is and will continue to be significant honesty, the IRV results could be slightly more satisfying, based on Smith's simulation data.
But only slightly. And the elections will be more costly to count (perhaps only a little, perhaps 20% more). And this election marked a strong shift towards rallying around the party's would-be prime ministers (featuring the first-ever debate among them). And there's also a rising interest in local polling, partially caused by interest in IRV's possible benefits. Taking everything into account, there will likely be less election-satisfaction in the future no matter what, and if IRV is passed, it will probably be blamed on it, as a great deal of money will have been spent with little to show for it; which surely won't endear voters to the politicians who fought for— and over-sold the benefits of—it.
I wish someone with Nick Clegg's ear would tell him about approval and score voting. It would not be appreciably more costly, and would lead to markedly better outcomes (and probably more LibDems) regardless of voter honesty.