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In excitable pundits vs. political scientists, I’ll take political scientists every time

Political scientists will tell you that debates don't usually decide presidential elections, or even lead to noticeable changes in the polls. In their huge survey of every publicly available poll in the last 15 presidential campaigns, Robert Erikson and Christopher Wlezien concluded, “there is no case where we can trace a substantial shift to the debates.” A study by James Stimson came to much the same conclusion.

But Mitt Romney's lopsided win in the first presidential debate is causing some to turn their guns on the earnest researchers. "Political science proclaims, 'debates don't matter,'" writes David Frum. "After this election, we may need to retire a lot of political science."


For the record, I'll take political scientists looking at thousands of polls over multiple elections over pundits freaking out after the first few polls following the first presidential debate. This, in my view, is exactly what political science does well: Reminds you to calm down, to take the long view, and to ask yourself whether you think this election will break from historical precedent, and if so, why.

Frum doesn't actually link to any political scientists saying "debates don't matter," and to my knowledge, none have said that. What they have said is, in the words of George Washington University's John Sides, "when it comes to shifting enough votes to decide the outcome of the election, presidential debates have rarely, if ever, mattered."

Note that if this election proves to be the rare instance in which the debates really did decide the election, that doesn't make the political scientists wrong. The fact that something only happens rarely doesn't mean it doesn't happen. The fact that something hasn't happened before doesn't mean it can't happen in the future. The past is a guide, not a promise.

Perhaps last Wednesday's presidential debate will decide this election. According to Gallup, which has also thrown cold water on the idea that debates routinely decide elections, Romney's victory in the debate was more lopsided than any other debate victory in the history of their polling. So maybe this really is a game-changer.

But then again, maybe not. When political scientists say debates don't tend to decide elections, note that they use the plural. They're looking at the change in polls from the beginning of the debates to the end, not the change in polls from the beginning of the debates to the end of the first debate. 

There's a difference between a post-debate "bounce" that fades after a couple of days and a post-debate "bump" that persists until the next debate. Erikson and Wlezien, for instance, use the polls taken 8 to 14 days after the first debate in order to separate signal from noise. There's an even bigger difference between a post-debate bounce that fades within a few days and a post-debate bump that persists through all of the debates. We just don't know which Romney has as of yet.

Romney took home a big win in the first debate. But there's going to be a second debate between him and Obama. And a third. And there's going to be a debate between Joe Biden and Paul Ryan. It's possible that Romney and Ryan will dominate each and every one of these encounters, but given the unusual rout we saw in the first debate, I think it's likelier that we'll see some mean reversion: Obama will perform better than he did in his first outing, and make up a bit of the ground that he lost. But perhaps not.

Typically -- and this is something I only learned after playing around with Erikson and Wlezien's data for a while -- the change in the polls after the first debate is smaller than the change in the polls after all the debates. That might imply that Romney's lead will grow rather than shrink.

Note that the change after the final debate is typically larger than the change after the first debate. (Data: Erikson and Wlezien)

But another way to read this data -- the way Erikson and Wlezien read it -- is that elections tend to tighten in the homestretch. How that will interact with a first debate that may have flipped the polls, at least temporarily, is anyone's guess. Perhaps it means Romney will increase his margins. Perhaps it means that the tightening now will come from Romney's newfound support, putting Obama back in the lead. We'll need to wait for more data to see.

I would say that the last week has been an object lesson in why it's worth paying attention to the evidence gathered by political scientists and tuning out some of the more excitable pundits. Pundits have every incentive to make sweeping pronouncements based off incomplete data. The work political scientists have done gives us some body of past evidence against which we can check those sweeping pronouncements. It's too early to say how much this debate mattered, but the wild reaction it's generated among political pundits has convinced me, more than ever, that political science matters. 



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Brad Plumer · October 10, 2012

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