Last week at the outset of the vice-presidential debate, I offered a few thoughts at the New York Times Campaign Stops blog on how to use political science to inform your debate-watching.  Each of them could also prove useful tonight.

Popular people are more persuasive — an idea that social scientists refer to as “source credibility.” On this dimension, Biden and Ryan are essentially tied. Roughly equal proportions of people have favorable views of each. In Ryan’s case, negative opinions of him have becomemore prevalent than positive opinions since he was picked as Romney’s running mate. So neither starts the debate as the more popular figure.

The same is true for Obama and Romney now.

It’s easier to play on people’s existing opinions than try to convince them of something new. What’s the lesson here? Talk about the issues on which you’re already favored. For example, people tend to trust the Democratic Party and Obama to handle Medicare. The Republican line of attack on this issue—that Obama took money from Medicare to pay for Obamacare—may prove less persuasive. Better to talk about the deficit, for example, an issue on which voters tend to trust Romney rather than Obama.

As Biden might say, Biden “literally” confirmed this when, in talking about Medicare, he turned to the camera and said “trust your instincts” about who was telling the truth on Medicare.

I also thought of this point during the extended discussion about Afghanistan.  As I tweeted at the time, 60% of Americans and 47% of the public say we should not be involved in Afghanistan now.  The most effective policy from a public opinion perspective, no matters its specifics, is something that suggests we are leaving Afghanistan.

A lot of commentary during and after the debates is essentially a theater review—who “performed” better, who was more aggressive, or too aggressive, or whatever. We saw a lot of this after the first presidential debate. Unsurprisingly, then, voters’ subsequent reactions had little to do with policy and much more to do with personality. For better or worse, the candidates’ personalities and demeanor may count for more than their actual words.

And that was certainly true afterward, with the extended discussion of Biden’s smiles and interruptions.  I expect the same during and after tonight’s debate.

[Cross-posted at The Monkey Cage]

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John Sides

John Sides is an associate professor of political science at George Washington University.