DAVID GREENE, HOST:
As in all the previous Republican primaries and caucuses this campaign season, negative ads have played a very prominent role in Wisconsin. But what is the real impact of ads like these?
(SOUNDBITE OF AD)
UNIDENTIFIED WOMAN: On the economy, Rick Santorum says...
RICK SANTORUM: I don't care what the unemployment rate is going to be.
(SOUNDBITE OF AD)
UNIDENTIFIED MAN: Romney raised job-killing taxes and fees by over $700 million.
GREENE: Experts say these ads often do what they set out to do. Last week on this program, Joe Heim, a political science professor at the University of Wisconsin-La Crosse, put it this way.
JOE HEIM: I think they're effective. When people are undecided or they're not real strongly committed towards one candidate or the other, they can be very effective.
GREENE: Well, given the prominent role we know negative ads will be playing throughout the fall, we wanted to dig into this issue a little further.
NPR's Shankar Vedantam tracks the latest social science research, and he's back with us this morning. Hi, Shankar.
SHANKAR VEDANTAM, BYLINE: Hi, David.
GREENE: So are negative ads effective, based on the research you've seen?
VEDANTAM: You know, if you'd asked me this question a couple of weeks ago, David, I would have said absolutely yes. You know, I thought that negative ads were the atomic bombs of political campaigns, that they were highly decisive and highly effective.
But then I went and looked at the scientific studies on negative advertising and I found something actually quite different, which is that if you look at a large number of races and the role of negative advertising in them, it turns out that negative advertising is nowhere as effective as most people think, that at best you can say it's effectiveness is highly inconsistent.
GREENE: Well, Professor Heim, this political scientist in Wisconsin who we just heard from, he seemed to be making a distinction, saying that these negative ads can be effective earlier in a campaign. Tell us why that might be.
VEDANTAM: So I think that's exactly right. So negative ads might be very effective at shaping how voters think of candidates very early in a race. But once voters have a clear sense of what candidate they support, negative ads are not very good at reshaping those views.
And if you look at the current GOP race, I think you can show that Romney and his supporters have a much lower return on investment from negative ads being run now in states like Illinois or even Wisconsin today than they did back in Iowa and Florida, when the race was much more widely open.
And I spoke with this political scientist called John Sides, who works at George Washington University. He pointed out that when voters develop loyalties to a candidate, it is very, very difficult to dislodge them from those loyalties with facts, including negative facts. Here he is.
JOHN SIDES: When voters are confronted with inconvenient facts, it's oftentimes difficult to persuade them that those facts are in fact facts. When supporters of President Obama see negative information about Obama, they don't think that it's true; to the extent that it seems true, they find ways to explain it or rationalize it. They discount it.
GREENE: And I suppose that that might true for President Obama going into the fall, and if Romney is the Republican nominee, people will surely know him. He might be a lot less vulnerable to negative ads as well.
VEDANTAM: I think that's exactly right. So most of the country probably by September is going to know whom they're going to vote for and $500 million of negative ads, you know, probably is not going to dislodge them.
GREENE: OK. Well, if negative ads don't really pay off in the end, especially when we get towards the fall, why do politicians - why do their supporters always seem to go down that road? Why do they bother?
VEDANTAM: You know, I've heard a couple of theories, and the first is that this is an arms race and that candidates are unwilling to unilaterally disarm.
GREENE: The cold war of American politics, it sounds like.
VEDANTAM: Exactly. And so even if you believe that negative ads are not very effective, if you know that your opponent is going to be using them a lot, you're very hesitant to hang up your guns. But I heard a second explanation, and that comes from John Sides again at George Washington University. Here he is again.
SIDES: A concern than I have is that a lot of our accounts and understanding about the effectiveness of advertising comes from people who have a professional interest in selling advertising to candidates and who don't necessarily have the need or the time or the inclination to do a lot of rigorous research into the decisions that they're making.
GREENE: That almost makes it sounds like campaigns could be hood-winked by advertising firms who convince them the negative ads are a good idea and want to sell those ads.
VEDANTAM: Yeah, I mean hood-winked might be too strong a term. It may be these advertising executives genuinely believe that negative advertising works. But I think what's fair to say is that contention is really contradicted by the scientific literature.
GREENE: All right. Always interesting to talk to you, Shankar. Thanks for being here.
VEDANTAM: Thanks, David.
GREENE: That's NPR's science correspondent Shankar Vedantam. He joins us regularly to talk about the trends in social science research. And you can give him ideas and feedback if you'd like on Twitter @HiddenBrain. And while you're at it, follow this program @MorningEdition.