“What’s most important is: what gets the job done…. I don’t think that the Democratic Party has a monopoly on good ideas. I think that the Republicans have a lot to offer. And what I will do is listen and learn from my Republican colleagues, and any time they can make a case that this is something that would be good for the American people, just because Democrats didn’t think of it, and Republicans are promoting it, that’s not a good reason not to do it. If somebody presents to me a plan that they are ideologically wedded to, but they can’t persuade me that this will actually be good for the economy, then we’re not going to do it.”

President-elect Barack Obama made this remark when asked about his willingness to incorporate ideas offered by the Republican Party into his plan for improving the American economy (Harwood, 2009). And technically he is correct: Logic suggests that a good argument should be persuasive regardless of who delivers it. Although this view is admirable, messages delivered by likable, attractive, powerful, and famous sources are more often met with approval than the same messages delivered by sources who lack these characteristics (Chaiken, 1979, 1980; French & Raven, 1959; Whittler & Spira, 2002).

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Messages are especially persuasive when they are delivered by a person (or a group) perceived to have source credibility—that is, someone who is both expert and trustworthy. Consider a study by Hovland and Weiss (1951). Participants read articles that advocated various positions on different issues. For example, one article advocated that antihistamine drugs should be sold without a doctor’s prescription. Half the participants were told the article was from the New England Journal of Biology and Medicine (an expert source), whereas the others were told it came from a popular magazine such as those you see in the supermarket (a nonexpert source). As you might expect, participants were more likely to agree with the position advocated in the articles if they thought the articles came from expert sources rather than nonexpert sources (Pornpitakpan, 2004).

Although real expertise can convey strong arguments that persuade people through central-route processing, the appearance of expertise often persuades through the peripheral route. For example, a commercial advertising mouthwash may feature a spokesperson wearing a white lab coat and a stethoscope around his neck, giving him the appearance of a medical expert when in reality he is most likely an actor made to look like an expert. A viewer who takes the peripheral route without paying close attention to the substantive claims being made might just assume that the mouthwash is effective because the spokesperson appears to know what he is talking about. Indeed, speakers can appear more credible, and thus be more persuasive, simply by speaking with confidence, quoting statistics, or even just by speaking quickly—none of which necessarily means that they have expert knowledge (Erickson et al., 1978; Miller et al., 1976).

The source of a message can also gain credibility by being perceived as trustworthy or unbiased in his or her views. Most of us are aware that other people typically have something to gain from changing our attitudes, and so we often are skeptical about whether they are telling the whole truth and nothing but the truth. In fact, if people are forewarned that another person intends to persuade them about issues they care about, they are less convinced by that person’s arguments (Petty & Cacioppo, 1979).

How can the source of a persuasive message come across as trustworthy? One way is to express an opinion without the audience realizing that it is the target of persuasion. Imagine that you overhear some people at a coffee shop praising a new restaurant. You have no reason to believe that these people are biasing their opinions to influence you (they don’t even know you’re eavesdropping!), and so you might find their opinions more believable. In a study testing this intuition, Walster and Festinger (1962) arranged a situation so that participants were allowed to overhear a conversation between two graduate students in another room. (In actuality, they heard an audio recording.) Some participants were under the impression that the graduate students were aware that they were in the next room, whereas other participants assumed that their presence was unknown to the graduate students. Participants in the second group was more likely to change their attitudes in the direction of the graduate students’ opinions, presumably because they had no reason to believe that the graduate students were purposely altering what they said.

The power of overheard messages hasn’t been lost on advertisers. Just think of all the commercials you’ve seen that attempt to portray a private conversation between two people (“Gee, Tyler, your glasses really do sparkle! What dish detergent do you use?”). In fact, advertising and public relations agencies often pay writers to masquerade online as delighted consumers, writing positive reviews of their products or services on various web sites such as Amazon.com (Dellarocas, 2006). The writers are paid to appear like average people, for example by intentionally inserting typos into their reviews. In this way, advertisers hope that the fake reviewers appear trustworthy, and thus persuasive, because the writers do not seem to have a stake in the sale of the product or service.

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Another way in which a communicator gains in trustworthiness (and persuasiveness) is if he or she argues in favor of a position that seems to be opposed to his or her self-interest. Imagine that you heard a convicted criminal giving a speech advocating more lenient prison sentences. Even if he is making some good arguments, you might not be very persuaded because you know that he personally has something to gain from convincing you of his position. But now imagine that the same criminal is arguing for stricter prison sentences. Here you might conclude that, because he has nothing to gain, and perhaps something to lose, by convincing others of his point, he must believe strongly in the truth of his claims. This intuition was put to the test in another study by Walster and colleagues (1966). When participants read a statement by “Joe ‘The Shoulder’ Napolitano,” a habitual criminal and drug dealer, arguing for more lenient court proceedings, they were completely unconvinced: Of course a convict would argue in favor of more lenient courts! But when the same alleged criminal argued for stricter court proceedings, participants were much more likely to agree with his position.

Credibility can be a peripheral cue when people are not elaborating on the persuasive message. It’s a handy heuristic simply to decide, “She’s an unbiased expert, I’ll believe that,” or “I’m not buying anything that sleazeball says.” However, this kind of peripheral influence of source credibility may not last long. The decay of source effects can happen when over time, people forget the source of the message but remember the message content, a concept known as the sleeper effect. Suppose that you scan the front page of a sensationalistic gossip magazine while waiting on the grocery checkout line (come on, we all do it at least occasionally), and it claims that new evidence calls global warming into question. A few weeks later, the topic of global warming comes up in a conversation, and you remark that you read a story arguing that global warming may in fact not be happening, but you can’t remember where you read it. Might that argument be more compelling now that you’ve forgotten that it came from a not-so-credible source?

Hovland and Weiss (1951) found the first evidence of the sleeper effect. Recall the experiment showing that people were more likely to agree with high-credibility sources (e.g., a medical journal) than with low-credibility sources. When the researchers again measured participants’ attitudes four weeks after they were first exposed to the arguments, participants who initially had agreed with a position advocated by a high-credibility source showed less agreement, and those who disagreed with a low-credibility source had come to agree with the position (see FIGURE 8.4). Subsequent research has clarified that such sleeper effects are most likely when people learn about the credibility of the source after they have been exposed to the arguments (Pratkanis, 2007). When people know in advance that an argument comes from a low-credibility source, they are better able to discount the argument.

Your current author recently saw a commercial in which the actor Dennis Haysbert recommended buying insurance from Allstate Insurance Company. In this context, Haysbert is not particularly high in credibility. First, although he may be an expert actor, having appeared in many movies and television series (e.g., he played the president of the United States in the popular TV show 24), who knows whether or not he has expert knowledge of the relative merits of different insurance companies? Second, his trustworthiness is in doubt: It is obvious that Allstate is paying him handsomely to appear in these commercials and that he is trying to influence us. So why is Haysbert considered an effective spokesman? If you’ve seen the commercials, you probably know the answer: He just seems like a likeable and straightforward guy. He is handsome and well dressed, and he looks straight at you as he asks you in his authoritative baritone voice whether you are in “good hands.” This example suggests that communicators can be persuasive when they are attractive, even if their credibility is low.

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The most obvious way that a communicator gains in attractiveness is by presenting an attractive physical appearance. Certainly you’ve noticed how magazines, billboards, and pretty much every other commercial medium features attractive models (some of which are computer-generated images rather than actual people). But does it work? Yes. People are more persuaded by arguments that come from communicators they consider physically attractive, even when the arguments are wholly unrelated to the communicator’s attractiveness (Chaiken, 1979; Mills & Aronson, 1965).

If we tend to be persuaded by those who are trustworthy, likeable, and attractive, we must also be more persuaded by people who are similar to us, right? Here the answer is a bit more complicated. In some cases, people are more easily persuaded when the source of a message is similar to themselves. In one study (Mackie et al., 1990), college students were more likely to agree with rather extreme positions (e.g., to discontinue use of the SATs in college admissions) if those positions were endorsed by a student from their own university, but not from another university.

Sometimes, however, people are influenced more by the opinions of dissimilar others than those of similar others. The key factor is whether the issue at hand deals primarily with subjective preference or objective facts (Goethals & Nelson, 1973). For issues dealing primarily with subjective preference—that is, when there is no “correct” answer—people are more confident in their attitude when similar others agree with them than when dissimilar others agree with them. So if you believe that Beyoncé is the greatest recording artist alive, agreement from a similar other, such as a close friend, would make you more confident in that attitude than agreement from a dissimilar other, such as a foreign exchange student.

In contrast, when people are trying to determine whether something is objectively true or factual, their attitudes are influenced more by the opinions of dissimilar others than by those of similar others. For instance, if you believe that Beyoncé sold more albums than Rihanna (which is either factually correct or incorrect), agreement from a dissimilar other would make you more confident in that belief than agreement from a similar other. Why? When similar others agree with us on matters of fact, we are often uncertain whether they agree simply because they like us, or perhaps because they have been exposed to the same faulty information that we have been exposed to. But when a dissimilar other who does not have those biases verifies our belief, we assume that the belief must be objectively true. (In case you were wondering, at least when it comes to studio albums, Beyoncé has sold more albums than Rihanna.)

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