Who is more persuasive: A person who expresses great certainty about his or her views, or a person who is less sure? If you are like most people, your intuition is that certainty makes you more persuasive. And this makes sense. A person who expresses certainty seems better informed; perhaps more credible. Most of us have had the experience of being persuaded by someone simply because they were so sure about what they were saying.

As it turns out, uncertainty can also be a powerful tool of persuasion. Yes, certainty can work well. It’s a psychological force that leads people to act on their beliefs. People who support a political candidate, for example, are more likely to vote for that candidate if they feel certain rather than uncertain about their belief. But uncertainty also plays a vital role. In fact, persuasion research reveals that in some situations people can make their own message more persuasive by explicitly noting that they are unsure about what they’re saying! The reason: Uncertainty draws us in. It causes us to pay more attention; to think more deeply about what’s going on.
In one study investigating the persuasive power of uncertainty, Uma Karmarkar, a professor of marketing at Harvard Business School, gave consumers a review for a new restaurant. The author of the review was very favorable. He gave the restaurant 4 stars and had compelling reasons for doing so, highlighting the top-notch food, excellent service, and charming ambience. Unbeknownst to participants, however, the experiment varied how certain the author was about his review. In some cases, the author was very certain (e.g., “I can confidently give 4 stars…”). In others, he expressed some doubt (e.g., “I don’t have complete confidence in my opinion, but I suppose I would give 4 stars…”). The result? When study participants believed the author was an expert—a food and dining critic—they formed more favorable attitudes toward the restaurant when he expressed uncertainty rather than certainty.

A little more digging revealed that the uncertain message was more persuasive precisely because it came from an expert. People expect experts to be certain of their views. When they’re not, it’s surprising. This surprise leads people to tune in. They want to figure out what’s going on, and they process the message more carefully. This, it turns out, is the key: Numerous studies in persuasion have shown that as long as the arguments in a message are compelling, getting people to process them more carefully can enhance their impact. The novel insight here is that an expert can encourage greater consideration of his or her message by expressing uncertainty. In another version of the study in which the author of the restaurant review was a non-expert, a highly certain review was more persuasive than an uncertain one. In this case, study participants found it surprising that a non-expert would express such high certainty and that led them to perk up and process the message more deeply.

The restaurant studies offer just one example of the upside of uncertainty in persuasion. In another paper, Taly Reich, a professor at Yale University’s School of Management, focused the lens on how contradicting yourself might affect your persuasiveness. Conventional wisdom dictates that contradicting oneself—for example, first opposing something and later supporting it—undermines one’s persuasiveness. It makes a person seem untrustworthy. Just ask John Kerry and Mitt Romney… It’s a classic political don’t. But Reich’s research indicates that, under some circumstances, contradicting oneself has the potential to boost one’s persuasiveness. Most notably, if people already trust you, you can get them to process your message more carefully if you contradict something you have said in the past. Why? Contradictions tend to be unexpected. If people trust you, that cues them to think more about what you’re saying (e.g., “I wonder why she changed her mind… maybe she got some new information.”). And again, if you have a compelling message, getting people to pay closer attention can enhance its effect.

Of course, if people distrust you, this process backfires as people chalk up your contradiction to pandering, flip-flopping, or general hypocrisy. This could help explain the divergent reactions to shifts in political candidates’ views over time. If voters trust the candidate, perhaps because he or she represents their party, they are far more likely to assume the candidate has received new information and updated his or her views. If voters do not trust the candidate, shifting views are seen as shifty politics.

As a final example, Boston University marketing professor Daniella Kupor’s work on the psychology of interruptions further reflects the power of uncertainty in persuasion. Imagine watching a video online, say a 2-minute clip from your favorite news website touting the newly discovered health benefits of nuts. In the opening moments, right after you learn that nuts have health benefits but before you find out what those benefits are, the video pauses for buffering. Just a few seconds, but the screen is frozen; the ubiquitous loading wheel is spinning. Then the video continues and describes a litany of positive health effects and explains about how much you should consume each day. How might the interruption—that pause for buffering—affect your response? Kupor’s studies indicate that as long as the interruption comes early, happens just once, and is brief, it can increase your intentions to eat nuts in the future. In other words, interrupting a message can make it more persuasive. Why would this happen? Just like a cliffhanger, the interruption seems to build curiosity about what’s coming next. This elevates interest in the material when it arrives, causing people to pay closer attention to it. As we’ve noted, that’s often the ticket to persuasion.

Until recently, most of the research on certainty and persuasion had emphasized that building people’s certainty is a great way to get them to take action. We now know that while it is true that certainty can prompt people to act, it is often uncertainty that prompts people to think. The evidence comes from many fields of study. In a series of experiments on emotions, for example, Larissa Tiedens and Susan Linton found that people put more effort into thinking about a variety of tasks when they experience emotions associated with uncertainty (e.g., worry, surprise) rather than certainty (e.g., anger, disgust). In a study of attraction, Erin Whitchurch, Timothy Wilson, and Daniel Gilbert observed that participants thought more about individuals when they were unsure how those individuals felt about them compared to when they knew those individuals liked them.

In persuasion, getting people to think about what you have to say is often a key challenge. You might have strong arguments for your position, but if people don’t pay attention to you or reflect on those arguments, they won’t be moved. Although counterintuitive at first blush, research has shown that creating some uncertainty in your message can enhance your message’s impact. It can pull people in and get them to think more about what you’re saying. Of course, if your arguments are weak, getting people to pay closer attention to them could backfire. Thus, it is still important to build a compelling case. Once you have that, though, uncertainty might be the crucial conduit that turns it into persuasion.