Study: Nodding or Shaking Head Changes Opinion

— It goes without saying that the people who read this column are all rational folks who reach conclusions based entirely on the evidence and they can't be influenced by such unrelated events as their own body language. There's not a chance, for example, that your convictions are likely to be affected by anything as irrelevant as nodding your head in agreement with someone else's statement, or shaking it back and forth in disagreement.

Don't bet on it, says psychologist Richard Petty of Ohio State University, whose research shows that what we think about something can indeed be influenced by simply nodding or shaking our head.

But Petty's findings are a bit confusing. It turns out that simply nodding yes doesn't necessarily mean we are more likely to agree with whatever we are hearing. It may have just the opposite effect, reaffirming our own opposing views.

Myriad of Influences

And shaking no doesn't necessarily mean we will disagree more with what we are hearing. We may be more inclined to agree with arguments that we would normally oppose.

The findings also reaffirm something else. Old Sigmund was right when he concluded that we are very complex creatures, subject to influence by a vast reservoir of data that we may not even know is there.

"There's so much that goes on at the unconscious level," Petty says. "Freud was right about that. He was wrong about a lot of the details, but he certainly was right in that there's so much going on [in our brains] that we're just not aware of."

Petty's research, which he carried out with former doctoral student Pablo Brinol, now at the Universidad Autonoma de Madrid, began with a study that seemed to reaffirm something psychologists have known for years. When we nod yes, we tend to agree with what we are hearing.

"That has been shown before," Petty says.

The surprise came when a proposal read to the participants was supported by very weak arguments. The effect of the head movements was reversed.

"People who were nodding their heads [in agreement] were actually more negative towards the proposal than people who were shaking their heads side to side," Petty says. So nodding yes made them disagree more than people who were shaking no in disagreement.

That's a little hard to visualize, but hang around for a minute or two.

A Matter of Confidence

To carry out the research Petty had to resort to a little subterfuge, a common practice among psychologists. The researchers told 82 college students who participated in the program that their job was to determine if simple head movements would degrade the sound quality of a new set of stereo headphones.

Half were told to nod their heads up and down once every second, and the others were told to shake their heads back and forth once a second. A recording of a campus radio program was transmitted through the headphones, featuring music and an editorial.

The editorial advocated increasing tuition, a subject that was sure to command the attention of the students. Half the participants heard reasonable arguments in favor of the increase (lower class size, more individual attention, better scholastic performance leading to better jobs) and the others heard really lame reasons for why they should support the increase.

"We told them we only had daffodils on the campus, and we needed the money so we could plant some tulips for variety," Petty says.

And it turns out that nodding yes gave the participants more confidence in their own convictions that the argument was absurd and no increase was necessary. But shaking their heads in the negative made them less sure of their own convictions and less likely to oppose the need for tulips.

"Nodding your head doesn't mean you'll agree with whatever you hear," he says. "One of the most surprising things we found is that if you're thinking negative thoughts while you're nodding, this actually strengthens your disapproval. What the head nodding is doing is making you more confident in your negative thoughts."

And that counterintuitive conclusion brings us to the question that so frequently confounds psychologists: What the heck is going on here? Why?

"We don't know for sure," Petty says, "but here's our speculation."

The Nod’s Subconscious Meaning

We're accustomed to using head movements as a form of communication with others. We generally nod in approval when we hear something we like, and shake in disapproval when we don't like whatever we're seeing or hearing. That becomes ingrained in the subconscious. Nodding means yes, shaking means no.

So in addition to communicating with someone else, either agreeing or disagreeing with them, we're also looking inward to see if our convictions really do agree or disagree. If we believe something strongly, we believe it more deeply than ever if we nod in the affirmative, even in the face of strong arguments to the contrary.

But if we're not too sure of our convictions, we're more likely to lose confidence in our own beliefs, even if the arguments against our convictions stink.

Petty reached his conclusions through a standardized test designed to measure the difference between the groups that heard strong arguments and those who heard weak arguments. The result, he says, revealed "at least a 95 percent confidence level that these effects are not random." That was good enough to get the study published in the Journal of Personality and Social Psychology.

By the way, when Petty finally told the participants their convictions were influenced by simple head movements, they all had the same reaction: Ridiculous.

"Almost every subject, to a person, thought that," he says.

That might be true for the other participants, each one thought, "but it certainly wouldn't affect me," they told the researchers. They were all, to a person, purely rational beings.

Just like us, right?

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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