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.