The human mind is a very strange place.

Petty conducted the study with Pablo Brinol, a former doctoral student at Ohio State now at the Universidad Autonoma de Madrid in Spain. The research appears in the current issue of the Journal of Personality and Social Psychology.

In one study, the researchers told 82 college students that they were testing the sound quality of stereo headphones – particularly how the headphones would perform when they are being jostled, as during dancing or jogging.

Half the participants were told to move their heads up and down (nodding) about once per second while wearing the headphones. The other half was told to move their heads from side to side (shaking) while listening on the headphones.

All of the participants listened to a tape of a purported campus radio program that included music and a station editorial advocating that students be required to carry personal identification cards.

After listening to the tape, the participants rated the headphones and gave their opinions about the music and the editorial that they heard. The study found that head movements did affect whether they agreed with the editorial. But the effect is more complicated than might be expected.

The study found that nodding your head up and down is, in effect, telling yourself that you have confidence in your own thoughts – whether those thoughts are positive or negative. Shaking your head does the opposite: its gives people less confidence in their own thoughts.

So participants in this study who heard an editorial that made good arguments agreed more with the message when they were nodding in a “yes” manner than shaking in a “no” manner. This is because the nodding movements increased confidence in the favorable thoughts people had to the good arguments compared to shaking.

However, students who heard an editorial that made poor arguments showed the reverse pattern. These students agreed less with the message when they were nodding than when shaking. This is because the nodding movements increased confidence in the negative thoughts they had to the poor arguments compared to shaking.

Want to increase your self confidence on some subject? Think about it while nodding yes. Or listen or read someone else talk about it while nodding yes. Of course, if you are reaching your opinions on some subject without suffiicient critical thought or knowledge then it makes sense to decrease your confidence on that subject so that you try harder to learn enough to be right.

Also, if someone is trying to sell you on something and they are nodding yes while intereacting with you then keep in mind that their self confidence may be due more to the nodding than to their actually knowing what the heck they are talking about.