According to a study from Ohio State University, nodding your head to signal approval, and shaking your head to signal disapproval not only lets others know what you're thinking, but also influences your own thoughts.
The study found that if you nod your head, even if you do it on purpose, you become more confident about your own thoughts, and if you shake it, you become less confident about them.
For example, the study's participants were instructed to listen to one of two editorials-one that was well-argued, and one that wasn't. When asked about it later, those participants who nodded their heads while listening agreed more strongly with the good editorial and disagreed more strongly with the poorly reasoned editorial.
By nodding, they confirmed their thoughts, positive or negative. And the same thing happened when people were asked to write with their dominant hand, and then with their non-dominant hand. When asked how confident they felt about the ideas they wrote down, those participants who wrote with their dominant hand felt more confident than those who didn't.
"Nodding or Shaking Your Head May Even Influence Your Own Thoughts" (ScienceDaily)