Why do people shake their head?
Just like with the head nod, the speed and rhythm of the motion gives us hints about the manner of the listener:
A fast and rhythmic head shaking definitely says - “no, I disagree, this is not true.â€
A slow and irregular turning of the head usually signals misunderstanding. Something didn't get through quite right.
Slow and rhythmic head shaking can also signal disbelief. We cannot accept what we just heard.
So far so good, but sometimes the head shake is only barely visible. It can be quick or nonchalant, it doesn't require a “complete†head shaking from side to side , a single turn of the head still can signal “no†- as if the listener got slapped by the words he just heard. I mention this because most people avoid shaking their head like a cartoon character, and often culturally raised with the idea that it's impolite or inappropriate.
/Thread
Stolen from:
http://www.study-body-language.com/head-shaking.html