What is the origin of nodding and shaking our heads to signal yes or no?

by colourC

I was reading an askreddit thread about the biggest culture shocks people have witnessed when a Bulgarian reditor said that in Bulgaria, unlike the UK, nodding meant "no", and shaking your head meant "yes." How did moving your head in a certain direction come to mean something, and how did the meanings change from culture to culture?

Stoycho

to be more precise - nodding downwards means - yes, upwards mean - no .. maybe this is the reason why some people got confused. shaking you head to aether side means - no. as a Bulgarian that's all i can say about it.