Question

Where did the idea of the right/left side of the brain originate from?

Answer 1

The whole right/left brain idea is probably a myth as of research done in 2013

Explanation:

Recent research based on brain scans of over 1000 brains did not reveal any right or left brain preference in people. People use their entire brain regions. It is probably one of those myths that got going back in the 1960s, before modern neuroscience had the tools it had today.

See: http://www.livescience.com/39373-left-brain-right-brain-myth.html