What is an "Inner Chimp"?

Apparently Joey Barton has one. Why on earth would a supposedly rational person describe poor behaviour on his part as having an "inner chimp"? Do Chimpanzees behave worse than Gorillas? How has he managed to have a career that pays him far better than the average schoolteacher ever will be? We live in a strange world.

I'm guessing he got it directly or indirectly from http://www.amazon.co.uk/The-Chimp-Paradox-Management-Confidence/dp/009193558X/ref=sr_1_1?s=books&ie=UTF8&qid=1330876655&sr=1-1