Subject: How is spin in quantum systems a direct result of special relativity?

I know that Dirac was the first to invent a quantum model which 
successfully incorporated special relativity. After reading a 
quote by him saying that spin was a relativistic phenomenon, I 
wondered in what fashion this arises. I know of the Stern Gerlach 
experiments, and how the spin property must be included to 
explain an electron's trajectory through them, and of spin for a 
photon relevant when passing through a polarizer. But aside from 
obvious mass variations of an electron orbiting a nucleus how 
would special relativity "create" spin? I'm only slightly 
familiar with his Transformation theory with vectors in N 
dimensional space, so go easy on that side.

Re: How is spin in quantum systems a direct result of special relativity?