Spin half in classical general relativity
  It is shown that models of elementary particles in classical general relativity (geons) will naturally have the transformation properties of a spinor if the spacetime manifold is not time orientable. From a purely pragmatic interpretation of quantum theory this explains why spinor fields are needed to represent particles. The models are based entirely on classical general relativity and are motivated by the suggestion that the lack of a time-orientation could be the origin of quantum phenomena. 
Mark J. Hadley