The quadratic form of the Dirac equation in a Riemann spacetime yields a
gravitational gyromagnetic ratio \kappa_S = 2 for the interaction of a Dirac
spinor with curvature. A gravitational gyromagnetic ratio \kappa_S = 1 is also
found for the interaction of a vector field with curvature. It is shown that
the Dirac equation in a curved background can be obtained as the square--root
of the corresponding vector field equation only if the gravitational
gyromagnetic ratios are properly taken into account.Comment: 8 pages, RevTeX Style, no figures, changed presentation -- now
restricted to fields of spin 0, 1/2 and 1 -- some references adde