  We derive the spin-statistics theorem in both relativistic and
non-relativistic first-quantized form, extending considerably the earlier
proofs. Our derivation is based on the representation theories of the groups SU
(2) and SL(2,C), latter being the universal covering of the Lorentz group. We
define the identity of particles. We include theories that have an internal
symmetry group. We discuss consistency of the non-relativistic limit.
