See-saw and Supersymmetry or Exact R-parity

We show how the renormalizable see-saw mechanism in the context of supersymmetry and spontaneously broken B-L symmetry implies exact R-parity at all energies. We argue that supersymmetry plays an important role in providing a "canonical" form for the see-saw, in particular in grand unified theories that solve the doublet-triplet splitting problem via the Dimopoulos-Wilczek mechanism.