In this paper we present a theory which accounts for both the intensity and the shape of the experimentally observed giant polarization anisotropy of the optical absorption in (InGa)As-InP superlattices. By first-principles self-consistent calculations of the superlattice crystal potential and of the dielectric tensor elements we derive a selection rule'' by which the radiation polarization along the 110 and -110 directions choose only one optical transition of the doublet corresponding to transitions to the conduction band from the two upper crystal field split hole states which are degenerate in common atom superlattices. However, due to the similar intensity of the oscillator strength of these transitions, it turns out that the magnitude of the anisotropy depends only on the magnitude of the energy gap between the split states.