  PT-symmetric systems can have a real spectrum even when their Hamiltonian is
non-hermitian, but develop a complex spectrum when the degree of
non-hermiticity increases. Here we utilize random-matrix theory to show that
this spontaneous PT-symmetry breaking can occur via two distinct mechanisms,
whose predominance is associated to different universality classes. Present
optical experiments fall into the orthogonal class, where symmetry-induced
level crossings render the characteristic absorption rate independent of the
coupling strength between the symmetry-related parts of the system.
