  Empirical drops in ground-state nuclear polarizabilities indicate deviations
from the effect of giant dipole resonances and may reveal the presence of shell
effects in semi-magic nuclei with neutron magic numbers $N=50$, 82 and 126.
Similar drops of polarizability in the quasi-continuum of nuclei with, or close
to, magic numbers $N=28$, 50 and 82, could reflect the continuing influence of
shell closures up to the nucleon separation energy. These findings open a new
avenue to investigating magic numbers at high-excitation energies and strongly
support recent large-scale shell-model calculations in the quasi-continuum
region, which describe the origin of the low-energy enhancement of the photon
strength function as induced paramagnetism. The nuclear-structure dependence of
the photon-strength function asserts the generalized Brink-Axel hypothesis as
more universal than originally expected.
