  We study the dominant forbidden transitions in the antineutrino spectra of
the fission actinides from 4 MeV onward using the nuclear shell model. Through
explicit calculation of the shape factor, taking into account Coulomb
corrections, we show the expected changes on cumulative electron and
antineutrino spectra. Compared to the usual allowed approximation this results
in a minor decrease of electron spectra from 4 MeV and onward, whereas an
increase of several percent is observed in antineutrino spectra. We show that,
despite their limited number, forbidden transitions dominate the spectral flux
for most of the experimentally accessible range. Based on the shell model
calculations we attempt a parametrization of forbidden transitions and propose
a spectral correction for all forbidden transitions. We enforce correspondence
with the ILL dataset using a summation+conversion approach. When compared
against modern reactor neutrino experiments, the resultant spectral change is
observed to be of comparable magnitude and shape as the reported spectral
shoulder, drastically decreasing the statistical significance of the latter.
