  We analyze within a nuclear database framework the shoulder observed in the
antineutrino spectra in current reactor experiments. We find that the
ENDF/B-VII.1 database predicts that the antineutrino shoulder arises from an
analogous shoulder in the aggregate fission beta spectra. In contrast, the
JEFF-3.1.1 database does not predict a shoulder for two out of three of the
modern reactor neutrino experiments, and the shoulder that is predicted by
JEFF-3.1.1 arises from $^{238}$U. We consider several possible origins of the
shoulder, and find possible explanations. For example, there could be a problem
with the measured aggregate beta spectra, or the harder neutron spectrum at a
light-water power reactor could affect the distribution of beta-decaying
isotopes. In addition to the fissile actinides, we find that $^{238}$U could
also play a significant role in distorting the total antineutrino spectrum.
Distinguishing these and quantifying whether there is an anomaly associated
with measured reactor neutrino signals will require new short-baseline
experiments, both at thermal reactors and at reactors with a sizable epithermal
neutron component.
