  Bulk nuclear observables such as charge radii and binding energies are well
described by both nonrelativistic and covariant mean-field models. However,
predictions of neutron radii, which are not tightly constrained by reliable
data, vary significantly. The nature of this variation is investigated using
correlations between basic properties of the models and the neutron skin
thickness in lead. The results suggest that conventional covariant models are
too limited. The study is guided by principles and insights of effective field
theory (EFT), such as power counting, and the relation of mean-field models to
a more general EFT approach to nuclei is discussed.
