  The Adler sum rule for deep inelastic neutrino scattering measures the
isospin of the nucleon and is hence exact. By contrast, the corresponding
Gottfried sum rule for charged lepton scattering was based merely on a valence
picture and is modified both by perturbative and non-perturbative effects.
Noting that the known perturbative corrections to two-loop order are suppressed
by a factor 1/N_c^2, relative to those for higher moments, we propose that this
suppression persists at higher orders and also applies to higher-twist effects.
Moreover, we propose that the differences between the corresponding radiative
corrections to higher non-singlet moments in charged-lepton and neutrino deep
inelastic scattering are suppressed by 1/N_c^2, in all orders of perturbation
theory. For the first moment, in the Gottfried sum rule, the substantial
discrepancy between the measured value and the valence-model expectation may be
attributed to an intrinsic isospin asymmetry in the nucleon sea, as is indeed
the case in a chiral-soliton model, where the discrepancy persists in the limit
N_c-->infinity.
