  We study the quark helicity distributions at large x in perturbative QCD,
taking into account contributions from the valence Fock states of the nucleon
which have nonzero orbital angular momentum. These states are necessary to have
a nonzero anomalous magnetic moment. We find that the quark orbital angular
momentum contributes a large logarithm to the negative helicity quark
distributions in addition to its power behavior, scaling as (1-x)^5\log^2(1-x)
in the limit of x\to 1. Our analysis shows that the ratio of the polarized over
unpolarized down quark distributions, \Delta d/d, will still approach 1 in this
limit. By comparing with the experimental data, we find that this ratio should
cross zero at x\approx 0.75.
