  The first moment of longitudinal and transverse spin densities of quarks in
the nucleon are calculated in a light-front constituent quark model for the
different cases of quark and nucleon polarization. Significant distortions are
found for the transverse spin densities. In particular the Sivers function is
predicted with opposite sign for up and down quarks and the Boer-Mulders
function is predicted large and negative for both up and down quarks, in
agreement with lattice calculations. Quite a different spin distribution is
obtained for up and down quarks in the cases of quarks and proton transversely
or longitudinally polarized in the same direction.
