  The observed deep inelastic nucleon spin asymmetries $A_{1}^{p,n}$ and the
spin dependent quark distributions break SU(6) spin - flavour symmetry. In
quark models two mechanisms for breaking SU(6) symmetry are well known: the
hyperfine interaction and the pion cloud of the nucleon. I re-evaluate these
mechanisms and show how the breaking of SU(6) in each case affects spin
dependent valence quark distributions of the proton. In particular I
investigate the properties of these mechanisms in the kinematic region $0.3 < x
< 0.7$ and attempt to test these against known data. Both mechanisms are able
to explain the quantitative features of the spin asymmetries.
