  We discuss the magnetic moments of the baryons allowing for flavor symmetry
breaking in the quark magnetic moments. We show that there is a correlation
between isospin symmetry breaking and data for the nucleon spin structure
obtained from deep inelastic scattering. For small values of the isospin
symmetry breaking, of the order of $5\%$, the magnetic moments and weak
axial-vector form factors alone indicate a value of the spin polarization
$\Delta\Sigma \leq 0.20$. Larger values of the spin polarization are compatible
only with large isospin symmetry breaking. We also calculate weak axial-vector
form factors, which are independent of the symmetry breakings, from magnetic
moment data and find good agreement with experiment.
