Anomalous Magnetic Moments and Quark Orbital Angular Momentum
  We derive an inequality for the distribution of quarks with non-zero orbital angular momentum, and thus demonstrate, in a model-independent way, that a non-vanishing anomalous magnetic moment requires both a non-zero size of the target as well as the presence of wave function components with quark orbital angular momentum L_z>0. 
Matthias Burkardt and Gunar Schnell