Anomalous magnetic moment and limits on fermion substructure

Experimental constraints on possible lepton and quark substructure are analyzed and expressed in terms of a general formalism for describing composite particles in terms of their constituents. In particular, the measured gyromagnetic ratios may very severely restrict possible internal structure of light leptons (electrons and muons) in some models. Simple expressions for hadronic $g$ values and electromagnetic radii are given in terms of their quarkgluon infinite-momentum-frame wave function. The contribution to the anomalous moment of a fermion due to internal structure is shown to vanish as the mass or inverse-size scale of the internal state becomes infinitely large.