Saturation of the Drell--Hearn--Gerasimov sum rule

The Drell-Hearn-Gerasimov sum rule for the forward spin-flip amplitude of nucleon Compton scattering is decomposed into separate sum rules originating from different isospin components of the electromagnetic current. The resulting sum rules are reexamined using recently available analyses of single-pion photoproduction in the region up to photon laboratory energies of 1.2 GeV. All three sum rules receive important nonresonant as well as resonant contributions. The isovector sum rule whose contributions are known best is found to be nearly saturated, lending support to the assumptions underlying the sum rules. The failure of the isoscalar-isovector sum rule to be saturated is then presumably to be blamed on inadequate data for inelastic contributions.