Substituent effects on the pi-pi interactions of aromatic rings are a topic of much recent debate. Real substituents give a complicated combination of inductive, resonant, dispersion, and other effects. To help partition these effects, we present calculations on fictitious "pure" sigma donor/acceptor substituents, hydrogen atoms with nuclear charges other than 1. "Pure" sigma donors with nuclear charge <1 weaken pi-pi stacking in the sandwich benzene dimer. This result is consistent with the electrostatic model of Hunter and Sanders, and different from real substituents. Calculated inductive effects are largely additive and transferable, consistent with a local direct interaction model. A second series of fictitious substituents, neutral hydrogen atoms with an artificially broadened nuclear charge distribution, give similar trends though with reduced additivity. These results provide an alternative perspective on substituent effects in noncovalent interactions.