We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, delta, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the pi-bond-order alternation (pi-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, delta peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of delta obtained via the converged sum-over-states approach to the results of a simple three-state model.