The Cu and Ag ion displacements in gamma-CuX (X = Cl, Br, I) and gamma-AgI are calculated by using the transverse effective charge based on a lattice-distortion model. These cation displacements are compared with the root-mean-square displacements of cations obtained from the anharmonic model for CuX and AgI. It is found that the lattice-distortion model is consistent with the anharmonic model.