Molecular dynamics (MD) simulations of bulk liquid water at different thermodynamic conditions or of biomolecules in aqueous solution require a molecular mechanics (MM) force field that accounts for the sizable electronic polarizability α of the water molecule. A considerable number of such polarizable water models has been suggested in the past.   Most of them agree that one should employ the experimental value α_exp for the electronic polarizability and compute the induced dipole moment μ_i through linear response from the electric field E at the position r_O of the oxygen atom. However, several more recent models have suggested somewhat smaller values for α. Using a hybrid method that combines density functional theory for a selected water molecule with an MM description of its liquid water environment, here we show that the choice of α_exp is solely correct if the induced dipole moment μ_i is calculated from the average electric field E within the volume occupied by the given water molecule. Because of considerable field inhomogeneities caused by the structured aqueous environment, the average field E is much smaller than the local spot check E(r_O). However, as opposed to E(r_O), the average field E cannot be easily calculated in MM−MD simulations. Therefore, in polarizable MM water models, one should calculate the induced dipole moment μ_i from E(r_O) through the reduced polarizability α_eff = 0.68α_exp, which then effectively accounts for the inhomogeneities of the electric field within the volume of a water molecule embedded in liquid water.