The theory of molecular polarization in dense systems is presented on a quantum mechanical basis for spherical, nonpolar molecules. This permits the calculation of the effect of molecular interaction on the polarizability of the molecules, which is considered constant in the statistical theory of the dielectric constant. The interaction Hamiltonian is restricted to the induced dipole term and the polarizability is obtained from perturbation theory as a power series in the interaction tensor Tik = ▽i▽k(1/rik) where rik is the distance between the centers of molecules i and k. As a special case it is shown that in a system of isotropic harmonic oscillators, interacting through induced dipole forces, the polarizability remains constant in any order of approximation. For hydrogen atoms and helium atoms, however, the polarizability increases in the order T2 due to interactions between pairs of atoms. This effect gives rise to a deviation from the Clausius-Mosotti expression which is of the same form and of the same order of magnitude as the effect of statistical fluctuations.