The Kerr electro-optic effect has been measured at a series of pressures in helium, neon, argon, krypton, xenon and sulphur hexafluoride. In ideal atomic gases the electric-field induced birefringence is directly proportional to the molecular second hyperpolarizability γ and the gas density ρ. Values for the second hyperpolarizabilities are recorded. Deviations from a linear dependence of the birefringence on ρ have been measured in argon, krypton, xenon and sulphur hexafluoride, and values for the “second Kerr virial coefficients” obtained. These are compared with values calculated from the polarizability anisotropy of an interacting pair of atoms at large separations; agreement is fair, but apparently there are shorter range contributions to the anisotropy.