We hypothesize that the shift of the glass transition temperature of polymers in confined geometries can be largely attributed to the inhomogeneous density profile of the liquid. Accordingly, we assume that the glass temperature in the inhomogeneous state can be approximated by the T-g of a corresponding homogeneous, bulk polymer, but at a density equal to the average density of the inhomogeneous system. Simple models based on this hypothesis give results which are in agreement with experimental measurements of the glass transition of confined liquids.