The performance of the local spin density approximation (LSD) of Kohn-Sham density functional theory when applied to partitioned electron densities has been explored. Spin partitioning and core/valence partitioning have each been examined and the results compared with conventional Moller-Plesset (MP2) theory based on the Kohn-Sham determinant. It is found that the LSD energy contributions from such subsets of electrons are generally in poor agreement with their conventional counterparts, and it is proposed that such differences be employed as a diagnostic measure of the quality of other density functionals.