Despite the high power conversion efficiency and ease of fabrication, planar-junction organolead halide perovskite solar cells often exhibit anomalous hysteretic current-voltage (I-V) characteristics. In this work, the origin of the I-V hysteresis is studied by fine-tuning the precursor ratio of methylammonium lead iodide and thus varying the native defects in the material. It is shown that the perovskites synthesized from "PbI 2 excess," "methylammonium iodide excess," and "stoichiometric" precursors exhibit identical film morphology but different I-V hysteresis in a planar solar cell configuration. Through a comparative analysis on the temperature-dependent continuous and stepwise-stabilized I-V responses of the three devices, a model involving transport and trapping of the ionic native defects is proposed. The active energy of the transport process is estimated to be between 0.10 and 0.18 eV, most likely associated with the vacancy-mediated iodide ion migration. The lower activation energy of the "PbI 2 excess" and "Stoichiometric" samples indicates that the presence of methylammonium vacancies may provide a favorable pathway for the migration of iodide ions due to reduced steric hindrance. Furthermore, the slow trapping and release processes of iodide ions at the TiO2/perovskite interface are accounted for the long time scale current decay (or raise) following a voltage change.