Subduction of the lithosphere at convergent-plate boundaries takes place asymmetrically—the subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). We carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Our experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[φ] < 0.15, where φ is effective internal friction angle), regardless of the extent of hydration. In contrast, steady-state one-sided subduction requires a weak hydrated slab interface and high slab strength (sin[φ] > 0.15). The weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes.