Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. Understanding this flow field remains one of the important unsolved problems in the solid Earth sciences, since it has implications for mantle dynamics, the tectonics of the back-arc region, the thermal structure of subduction zones, and the physical and chemical characteristics of arc volcanism. An important constraint is provided by observations of seismic anisotropy, as manifested by shear-wave splitting. However, the wide diversity of shear wave splitting behavior observed in various regions has posed challenges for interpretation, and a wide variety of models has been proposed to explain the observed splitting. A promising approach to this problem lies in a systematic study of splitting constraints in subduction zone regions and a search for relationships between splitting parameters and parameters that describe subduction. We have compiled shear wave splitting measurements from subduction zones worldwide, and have tested for relationships with indicators of tectonic processes such as convergence velocity and obliquity, trench migration velocity, and slab dip, curvature, seismicity, thickness, and morphology. Our hypothesis testing supports a model in which mantle flow in subduction systems is controlled by the interplay between three-dimensional flow, induced by trench migration, and two-dimensional corner flow, controlled by the downdip motion of the slab.