Earth's subducting plates move 3–4 times faster than its overriding plates, but it remains unclear whether these contrasting plate speeds are caused by additional pull from subducting slabs or by increased viscous drag on the lithosphere-asthenosphere boundary beneath deeply-protruding continental roots. To investigate the relative importance of plausible controls, we predicted global patterns of plate motions using numerical models that incorporate the influence of subducting slabs, convective mantle flow, and continental roots. From the mantle convection models, we computed a set of dynamically consistent plate velocities by balancing forces that drive and resist the motion of each plate. When deep continental roots anchor to the sub-asthenospheric upper mantle, the calculated patterns of plate motions are close to the observations if only ∼20% of (excess) upper mantle slab weight contributes to the slab pull force. However, this small contribution causes plates to move too slowly on average unless mantle viscosity is a factor of ∼2 lower than expected from post-glacial rebound. In contrast, we show that predicted and observed plate motions are more easily reconciled if even the deepest continental roots are underlain by a low-viscosity layer and at least half of (excess) upper mantle slab weight contributes to the slab pull force. This preferred scenario agrees with recent seismological evidence for a global asthenosphere and previously proposed mechanisms for partial decoupling of slabs from surface plates.