A primary feature of subduction zones is the point at which the slab and mantle viscously couple—defining the initiation of mantle wedge circulation. Here we consider the effects of the backarc thermal structure on coupling depth using two-dimensional thermomechanical models. Models show a strong correlation between coupling depth and backarc lithospheric thickness and coupling is always associated with the breakdown of antigorite. Backarc lithospheric thickness is a good predictor of coupling depth and can be indirectly estimated from surface heat flow. Consistently high backarc heat flow in circum-Pacific subduction zones may indicate a common depth of slab-mantle coupling globally.