The role of Earth’s spherical geometry in modulating the evolution of subduction zones is poorly understood. Here, we simulate multi-material free-subduction in a 3-D spherical shell domain, to investigate the effect of plate thickness, density (combined approximating age) and width on the evolution of subduction systems. To isolate the role of sphericity, we compare results with equivalent Cartesian models. The first-order predictions of our spherical cases are generally consistent with existing Cartesian studies: (i) slabs retreat more, at a shallower dip, as plate age increases, due to increased bending resistance and sinking rates; and (ii) wider slabs can develop along-strike variations in trench curvature, trending towards a ‘W’-shape, due to toroidal flow at slab edges. We find, however, that these along-strike variations are restricted to older, stronger, retreating slabs. When compared to slabs in Cartesian models, in a spherical domain: (i) slabs descend faster, due to the convergence of downwelling material with depth; (ii) these faster sinking rates reduce the time available for bending at the trench, resulting in effectively stronger slabs; (iii) the curvature of slabs increases their effective strength; and (iv) the curvature of the transition zone tends to enhance slab stagnation. These differences between spherical and Cartesian cases become more prominent as slab width increases. Taken together, our results suggest that Cartesian models are suitable for simulating narrow subduction zones, but spherical models should be utilised when investigating subduction zones wider than ~ 2000 km: at such length-scales, the consequences of Earth’s curvature cannot be ignored.