There is an anti-correlation in stress field between the back-arc area and the shallow portion of the slab. Generally slabs showing down-dip compression have tensional back-arcs, and the reverse. We explain this anti-correlation by a force balance in the fore-arc wedge between the slab pull, collision force in the upper plate, and the ridge push. Back-arc spreading can occur as a passive response to the down-dip compressional slab. The Mariana, Kyushu and Aegean arcs are the exceptions to the anti-correlation; in these arcs, the back-arc is tensional although the slab is in down-dip tension. This may be because the fore-arc is driven by the mantle drag toward the trench, resulting in compression balanced with the slab pull. The flow in the mantle would cause back-arc spreading in this case. ¿ 1998 American Geophysical Union