Governments' desire to ameliorate environmental problems may conflict with other goals. Policy levels which balance different objectives can be altered by policy changes in other countries. A decrease in the importance of the pollution problem, or an increase in its global extent, increase the likelihood that tighter environmental regulations in one region induce laxer policies elsewhere. The transboundary character and the importance of environmental externalities also affect the amount of cooperation needed to improve members' welfare in a coalition. More global pollution problems require a larger coalition. However, the critical coalition size may be larger or smaller for more severe problems.