International environmental accords have become important mechanisms by which nations make promises to administer natural resources and manage the global environment. Previous studies, relying mainly on single cases or small- n data sets, have shed light on the proximate political causes of participation in these agreements. However, no study has yet systematically explained the deeper social determinants of why nations sign, ignore or resist environmental treaties. We offer a theoretically-sequenced model that exploits complementarities between rational choice institutionalism and world-systems theory. Key variables posited by realists and constructivists are also examined, using a new environmental treaty participation index based on ratifications of 22 major environmental agreements by 192 nations. Cross-sectional OLS regression and path analysis strongly supports the institutionalist claim that credibility—the willingness and ability to honor one's international environmental commit-ments—“matters.” But these measures also lend considerable support to the world-systems hypothesis that state credibility is strongly influenced by a legacy of colonial incorporation into the world economy. Narrow export base—our proxy for disadvantaged position in the world-economy—directly and indirectly (through institutions and civil society strength) explains nearly six-tenths of national propensity to sign environmental treaties. A nation's natural capital, its ecological vulnerability, and international environmental NGO memberships had no explanatory power in the path analysis. Our results indicate that new theoretical, methodological and policy approaches are needed to address structural barriers to international cooperation.