Can social externalities solve the small coalitions puzzle in international environmental agreements ?[Les externalités sociales permettent-elles de résoudre la controverse théorique des "petites coalitions stables" dans les accords environnementaux ?]

A puzzle in the literature on the formation of coalitions supporting International Environmental Agreements (iea) is that if an iea leads to substantial gains, then it will not be supported by many countries. The non-cooperative game theoretic literature highlights the « small coalitions » puzzle by which only a small number of countries are willing to sign an environmental convention. In these models, a global coalition comprising all countries or most of the countries and generating significant global benefits is not sustainable. In effect, greater the number of countries in the coalition, higher the incentive of signatories to not respect their engagements. Introducing social externalities, we explain why in reality, some treaties can be sustained by nearly all the countries in the world, and others can be sustained by a handful of countries.