Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I say who wrote them.
Reus-Smit. 1997. The constitutional structure of international society and the nature of fundamental institutions. International Organization 51 (autumn): 555-89.
International society has a constitutional structure (informal) by which states relate with one another. Neorealists view institutions as a reflection of the current power distribution; neoliberals see institutions as an attempt to deal with coordination problems; neither can explain the FORM that these institutions might take, only why they might be needed (in general).
Reus-Smit presents a constructivist theory to explain why states would adopt the particular institutional structure that they adopt. He traces it back to to ideas about the moral purpose of the state: "Because societies of states [e.g. Europe, not the whole world necessarily] emerge in different cultural and historical contexts, they evolve different constitutional structures characterized by different conceptions of the moral purpose of the state and different ideas about procedural justice. This in turn leads them to construct different fundamental institutions."
These different conceptions about the "moral purpose of the state and different ideas about procedural justice" explain why modern European states have developed international law and rules of diplomacy but ancient Greek states didn't (they developed arbitration systems). I don't see how this theory could really be tested, though, beyond this cute story.
Research on similar subjects