Allison Christians (Wisconsin) has posted Hard Law & Soft Law in International Taxation, 25 Wis. Int’l L.J. ___ (2007), on SSRN.  Here is the abstract:

Why do countries follow the rules they do for international taxation? Tax scholarship is increasingly using terms like hard law and soft law to explain the degree of global adherence to various tax practices. In this brief essay, I make the case that terms we use are important because they convey something about how tax norms are being formed and how nations will or should behave in response. Using a term such a customary or soft law may connote a method by which key players working within transnational networks achieve supranationalization of particular tax norms outside of conventional mechanisms (e.g., treaties). As such, the description is an important signal about expectations for participation, inclusiveness, and compliance in the development and diffusion of international tax law.