How is legitimacy to be secured for constitution-like legal orders operating beyond the state? Some scholars recommend connecting aspects of global law to human rights adjudication and enforcement by adopting their preferred method for resolving conflicts, namely, proportionality analysis. Adopting a frame of analysis widely embraced by apex courts might generate the requisite regime legitimacy, it is argued. This turns out to be a strategy that is difficult to pursue in the realm of international investment law, a global legal order (...) made up of over 3,300 investment and regional treaties, enforced by private systems of dispute resolution, that incorporates standards of treatment familiar to national constitutional law. By undertaking an analysis of statements made by relevant actors in the field, principally investment lawyers and arbitrators, we learn that there is not that much interest in taking up proportionality analysis, despite the legitimacy problems that continue to dog the system. Whatever the purported advantages of turning to proportionality, it turns out that investment lawyers and arbitrators are not so interested in embracing logics other than one preoccupied with owing solicitude to foreign investors. This reluctance renders investment law a less legitimate, and therefore less sustainable, form of constitution-like global law. (shrink)