The Arbitrary Deprivation of Dual Nationality in Investment Arbitration. Recently, some investment tribunals have held that the doctrine of “effective and dominant nationality” is a “relevant rule of international law” according to which a dual national can only invoke the protection of the respective bilateral investment treaty to the extent that it invokes its effective and dominant nationality, in order to sue the State of their non-dominant nationality. However, the idea that multiple nationality is an evil that must be avoided in the interest of States has ceased to be valid. In 1960, less than a third of the States accepted dual nationality. By 2018, already three-quarters of the States allow their nationals to possess another nationality. We are currently witnessing a paradigm shift that some arbitrators are still reluctant to acknowledge. When an investment tribunal claims to apply the “effective and dominant nationality principle,” what it is actually doing is refusing to recognize one of the claimant’s nationalities. The nonrecognition of a nationality that does not correspond to an express legal norm and does not respond to a legitimate purpose is arbitrary and, therefore, is prohibited as it violates a fundamental human right.