The question of the existence of legal protection for foreign investors under customary international law (or "custom") has always been controversial. As a result of this perceived lack of established customary principles, States concluded thousands of bilateral investment treaties in the 1990s for the promotion and the protection of investments ("BITs"). The number of BITs is now so overwhelming and their scope so comprehensive that a new debate has recently arisen in doctrine about the impact of these treaties on the existence of custom in the field of international investment law. It has been recently argued in doctrine that these BITs represent the "new" custom in this field. For some writers, the content of both custom and BITs is now simply just the same.
Patrick Dumberry, Are BITs Representing the "New" Customary International Law in International Investment Law?, 28 Penn St. Int'l L. Rev. 675 (2010).