Since 2002, the U.S. Supreme Court has consulted contemporary foreign legal judgments to help interpret, and dramatically expand, the substantive scope of the Bill of Rights in three landmark cases. It has not, however, explained when and why contemporary foreign legal materials are relevant to a principled, objective mode of constitutional interpretation. This Note represents an attempt to do so. It postulates two rationales that could retrospectively justify the Court’s methodology in Atkins v. Virginia (2002), Lawrence v. Texas (2003), and Roper v. Simmons (2005). One is grounded in a theory of Anglo-American common law, the other rests on jus cogens and customary international law. This Note then compares the two and concludes that the jus cogens theory could best address critics’ concerns that the use of foreign law will undermine U.S. sovereignty, reduce civil liberties in this country, and vastly increase judicial discretion.