Regulatory efforts in the United States and European Union have recently refocused on materiality, an essential but elusive securities law concept. The renewed focus is due in part to increasing globalization of capital markets, data, and information channels that has highlighted inconsistencies in theory and application. In the United States, the Supreme Court has loosely defined materiality through a line of cases beginning with TSC Industries v. Northway and Basic, Inc. v. Levinson, whose reasonable investor rubric is frequently disregarded by lower courts, prosecutors, and the Securities and Exchange Commission. Meanwhile, in the E.U., courts have only lightly addressed materiality. Scholars in law and behavioral economics have criticized the TSC-Basic rubric for its ambiguity, unpredictability, and disconnection from market psychology. Recognizing these criticisms, this Article conducts an international comparative investigation of materiality in the legislation, regulation, and case law of the United States and European Union, revealing a shared, probabilistic Bayesian infrastructure. The Article then proposes a flexible Bayesian framework that harmonizes the substantive evaluation of materiality under existing U.S. and E.U. law. Finally, it models application of the framework using Bayesian network analysis in the context of a hypothetical stock transaction.