What is the relationship between war propaganda and nationalism, and what are the effects of each on support for, or participation in, violent acts? This is an important question for international criminal law and ongoing speech crime trials, where prosecutors and judges continue to assert that there is a clear causal link between war propaganda, nationalism, and mass violence. Although most legal judgments hinge on the criminal intent of propagandists, the question of whether and to what extent propaganda and nationalism (...) interact to cause support for violence or participation remains unanswered. Our goal here is to contribute to research on propaganda and nationalism by bridging international criminal law and the behavioral and brain sciences. We develop an experiment conducted with Serbian participants that examines the effects of propaganda as identified in the latest international speech crime trial as causing mass violence, and thereby test hypotheses of expert witness Anthony Oberschall’s theory of mass manipulation. Using principal components analysis and Bayesian regression, we examine the effects of propaganda exposure and prior levels of nationalism as well as other demographics on support for violence, ingroup empathy, and outgroup empathy. Results show that while exposure to war propaganda does not increase justifications of violence, specific types of war propaganda increase ingroup empathy and decrease outgroup empathy. Further, although nationalism by itself is not significant for justifying violence, the interaction of increased nationalism and exposure to violent media is significant for altering group empathies. The implications of these findings are discussed with respect to international criminal law and the cognitive science of nationalism. (shrink)