His later research focused on the science of political parties, public opinion, international relations and political behavior. with Harold D. Lasswell, Ralph D. Casey: Propaganda and Promotional Activities: An Annotated Bibliography .
Seine Forschungsschwerpunkte als Wissenschaftler waren Politische Parteien, Öffentliche Meinung, Internationale Beziehungen und Political Behavior. mit Harold D. Lasswell, Ralph D. Casey: Propaganda and Promotional Activities: An Annotated Bibliography.