Neofunctionalism is a theory of regional integration, building on the work of Ernst B. Haas, a German-born American political scientist, and also Leon Lindberg, an American political scientist. The explicit purpose of the neofunctionalists was to utilize the pioneering European experience of integration to generate hypotheses for testing in other contexts. Jean Monnet's approach to European integration, which aimed at integrating individual sectors in hopes of achieving spillover effects to further the process of integration, is said to have followed the neofunctional school's tack. Haas later declared the theory of neofunctionalism obsolete, a statement he revoked in his final book, after the process of European integration started stalling in the 1960s, when Charles de Gaulle's "empty chair" politics paralyzed the institutions of the European Coal and Steel Community, European Economic Community, and European Atomic Energy Community. The theory was updated and further specified namely by Wayne Sandholtz, Alec Stone Sweet, and their collaborators in the 1990s and in the 2000s (references below). The main contributions of these authors was an employment of empiricism.