Combining ideology with narrow self-interest in positive political theory

Ideology and self-interest are often in tension. If positive political theorists assume broad self-interest as the standard of human behaviour, they can accommodate people's ideological motivation to act for the common good. Yet, their theories may become tautological and empirically untestable. Conversely, if these theorists assume narrow self-interest, they increase testability. But because ideology seems incompatible with narrow self-interest, they often rule ideology out as a motivational driver. Positive political theorists can, however, combine ideology with narrow self-interest by assuming broad egoism. And in so doing, they should assume that ideology motivates political agents until it becomes too costly for these agents' material interests.