33Party Effects in Legislative Voting LEGISLATIVE STUDIES QUARTERLY, XXXI, 1, February 2006 33

Political scientists have long attempted to measure and describe the modest and contingent effects of party on the behavior of members of Congress. Recent efforts have extended the debate to the more specific question of whether or not party influences are sufficiently strong to move policy outcomes away from the median position. In this article, we specify four theories of legislative behavior. One is a preference-based, or partyless, theory of behavior. This theory posits that there are no party effects independent of preferences and that equilibrium outcomes are located at the chamber’s median. The other theories rely on different conceptions of the foundations of party effects and yield distinctive predictions about the legislators who will support bills on final passage votes. After testing, our conclusion is that strong party influences can be found in final passage voting in the House: the partyless theory receives little support, but a model based on majority party agenda control works well. Legislative outcomes are routinely on the majority party’s side of the chamber median. Since the beginning of the twentieth century (see Lowell 1902)