ABSTRACT
While it has long been observed by students of western European politics that
deputies from the same party caucus generally take identical positions when
votes are cast in legislatures there, the explanation for this commonly occur-
ring party voting agreement is more controversial. Scholars taking a sociologi-
cal approach have argued that MPs hold together for normative reasons
including party solidarity, socialisation in proper behaviour by caucus
leaders, or simply because members of the same party families hold the
same ideological convictions. In contrast, scholars from the rational choice
or institutionalist tradition have argued that discipline emerges as rational
actors discover self-interested reasons to vote en bloc.1 The study of well-
established western European legislatures has not enabled political scientists
to resolve this debate.
