The institutional arrangement of bicameralism has
often been defended as a solution to the problem of legislative instability. It
is claimed that equilibrium outcomes are more likely to exist under bicameralism,
thereby making decision making more stable and predictable. The complex legislative system of the European
Union, which is occasionally bicameral, provides a unique opportunity to
empirically test this claim. The ‘codecision’ procedure involves intensive
negotiation between the two legislative chambers, the European Parliament and
the Council of Ministers; final decisions made under this procedure must have
the support of concurrent majorities in both chambers. In contrast, decisions
reached in the Parliament under other procedures are not necessarily supported
by the Council. In line with
expectations, the results show that coalition formation in the European
Parliament is more stable and predictable when bicameral agreements are
required. In particular, the median party on the left-right dimension is
pivotal to agreements under codecision.