If we follow the reasoning from Chapter 2, a strong party system requires that parties are cohesive, in other words that the elected members of the same party vote in a disciplined way, following the position reached by their party leaders in the parliament. A political party will attract support by making commitments to particular policies or goals that are distinct from the commitments of other parties. If a party is able to work as a highly organised team, it will be able to turn these promises into policy outcomes. If a party is not able to ‘rally its troops’, it will have little chance of shaping policy outcomes, and so will leave its supporters disappointed. Scholars of the EU have hence argued that for transnational parties to increase the democratic accountability of the EU they must be able to act cohesively to implement the policy platforms they announce (e.g. Attinà, 1992; Andeweg, 1995; Hix and Lord, 1997). If voting in the European Parliament breaks down along national lines rather than party lines, the transnational political groups cannot claim to be able to articulate the classic ideological viewpoints in the EU policy-making process with any real degree of effectiveness. Testing for the cohesiveness of parties is the first natural step towards understanding the effectiveness of parties. As in the previous chapter, where we looked at the long-run and short-run determinants of participation, we analyse in this chapter the long-run determinants of cohesion.