The question of what drives the voting behaviour of legislators is central to representative democracy and key to understand how the linkage function is performed. Legislators are often said to arbitrate between several principals when making (voting) choices: their constituents, their voters, their party. This is especially true of Members of the European Parliament (MEPs) for whom a further distinction needs to be made between their EP parliamentary group and their national political party. Previous research has most often concluded that MEPs are above all loyal to their national party. To explain this behaviour, a widespread assumption is that MEPs abide by their respective national parties since these parties control their (re)selection as candidates. Yet, the mechanisms underlying these claims have not been investigated, in part because of the lack of data. The question we ask is thus: Does the way political parties select their candidates impact the behaviour of elected representatives? The relevance of EP (s)election processes to test the linkage function is two-fold. Theoretically, they question not only the horizontal, but also the vertical linkage between the European and the national electoral arenas. Empirically, they provide a good setting for a cross-sectional research, allowing to include most parties of most European states, but acting under a broadly similar electoral system (PR). Our statistical models test whether more open processes of candidate selection lead to more disloyalty of the MEPs toward their national party. To measure the independent variable, we rely on a new dataset recoding the formal candidate selection processes for European elections used in all national political parties having gained representation in the 8th EP legislature. The dependent variable is operationalized by means of the roll call votes for all MEPs belonging to a party having received one than one seat in 2014 (N = 642).