A significant and influential body of research suggests that electoral systems influence legislators’ behaviour. Yet, individual legislators are potentially motivated by other concerns, such as policy and office. What happens when competing goals predict contradictory behaviour, for example, when electoral incentives clash with enticements to win prized post-election positions (mega-seats)? When party leaders cartelize the allocation of mega-seats, the anticipated effects of the electoral system on legislators’ behaviour may dissolve – creating strong parties in the legislature despite a candidate-centred electoral system. New data on mega-seats and voting behaviour in the Irish parliament between 1980 and 2010 supports the notion that mega-seat considerations trump the impact of the electoral system on roll-call behaviour. The implication is that what goes on within the legislature may be more important for influencing legislators’ behaviour than what goes on at the ballot box.