This contribution investigates how ballot complexity shapes vote concentration in preferential list PR systems – a growing electoral system used in over 30 democracies. These systems impose a dual cognitive burden on voters: choosing a party list and then selecting candidates within that list. Drawing on original data covering 158 elections across 28 countries (1994–2024), we test the effects of three electoral parameters: the number of candidates (C), the number of preference votes allowed (PV), and the number of competing parties (N). We find that larger candidate choice (C) consistently increase vote concentration, confirming the « paradox of choice » hypothesis. Additionally, high party fragmentation (N) amplifies cognitive overload, reinforcing reliance on simple heuristics (primacy effect). However, systems allowing multiple PVs substantially mitigate this effect. These findings highlight key trade-offs for electoral engineers between voter empowerment, cognitive feasibility, and proportional representation, offering guidance for the design of democratic and competitive electoral systems.
