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©, the number of preference votes allowed (PV), and the number of competing parties (N). We find that larger candidate choice© 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.