Mixed-Member Electoral Systems

A mixed-member system splits a legislature into two pools of seats, filled two different ways, on one ballot. The voter marks a candidate for their local district, which is decided by whoever gets the most votes there, usually first-past-the-post. The voter also marks a party on a separate line, and those votes get tallied nationally to hand out a second batch of seats in proportion to each party's vote share. One ballot, two counts, two logics: local accountability from the district tier, fair proportional representation from the list tier.

Russia's State Duma has 450 seats total, split evenly: 225 go to single-member districts where the top vote-getter wins outright, and 225 go to parties based on their share of the nationwide party vote, usually with a minimum threshold a party must clear to qualify. Mexico's Chamber of Deputies works the same way at a different ratio: 300 seats come from single-member districts, and 200 come from regional party lists, for 500 seats total. In both countries the two tiers are counted independently, not blended into one formula.

That independence matters most for a dominant party. If a party sweeps nearly every district race, those wins do not reduce or offset its list seats. The list tier is allocated separately, based purely on the national party vote, so the party's list seats simply stack on top of its district seats. A party that wins big in both tiers ends up with a seat share well above what pure proportional representation would have given it, which is why mixed systems sit between the plurality systems and the proportional systems that bracket them in this unit.