Majority bonus system

A majority bonus system (MBS, also called a minority-friendly majoritarian system) is a mixed-member, partly-proportional electoral system that gives extra seats in a legislature to the party with a plurality or majority of seats. Typically, this is done with the aim of providing government stability, particularly in parliamentary systems.

The size of the majority bonus can vary substantially, is usually a fixed number of seats, and may be conditional on the number of votes for each party. However, a relatively small majority bonus (such as in the reinforced proportionality system of Greece) may not always guarantee that a single party can form a government. At the same time, as the majority bonus is allocated in a non-compensatory manner, if the majority bonus is as high as 50%, and the largest party which has 50% of the popular vote receives it, this party may win as many as 75% of all seats available. This differentiates it from the similar majority jackpot system.

It is currently used in Greece and on a local level in Italy and France.^{[citation needed]} In Argentina, it is used in the Chamber of Deputies of Santa Fe, Chubut, and Entre Ríos.^{[citation needed]}

The bonus system adds a certain fixed number of additional seats to the winning party or alliance. In the Greek Parliament up to a sixth of the assembly seats are reserved as extra seats for the winning party. In the Sicilian Regional Assembly, a tenth of the assembly seats are granted to the winning coalition on top of those allocated proportionally. The size of majority bonuses may vary greatly, from as low as a few seats to up to 50%. In case of a bonus of 50%, the party also received their proportional share of the other 50% seats, which make a supermajority almost certain.

The following table shows how small (10%), medium (25%) and large (50%) majority bonuses would work without any additional distortions of proportional systems. Using an electoral threshold or an apportionment method favoring large parties would give an even larger bonus to the largest party. The reason why the difference of the seats share and vote share (the effective bonus) is lower than the bonus is that the number of non-bonus seats to be allocated proportionally is less than the total number of seats. If all parties would get their full proportional number of seats, and one party would get the (nominal) bonus on top of that, the total number of seats would increase. While this means the size of the effective bonus (without other factors giving an effective bonus) if always smaller than the nominal one in terms of percentages, this naturally means all the parties who don't receive a bonus have a naturally less seats than they would proportionally.

The bonus system is unconditional and non-compensatory, while its goal in a political science sense is to provide for stable majorities (a bonus lower than 50%) does not ensure it and applies also when a stable majority can already be formed. This is the main difference between a majority bonus and a majority jackpot.

The difference between the majority bonus and a majority jackpot is shown in the following table, where the largest party receives a majority bonus/jackpot.

The jackpot system essentially gives the size of the jackpot or the vote share (whichever is higher, making it a conditional system and an effectively flexible "bonus"), while the bonus system gives the bonus and a proportional share of the rest of the seats. The jackpot only modifies the seat share when the largest party's proportional seats count based on its vote share is below the size of the jackpot.