Did Thailand’s parliament just vote to switch to MMP?

Via the Nation from Thailand, it seems that a joint sitting of parliament has voted to adopt mixed-member proportional representation (MMP) instead of a mixed-member majoritarian (MMM) system. The headline reads, “Parliament votes for party-list MP calculation method to be divided by 500.” SEE UPDATE.

The meeting, which comprised senators and members of the House of Representatives, chose whether to divide the party-list MP calculation by 500 or by 100. The first choice won with 354 votes in favour, 162 votes against, 37 abstentions and four no votes.

The meeting rejected the use of 100 to calculate party-list seats, which is the current method, by 392 votes to 160, with 23 abstentions and two no votes.

This is a somewhat awkward way of stating the choice between MMP or MMM, but it works. Let’s take a couple of examples to demonstrate. Suppose there are 36 million valid votes, and there is a party with 6 million votes, or one sixth. If the division is by 500, then a simple quota for a seat is 72,000 votes. This party has 83 full quotas (6M/72k=83, discarding the fraction, 1/3). If instead the division is by 100, then a simple quota is 360,000 votes. Our hypothetical party with six million votes now has 16 quotas.

The first scenario logically implies MMP. The party might have won, let’s say, 30 nominal seats (single-seat district contests in which the plurality wins the seat). It is entitled to 83 seats. So it gets 53 list seats, to bring its nominal total of 30 up to its full share of 83 quotas out of 500. It is thus proportionally represented.

The second scenario logically implies MMM. Again, let’s say it has 30 single-seat wins. Its 16 quotas mean 16 list seats, which it will add to its 30, for a total of 46 seats. This is a little short of ten percent (9.2%, to be precise). It is under-rerepsented.

Note that in the first scenario, it has one sixth of the total 500 seats. In the second it has one sixth of the 100 list seats. This is precisely how MMP vs. MMM works.

Now let’s take a somewhat larger party, with nine million votes. If the division is by 500, it has 125 full quotas. The hypothetical vote total has been chosen to roughly reflect the For Thais Party (PPT) in 2019, which actually won 7.8 million votes. That made it second largest party in votes, but it won the most nominal seats, 136. So let’s give it in our hypothetical that number as well.1 It is over-represented already! It is entitled to 125, based on full quotas with the denominator of 500. In other words, it has 11 overhangs. It will keep these, but it will get no list seats. Even though the system is MMP, the other parties that need compensation from the list seats will remain somewhat under-represented as a result.

If instead the division is by 100, this party has 25 full quotas. It thus wins a quarter of the 100 list seats, and adds these to its 136 nominal seats, for a total of 161. This is 32.2% of the total, under the hypothetical MMM system. It is over-represented, given it had 25% of the vote. It was also over-represented under MMP, as we saw in the outcome with division by 500. However, in that case, its seat percentage is 136/500=27.2%, which is much closer to its vote percentage than the MMM example.2

The Nation article concludes with this puzzling note:

However, Thammasat University public law specialist Prinya Thaewanarumitkul expressed concern on Facebook on Wednesday that the new formula would result in the number of list MPs exceeding the number allowed by the constitution.

No, it would not. Or, rather, it would not have to do so. The public law specialist seems to be assuming the overhangs would be compensated, by adding further list seats. But MMP does not necessarily do this, and if the list seats are restricted to 100, it will not generate more than 100 list seats for the simple reason that it can’t. The “extra” implied seats do not exist, and we simply reduce the total compensation potential when there are overhangs in the nominal tier. You keep 100 list seats, but the parties that have not already won at least their full quotas worth in the nominal tier get fewer of the list seats. This can be accomplished by recalculating the quota. In the scenario here, assuming those 11 overhangs for the party with nine million votes are the only overhangs the election produced, the quotas would be recalculated with a denominator of 500-11=489. Now parties get a list seats for each 73,619 votes they have earned (instead of 72,000). Our party with the six million votes is now entitled to 81 total (again, discarding fractions) rather than 83. And other parties–not considered in these illustrations–get similar reductions in what would have been their entitled final shares had there been no overhangs generated by parties that got over-represented due to success in the nominal tier.

In conclusion, it seems Thailand has chosen to shift to MMP. However, it will not be a highly proportional version of MMP, given that 20% of total seats in a compensation tier is on the small side. It is still MMP, and need not permit expansion of the list tier in order to so qualify.

I should also not that this was the second reading of the bill, not final passage. SEE UPDATE.



1. In the 2019 election, there were 350 nominal seats and 150 list, but rather than weight the seats to totals out of 400 nominal seats, I will just go with the raw numbers of seats PPT won in 2019.

2. In all these scenarios, for simplicity, I have ignored remainder seats, which result from the fractions that are ignored after seats are assigned via full quotas.

12 thoughts on “Did Thailand’s parliament just vote to switch to MMP?

  1. Presumably this is the legislative meat upon the bones <a href="https://thediplomat.com/2021/09/with-eye-to-next-election-thai-government-tweaks-election-rules/&quot; created by the 2019 electoral reform. the details of which I think we interpreted as a switch to MMM but which may just have been the introduction of a split ballot and the reduction in the number of list seats from 150 to 100.

    This BBC Thailand story is remarkably detailed, but does need Google Translate (pro-MMP t-shirts are featured). It seems as though the incumbent party of former coup leader Prayuth Chan-ocha was in favour of MMP, as was the right-wing Democrat Party, and it seems like the Pheu Thai party (successor to the successor to the successor of Thaksin) was opposed.

    Liked by 1 person

    • For MMP systems, what is the best ratio of SMD vs List Seats? And what ratio should be chosen to avoid overhang seats? Does a two vote MMP make overhang more or less likely than a one vote system?

      I believe that an open party list system with a variable district magnitude with the smallest district no smaller than 3 (the natural threshold being 25%) and 24 (4%) especially if one is concerned about overhang. 10% of the seats could be reserved for the national tier. SMD is a lot of work to maintain the districts lines, open party list with a variable district magnitude where the district magnitude is adjusted is less costly to manage.


      • The usual answer to questions like Rob’s regarding what is “best” in electoral-system design is, best at what? It depends on one’s goals. Is the goal full proportionality? If so, then normally for MMP, I’d say at a minimum you want the share of compensation seats to be no smaller than the expected deviation from proportionality arising out of the nominal tier. What might that be? Well, for all FPTP-parliamentary systems that have at least 250 seats, the mean deviation (Gallagher index) is just over 12%. So, 20% is not obviously too small, although it sure feels like it.

        In Thailand’s election of 2019, the largest party in terms of seats won (the PPT) had a difference between vote and seat percentages of just under 17%. If your goal was full proportionality, you might consider 20% cutting it close. But we should not assume that full proportionality is the designers’ goal!


        • 80% of the seats are SMD and 20% is party list, that seems like that is cutting it close, would 75% SMD and 25% party list is more optimal. Are there any countries using MMP that have 60% List Seats, and 40% SMD?


    • They did. The story here seems to be that they used MMP in 2019 with a single vote and a 350:150 split of list to SMD seats. The government then amended the Constitution to provide for a two-vote system and a 400:100 split. This amendment seems to have opened the door for the consideration of MMM (since it presumably involved the complete repeal of the 2019 system), but the legislature, given the choice, has opted for MMP.


  2. My sources tell me that the next election in Thailand will be by MMM.

    So apparently it was good of me to have added that caveat in the last sentence of the main text above.


  3. Pingback: 14 May 2023: Thailand and Turkey elections | Fruits and Votes

  4. Pingback: Correction: Thailand is using MMM, not MMP | Fruits and Votes

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