Countries with a very plural congress/parliament

Question

As Brazil heads toward it's runoff, one striking feature is that it's very multiparty. No party has more than 13/81 and 77/513 senate and house seats, respectively (about 15%). Are there any countries that are similar to Brazil? Are there any that are even more plural?

Defining plurality

There are several metrics of how multi-party a congress is. An obvious choice is the number of parties. But this metric does not care about size: A single dominant party plus lots of single-seat parties would score highly.

A more robust choice is how few seats (proportionately) the most popular party has. However, this metric still neglects the distribution between the smaller parties. For example, a two-party dominant 300:250:10 house scores the same as a 300:150:110 house even though the latter has an important third party.

Fortunately there is a metric (one of several) that doesn't have such "blind spots". It is a the HHI index, a mathematical measure of concentration which "combines" the number of parties with how evenly distributed they are. It does not neglect smaller parties. Note: the linked formula rounds the numbers to the nearest percent, but there is no reason to.

Another metric that tries to "cover all the bases" is the Gini index with "wealth" measured in seats. This metric can only compare two countries if they have the same number of parties. To make it work we can add zero-seat parties to countries with fewer than the maximum party count.

Defining a party

Is the Brazil of Hope one party or three? A party should have a fairly uniform platform which is meaningfully different from the other parties. Brazil of Hope contains the center-left green-party and the far-left communist party as well as the standard-left workers party. These are different enough on a left-right scale that in my opinion they are three separate parties.

On the flip-side, US Southern democrats were traditionally fairly right wing and could be considered a separate "party" from the other democrats. The more conservative members have since moved to the republican party.

Of course, left vs right is the first "principle component" of political platforms but there are other "axes" as well. There are also multiple left vs right axes (social, economic, etc).

Defining a congress

Congress or parliament must have a meaningful amount of legislative power. This roughly means at least "partly free" under freedom house.

Given these caveats, we can still make a best-guess as to what a "party" is and calculate the HHI index of a given country (averaging the house and senate for bicameral systems). Are there other countries similar to Brazil? Are there any that are substantially higher?

Answer 1

I don't know if it's the mostest, but Israel's Knesset w 120 seats has a long history of fragmentation. Likud has 29 seats, but is in opposition while the lead in the governing party is 17 and it's steeply down from there.

Don't know enough how different each party is, but between right-wingers, religious conservatives, Russian immigrant party, Arab parties, Communists, Labor and the like, seems plenty diverse to me.

Answer 2

As mentioned by Roland in a comment, Belgium's House of Representatives does pretty well under the HHI metric; 150 seats in the chamber with the largest party having 24 seats, giving a score of 993. As Italian Philosophers points out, the Israeli Knesset is also pretty good, scoring 1056.

The best I've found so far without counting independent politicians as their own party is the Netherlands' Senate, which scores 901. On the issue of independents, though, these metrics fall down rather when used to measure non-partisan democracies; where representatives are elected with no reference to political parties. For example, Palau's House of Delegates consists of 16 non-partisan representatives, giving a score of 625. Another example might be Indonesia's Regional Representative Council, which has 136 non-partisan members. However, many of these members, while officially non-partisan, have links to political parties in reality.

Answer 3

The Spanish Congress houses 17 different political groups (15 actual parties or coalitions + 2 individual deputies who abandoned their respective groups in the middle of the legislature).

With "just" 350 seats, it has an HHI score of 2155.

If we count the smaller groups as a single 11-member group (which, for purposes like intervention time and expenditure budget, they are: it's called the "Mixed Group"), that'd be an HHI score of 2205.

Answer 4

A good contender would be the Indian parliament with its two chambers the Rajya Sabha and the Lok Sabha, the latter of which is elected in general elections via FPTP.

If I have done the math right (I'm not versed with Indian parties, so I don't know which are related. I also counted independents separately from each other) with the data from the last election, then the Rajya Sabha (with 35 parties and one independent) would get a HHI of 1729 and the Lok Sabha (with 37 parties and 2 independents) a HHI of 3314.

It is also one if the biggest, if not the biggest election. The 2019 general election for the 17th Lok Sabha had over 900 million voters and 8039 candidates from over 650 parties in 542 constituencies.

Answer 5

If you're willing to accept supranational parliaments, after the 2019 European elections (i.e. pre Brexit) the largest grouping in the EU Parliament had 24.2% of the seats. In total there were 7 groupings and 12 unaligned parties for an HHI of 1,522.

If you wanted to break it down a bit there were 171 national delegations sitting in those groups for a HHI of 150. The largest national delegation here was the Brexit Party with 29 of 751 seats, followed by Lega Nord with 28 and (the reason why I'm going this far) a coalition of Law and Justice and United Poland with 26.