Voting systems and what they achieve

Inspired by this tweet, I thought it might be good to quickly go over various voting systems and what they achieve. What they do well, and what they do not do well.

MIXED MEMBER PROPORTIONAL:
This system does one thing better than any other; achieves balance. It is likely why this system is one of the, if not the, most widely uses electoral system on the planet. It is great at balancing the need for a proportional result with the need for local representation. The two downsides are that two 'classes' of member get elected - local and list - which some voters feel is unfair as it means two standards for 'winning' a seat; and that the system can be complex, potentially requiring two ballots be cast and counted.

SINGLE TRANSFERABLE VOTE:
This system does one thing better than any other; allowing a voter to express nuance. Your vote will count at some point, even if your first-placed choice is not elected. People are complex, and this system reflect that. Sadly that complexity is also its largest downside; it is the most complex of all the systems (that we are looking at here) and counting ballots for such a system often takes days.

CLOSED-LIST PROPORTIONAL:
This system does one thing better than any other; turning public share of vote for each party into the party share of seats in the parliament. A party that takes 40% of the vote will end up with 40% of the seats. Roughly. By being a closed-list system, however, it means that the party, not voters, directly select the candidates. Additionally, such a system may not include any local component at all (as MMP does) and thus may lack that local aspect.

INSTANT RUNOFF:
This has some of the benefits of STV, and ensures that the winning candidate will always have a majority of the valid ballots cast for them. Its simplicity and ease of understanding often make it a popular choice for countries looking to switch away from FPTP. That simplicity however can be a negative, as this system is not much more proportional than FPTP, and by its nature tends to skew towards the middle.

FIRST PAST THE POST:
Quite simply, the simplest of all of the systems. The person with the most vote wins. An ancient system, it was even used, although in a strange form, by the roman republic over 2000 years ago as part of its tribal assembly. The largest flaw is its lack of a direct connection to the popular vote. It is also not well suited to represent views of voters and can allow a party with a minority viewpoint to defeat a combination of parties representing the majority view so long as that minority party is well united.

PARALLEL VOTING:
My favorite of the options, it excels at balancing the desire for majority governments (which aids stability) and allowing for a well represented opposition (inviting in smaller parties, and ensuring larger parties have good geographic representation). It achieves this, however, by failing accurately turn the popular vote into seats in parliament, as well as allowing a united 'minority' party to still defeat a sum of disunited 'majority' parties


There are still other systems with their own pros and cons. The block vote (US electoral college) is like a hyped up FPTP where instead of just winning one seat, you win all of them. Sortition literally randomly selects a winner. Condorcet compares all the candidates in a ranked system to find the greatest consensus. Like the above, they all have good and bad points.

No system is "broken" nor does any system "fix things".

However

Systems can be "broken in the context of our electoral culture" as well as "fix things wrong with our electoral culture"
That being said, the electoral culture in your country, or even region of your country, may differ widely from the electoral culture in other countries, or other regions of your own country. No one system can magically solve all problems, nor will a system that can 'fix things' for now, 'fix things' forever.

In the end, like so many other things, this is a nuanced and complex issue without a simple one-size-fits-all solution.