Although the SNP benefitted hugely from First Past The Post (FPTP) in May (gaining almost all the Scottish seats on 50% of the votes), I remain committed to proportional representation — I believe FPTP is poison for popular engagement, at least in a multi-party system, because so many people feel their vote doesn’t count.
Proportional representation comes in many varieties, however (and some are more proportional than others). We’re already using three different systems in Scotland: (1) The Additional Member System (AMS), which we use for electing the Scottish Parliament; (2) the Single Transferrable Vote (STV), which we use for electing the councils; and (3) d’Hondt, which we use in elections for the European Parliament.
The SNP opted for STV in their recent Westminster manifesto. I can understand why — STV is a decent system in many contexts, especially when the candidates aren’t organised into parties (for instance, it’s a great system for electing members for a committee in an political party). However, it has some shortcomings which makes it less than ideal for Westminster elections.
Firstly, STV benefits those parties who are good at predicting their support. For instance, if May’s election had been held using this system, Labour and the Liberal Democrats would probably not have predicted the scale of their losses, so they would have put forward too many candidates, which could have exaggerated their losses; in the same way, the SNP might not have been bold enough, which again would have harmed them. (This problem can be alleviated by forcing the voters to prioritise all the candidates and not just one or two, but we don’t tend to do that in Scotland.)
Secondly, STV doesn’t help parties with varying levels of support in different areas. In particular, whereas the SNP’s 50% support resulted in nearly 56 out of 59 seats under FPTP, it would probably only have resulted in around 30 seats in Scotland under STV; the fact that the SNP also had supporters in England wouldn’t have led to any additional seats.
The Danish electoral system would be much better for the SNP. Denmark uses a variant of d’Hondt (Sainte-Laguë to be precise) in multi-member constituencies, but crucially all the votes get added up nationally afterwards, and additional seats are allocated in order to ensure that every vote counts. In other words, if the SNP got 50% of the votes in Scotland and about 5% in the rest of the UK so that the UK-wide support was exactly 10%, the SNP would have received 10% of the seats, which would actually be even better than the current 56 seats.
Some years ago I made a simulation of the 2005 Westminster election using the Danish electoral system. I didn’t at that time assume the SNP would have received any votes outwith Scotland, but Nicola Sturgeon would definitely have appealed to many voters down south after her phenomenal performance in the TV debates.
My guess is the SNP chose STV for their manifesto in order to tempt the Lib Dems, and that’s of course a completely valid reason to opt for this, but the Danish system would be much better for the SNP.