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This is the third in a short series of blog posts about the impact of electoral systems on electoral and party politics in Wales, drawing on research for my forthcoming book on Elections in Wales.

In a previous blog post looking at the impact of the First Past the Post and AMS electoral systems in Wales, I explored their impact on Wales’ dominant Party, the Labour party. Of course, the impacts of electoral systems are felt by all parties, not just the largest one, and in this blog post I broaden the attention out from Labour.

Scholars have come up with various ways to measure the extent to which electoral systems produce outcomes in terms of seats that vary from the share of the vote that political parties win. Probably the most widely-used such measure is the Gallagher Index – developed by Prof Michael Gallagher of Trinity College Dublin. His index produces an overall score for each election. Those scores can potentially run from 0 (which would mean that the percentage of seats allocated to all parties matched exactly the percentage of votes that they had each won) to a theoretical maximum of 100 (albeit in the somewhat unlikely scenario where all of the seats went to parties that had won none of the votes!). In practice, scores in double figures indicate a significant degree of dis-proportionality in an election outcome.

As can be seen in the Figure below, every Assembly election has produced a lower Gallagher Index score than any of the Westminster general elections during the same era. This confirms that the proportional element in AMS does indeed make the outcomes in terms of seats more proportional to the share of votes that parties win.

Gallagher Index Scores: Wales 1997-2017 (click on picture for larger version)

But the recent trend in Westminster elections under First Past The Post has actually been for the disproportionality of election outcomes in Wales to reduce somewhat. This reflects primarily the decline in Labour seats in the House of Commons between 2001-2015 (the party’s total of Welsh MPs fell from 34 to 25), which reduced the extent to which Labour was over-represented at the expense of other parties; and then the large rise in Labour vote share in 2017, which brought the party’s share of popular support closer to their level of parliamentary representation.

In contrast, NAW elections have generally become more disproportional. Labour’s success in holding all but one of its previous seats in 2016, even as its vote share fell considerably from that in 2011, helped make that the most dis-proportional Assembly election yet. The absolute level of disproportionality in Wales at that election was also strikingly high for an allegedly ‘proportional’ electoral system, with a Gallagher Index score for 2016 well into double figures (at 11.5). In Scotland – where AMS is also used to elect the Scottish Parliament, but where 43 percent of the seats are allocated proportionally via the regional lists, rather than the 33 percent in Wales – levels of disproportionality have been consistently lower. The 2016 Scottish Parliament election, held on the same day as the Welsh contest, produced a result which generates a Gallagher Index score of only 6.2.

The AMS electoral system used for NAW elections has thus moderated the impact of First Past The Post but not overturned it. Labour have still won the most votes on both ballots in every NAW election thus far, so it is unsurprising that they have always been the largest party in the Assembly. But Labour have never won an outright majority in a devolved election, and AMS has ensured decent levels of representation for the main opposition parties. Nonetheless, those parties have remained a long way from offering a serious challenge to Labour dominance. At no Assembly election has any of its opponents ever come within even ten seats of Labour. That is partly due to the way AMS was applied in Wales: with only a modest proportional element to the system, it is not able to fully balance out Labour’s dominance of the constituency seats. But the absence of a serious challenge to Labour in the Assembly is primarily due to a more basic political factor – the failure of the opposition parties to win sufficient electoral support.