UK Constituency Boundary Changes: Who Does It Impact?

Earlier this month, the last of the four boundary commissions have returned to Parliament with their recommendations for redrawing the boundaries between parliamentary constituencies. The review was triggered by the Parliamentary Voting System and Constituencies Act, 2011, which made some changes to the rules on how these constituencies were to be created.

Labour supporters have claimed that these new rules result in undue gerrymandering towards the Conservatives, while Conservative supporters argue that these rules undo existing gerrymandering in favour of Labour. So who’s right?

The Rules

The new rules for constituency boundaries, as set out by the Act, are:

  • The total number of constituencies is reduced from 650 to 600
  • Four constituencies will be protected: two on the Isle of Wight, one covering Orkney and Shetland, and one covering the Western Isles
  • None of the remaining 596 constituencies may not deviate from the average electorate by more than 5% either way, unless the constituency would otherwise be too geographically large, or under certain circumstances in Northern Ireland

The 596 constituencies were allocated proportionally, using the Sainte-Laguë method, between the four nations of the UK. The Boundary Commission for England then further sub-divided its seats between its nine regions using the same method. The Boundary Commissions then attempted to define parliamentary constituencies to coincide with local council wards, as they have attempted to do in the past, though they did need to make a few exceptions to this to fit other rules.

Notional Results

Once the final recommendations were published by the four boundary commissions, UK Polling Report published an estimate of what would have happened if the 2017 election were fought on the new boundaries. While a loss of 50 seats would certainly result in all parties losing seats, Labour has lost the most by far, with 28 of their seats disappearing. The Tories would lose only ten, and would go from a minority to a majority government as a result.

So is this gerrymandering on the part of the Tories? Or is it evening out an already unlevel playing field?

The 5% Rule

Under the previous rules, there were no specific rules on constituency sizes, other than that they should be as even “as practicable”. Regionally, however, we do see significant disparities in average constituency sizes with regard to the current constituencies.

For the current constituencies, the average electorate is 72,055 voters. Thus, if the 5% rule applied, we would expect a constituency to have between 68,452 and 75,657 voters. However, nearly two thirds of constituencies (421 out of 650) fall outside of this range. The most over-represented regions are Wales, North East England, and Scotland, where you would generally expect to see more Labour support. The most under-represented regions are South East England, East of England and South West England, where you would generally expect to see more Tory support.

If you were to pivot these stats based on the party holding the seats, you would see that the average electorate in a Labour seat (70,559) is smaller than in a Tory seat (74,401). While both of these are within the 5% range, Labour holds more over-represented seats (105 vs 61) and fewer under-represented seats (68 vs 132).

Of the proposed constituencies, 571 out of 600 are within 5% of the average size of 74,537 (i.e. between 70,810 and 78,263). Note that this is slightly lower than the national quota (74,769) as the latter doesn’t count the four protected constituencies. No constituencies have an electorate more than 5% above the quota, but 21 are more than 5% above the average (14 Tory, 6 Labour, 1 SNP). Meanwhile, apart from the four protected constituencies, four additional constituencies in Northern Ireland have an electorate more than 5% below the average due to statistical difficulties in keeping to the 5% rule there.

Of the new constituencies, the average electorate in Labour seats (74,825) is far closer to the average electorate in Tory seats (74,576) that is the case presently. Both are within 1% of the nationwide average.

Formation of a Government

So how easy or difficult would it be then for the two parties to form majority governments? For this we’ll go to a slightly modified version of the infamous Swingometer, though instead of defining 0% as the result of the last election, we will define 0% as the point at which the Tories and Labour have the same number of votes nationwide.

At the last election, the Conservatives finished with a 2.4% lead over Labour in Great Britain, and it left them nine seats short of an overall majority. If the lead had been for Labour instead, and all other parties’ votes were equal, Labour would be 25 seats short of their majority.

Using the 2017 election as a baseline, the lead that Labour would need to get a majority is significantly larger than it would be for the Tories. With these new constituencies, the lead that the Tories would need to get a majority would halve, while the lead that Labour would need to get a majority would bump up slightly. A slight shift in this direction was to be expected, given the differences across constituency sizes between the parties, but it doesn’t appear to be a significant difference.

It’s perhaps worth noting that it’s theoretically easier for the Tories to earn a majority than Labour is relatively new; the 2010 election saw the Tories fall short of a majority despite a 7.1% lead over Labour, while 2005 saw a comfortable Labour majority with a lead of only 2.8%. Further analysis will be required to see whether the cause of this reversal was the collapse of the Lib Dems in Britain, the collapse of Labour in Scotland, or something else.

So is it gerrymandering or un-gerrymandering?

Yes, these new constituencies do make a Tory majority slightly easier, and a Labour majority slightly harder, than it would have been otherwise. But the shift of 1–2% is within the margin of error for a standard poll, and within the variance of swings across constituencies in a general election. Further, that shift would not have been anywhere near enough to make Tory and Labour majorities on level playing fields in 2005 or 2010.

So quite simply, this redistricting would only make a material difference to an outcome if it was close enough to begin with.

I am Joe. I am a techy at heart, a self-taught psephologist (political number cruncher), a pleasure cyclist, and someone who just calls things as he sees them.

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