Each of the 650 constituencies in the UK has roughly (in a few cases, very roughly) the same number of electors, meaning that the effect of people's votes is roughly the same. Over time, however, people move about the country, the population is some areas falls, in some areas it rises, in some areas it rises faster than other areas. In recent decades the trend has been for the population in declining industrial areas to fall, while the population in the south-east grows. This means that, over time, seats become increasingly unequal in electorate - to given an extreme example, before the 3rd review was implemented in 1983 there were seats in inner-city Glasgow where the electorate had declined to below 20,000 but seats containing booming newtowns like Midlothian (Livingston) and Buckingham (Milton Keynes) had grown to over 100,000.
Boundary reviews have been a feature of the British political system since the Great Reform Act, with several ad hoc reviews of seats in the nineteenth and early twentieth century. Since the second war these have been put on a statutory timetable with an initial review after the war and periodic reviews since then. These rules governing these reviews have undergone several amendments and tweaks since they were originally passed, but the fundementals have remained the same. Changes have largely been adjusting the balance between drawing up seats that reflect communities, and drawing up seats that have the same number of voters, or changing the timescales for when boundary commissions should report.
The boundaries are drawn up by independent boundary commissions, one each for England, Scotland, Wales and Northern Ireland. The Speaker is the notional chair of the Commissions, but in practice they are made up of senior independent figures such as Judges, QCs and retired senior civil servants. The rules require the Boundary Commissions to adjust the existing seats so they are roughly equal in terms of their electorate (based on the electoral register) at the start of the review, taking into account things like local government boundaries and "local ties". Since the 1950s these have happened, on average every 13 to 14 years.
Because of the demographic trends in Britain, with the population in (Labour voting) inner-city areas generally declining, and the population in (Conservative voting) suburbs and the semi-rural south growing, boundary changes normally favour the Conservative party, cancelling out a seat-size bias towards Labour that gradually develops between reviews. In the past this has meant the Conservative party has often sought to speed up or bring forward reviews (such as in the Boundary Commissions Act 1992 and the Parliamentary Voting System and Constituencies Act 2011) and the Labour party has often sought to delay or prevent the implementation of reviews (such as the Callaghmander of 1969, R v Boundary Commission for England, ex parte Foot 1983 and the amendments to the Electoral Registration and Administration Act 2013).
There have been five reviews since the regime of periodic reviews was introduced after the war, the last being carried out between 2001 and 2007 and introduced for the 2005 in Scotland and the 2010 election for England, Wales and Northern Ireland. The sixth review was brought forward by the Parliamentary Voting System and Constituencies Act 2011 and had been due to report by 2013, in time for the 2015 election, but was aborted part-way through by the Electoral Registration and Administration Act 2013.
|Date Started||Date Reported||Date Implemented|