Contamination from below: devolution and party systems in the United Kingdom

This paper examines if voters in Scotland, Wales and Northern Ireland are considering results from devolved level elections when casting general election ballots. It questions if this could lead to a change in the two party systems which had been relatively steady in Scotland and Wales since the 1920s, and Northern Ireland since the 1970s. This paper suggests that some voters are indeed taking cues from the devolved tier, which operate under proportional rules, and adapting their voting behaviour at majoritarian Westminster elections. This paper contends that this has removed an important barrier preventing historically smaller partiers from upsetting the long standing two party systems in two of the three cases, with only the system in Wales remaining in place. This paper consists of constituency level tests of all general and devolved elections from 1997-2022, across three distinct political entities using methods devolped to examine contamination effects and strategic voting in other jurisdictions. It accounts for important differences between the devolved authorities as well as differences in turnout and Northern Ireland's use of STV at the devolved tier.