We present a systemic threat theory to explain the introduction of Proportional Representation (PR).
If facing a revolutionary threat, incumbents agree to enact electoral reforms such as PR to secure the stability of the parliamentary system, even if this could imply their own personal electoral loss. We argue that the theory can help explain the largest wave of PR adoptions in history, namely in the years immediately after the Bolshevik revolution in 1917. Incumbents came to over-estimate the true revolutionary threat in Europe. Simultaneously, reformist parliamentarian socialists came to push for PR to weaken the radicals within their party. Incumbents and reformist socialists could therefore support the same system. We test this using qualitative and quantitative data from Norway’s adoption of PR in 1919.