Demographic Stability of a Degressively Proportional Allocation

According to the Treaty on European Union, the composition of the European Parliament must be degressively proportional with respect to the population size of the individual Member States of the European Union. The reference point is always the population data from the year preceding the elections for the five-year parliamentary term. During the term, however, population sizes may change, which can lead to a violation of the principle of degressive proportionality. In this paper, the concept of demographic stability of a degressively proportional allocation rule is defined, and based on this definition, a coefficient is constructed whose maximisation leads to the identification of allocations that are stable in the sense defined above. A rule based on this maximisation has been empirically verified using data from the 2024 – 2029 parliamentary term and changes in population size that occurred by 2025. The verification confirmed the high effectiveness of this rule in preserving the property of degressive proportionality.Copyright© 2025 The Author(s). This article is distributed under the terms of the license CC-BY 4.0., which permits any further distribution in any medium, provided the original work is properly cited.Article’s history: Received 29th of July, 2025; Received in revised form 16th of August, 2025; Accepted 14th of September, 2025; Available online: 30th of September, 2025. Published as article in the Volume XX, Fall, Issue 3(89), 2025.