Fair Voting Methods and Aggregations in Digital Participatory Budgeting: Proportionality and Legitimacy

The past few years have seen the rise of digital Participatory Budgeting (PB) as a popular democratic approach for cities to directly involve citizens in the allocation of public funds. However, much of the process design is still being formulated, and cities often have to experiment with their process design to ensure fairness and legitimacy. This paper aims to address the lack of empirical research in the field and provide guidance to city practitioners on the design decisions they can make to optimize the fairness and political legitimacy of digital participatory processes. From a Social Choice perspective, fairness in PB often implies a proportional distribution of a public budget. However, most cities currently use a simple rule to choose winning projects, which can suffer from the negative effects of majority politics and minority interests being overlooked. To explore alternative solutions, this paper examines prevalent input and aggregation methods in recent social choice literature and real-world PB practices, specifically an emerging voting aggregation method called Method of Equal Shares that focuses on proportionality in PB. Using behaviour experiments, this paper investigates the economic and social trade-offs in the voting process of digital PB programs and explores the main challenges from the perspectives of social choice and political legitimacy. The objective is to identify the combination of input and aggregation methods that may be suitable for different voting scenarios in digital participatory processes. The findings highlight the importance of proportionality considerations and provide valuable insights for the design of digital PB systems that prioritize fairness and legitimacy in the real world.