Fairness in Federated Learning: Fairness for Whom?

Fairness in federated learning (FL) has emerged as a rapidly growing area of research, with numerous works proposing formal definitions and algorithmic interventions. Yet, despite this technical progress, fairness in FL is often defined and evaluated in ways that abstract away from the sociotechnical contexts in which these systems are deployed. In this paper, we argue that existing approaches tend to optimize narrow system-level metrics --- such as performance parity or contribution-based rewards --- while overlooking how harms arise throughout the FL lifecycle and how they impact diverse stakeholders. We support this claim through a critical analysis of the literature, based on a systematic annotation of papers for their fairness definitions, design decisions, evaluation practices, and motivating use cases. Our analysis reveals five recurring pitfalls: (1) fairness framed solely through the lens of server–client architecture; (2) a mismatch between simulations and motivating use-cases and contexts; (3) definitions that conflate protecting the system with protecting its users; (4) interventions that target isolated stages of the lifecycle while neglecting upstream and downstream effects; and (5) a lack of multi-stakeholder alignment where multiple fairness definitions can be relevant at once. Building on these insights, we propose a harm-centered framework that links fairness definitions to concrete risks and stakeholder vulnerabilities. We conclude with recommendations for more holistic, context-aware, and accountable fairness research in FL.