The Illusion of Specialization: Unveiling the Domain-Invariant "Standing Committee" in Mixture-of-Experts Models

TL;DR

This paper reveals that Mixture-of-Experts models rely on a domain-invariant 'Standing Committee' of experts, challenging the assumption of domain-specific specialization and highlighting a structural bias toward centralized computation.

Contribution

Introduces COMMITTEEAUDIT, a framework for analyzing expert routing, and uncovers a universal, domain-invariant 'Standing Committee' that dominates routing across models and domains.

Findings

Standing Committees consistently capture most routing mass across domains

Qualitative analysis shows Committees anchor reasoning and syntax

Peripheral experts handle domain-specific knowledge

Abstract

Mixture of Experts models are widely assumed to achieve domain specialization through sparse routing. In this work, we question this assumption by introducing COMMITTEEAUDIT, a post hoc framework that analyzes routing behavior at the level of expert groups rather than individual experts. Across three representative models and the MMLU benchmark, we uncover a domain-invariant Standing Committee. This is a compact coalition of routed experts that consistently captures the majority of routing mass across domains, layers, and routing budgets, even when architectures already include shared experts. Qualitative analysis further shows that Standing Committees anchor reasoning structure and syntax, while peripheral experts handle domain-specific knowledge. These findings reveal a strong structural bias toward centralized computation, suggesting that specialization in Mixture of Experts models is far less pervasive than commonly believed. This inherent bias also indicates that current training objectives, such as load-balancing losses that enforce uniform expert utilization, may be working against the model's natural optimization path, thereby limiting training efficiency and performance.