Reading Smiles: Proxy Bias in Foundation Models for Facial Emotion Recognition

--Foundation Models (FMs) are rapidly transforming Affective Computing (AC), with Vision-Language Models (VLMs) now capable of recognising emotions in zero-shot settings. This paper probes a critical but underexplored question: what visual cues do these models rely on to infer affect, and are these cues psychologically grounded or superficially learnt? We benchmark varying scale VLMs on a teeth-annotated subset of AffectNet dataset and find consistent performance shifts depending on the presence of visible teeth. Through structured introspection of -the best-performing model, i.e., GPT -4o, we show that facial attributes like eyebrow position drive much of its affective reasoning, revealing a high degree of internal consistency in its valence-arousal predictions. These patterns highlight the emergent nature of FMs behaviour, but also reveal risks: shortcut learning, bias, and fairness issues--especially in sensitive domains like mental health and education. Understanding and interpreting human emotions is fundamental to social interaction. From early developmental cues in infants, to high-stakes decision-making in adults, facial expressions serve as a primary channel for conveying affect.