Motor similarity sharpens affect representation by acting as an embodied prior during action observation

Social interaction requires inferring others' affect from body movements, yet how the observer's own motor repertoire shapes this process remains unclear. Here, we investigated whether motor similarity between observer and agent modulates affect decoding from whole-body movements in social interactions. Using motion capture to quantify individual motor similarity, combined with behavioral ratings and fMRI multivariate pattern analysis, we examined neural representations of affective valence across varying levels of affective intensity. Behaviorally, motorically dissimilar movements were perceived as more emotionally intense, possibly showing increased salience for unfamiliar kinematics. At the neural level, motor similarity attenuated responses in the inferior parietal lobule while enhancing affect decoding in the inferior frontal gyrus. Importantly, decoding accuracy in the inferior frontal gyrus peaked for motor-similar interactions with high affective intensity. Together, these findings suggest that motor similarity might function as an embodied prior that sharpens affective representations and supports affect understanding from movement. The results align with hierarchical predictive-coding accounts, highlighting how the observer's own body shapes social perception.