<p>The silent behaviour of children with selective mutism (SM) is often assumed to reflect an acute stress-related response to social-communicative anxiety, yet objective evidence remains limited, especially in young children. To explore the potential stress-related mechanisms underlying silence in socially challenging situations, we assessed both acute and chronic stress. Eighty-three children aged 4–8 years (37 with SM, 46 controls) completed two socially demanding interaction tasks - one verbal and one nonverbal - each with three phases: anticipation, performance, and recovery. Heart rate (HR) was measured at rest and continuously throughout all task phases. Chronic stress was indexed via hair cortisol concentration (HCC). Compared to controls, children with SM showed elevated resting HR and higher arousal during both verbal and nonverbal social tasks. During the verbal task, the SM group (80% remained silent) exhibited blunted HR reactivity from anticipation to performance, suggesting a silence related coping effect. No group differences emerged in arousal patterns over time during the nonverbal task. HCC levels were significantly higher in the SM group, indicating increased chronic stress, however HCC was not correlated with SM severity. These findings support the view that mutism serves as an emotion regulation mechanism to reduce arousal in anticipation of verbal demands. The results underscore the importance of stress-reducing interventions for young children experiencing high stress in daily life and support the need for exposure-based interventions for children with SM, enabling them to learn that anticipated threats often do not occur, leading to decreased arousal and improved regulation.</p>