Williams syndrome (WS) is a neurodevelopmental disorder associated with hypersociability and anxiety. However, little is known about how these salient aspects of the phenotype are related or their underlying physiology. We examined cortisol reactivity in WS because cortisol is responsive to psychosocial stress. Compared to typically developing adults, adults with WS had a significant cortisol decrease in response to a challenging cognitive battery. In contrast, cortisol levels in WS stayed stable in response to a solo musical performance, and baseline cortisol levels were significantly associated with musical skill. Results indicate that people with WS respond differentially to different socially-loaded situations. Implications for salience and arousal in cognitive and social situations are discussed.