Using observational methods, we examined the social influences on laughing and smiling behavior in children with Angelman syndrome by systematically manipulating aspects of social interaction. Seven boys and 4 girls who were between 4 and 11 years of age and who had a confirmed maternal deletion of chromosome 15q11-q13 completed the study. Each child was observed while repeatedly exposed to three conditions in which parameters of social interaction were manipulated. Laughing and smiling behavior varied across all children and was significantly heightened in a condition involving adult speech, touch, smiling, laughing, and eye contact. The findings highlight the importance of examining environmental and social influences on purported phenotypic behavior in genetic syndromes.