Selective mutism (SM) is a rare social anxiety disorder characterized by failure to speak in some situations despite normal verbal behavior in others. Previous work suggests a familial component for a predisposition to this disorder. The present article reviews the literature on twins with SM and examines the onset and progress of SM in a monozygotic female pair, age 5 years, 7 months. Information was gathered from parent and teacher interviews, birth records, direct observation, standard child assessment protocols and school performance reports. The variable symptom expression and severity in this concordant twin pair underline contributions from both genetic and environmental sources. Some unusual behavioral features observed in these twins, such as their popularity and non-verbal communication with classmates, underline the complexity of factors affecting the expression of SM. Pooling available data from affected monozygotic and dizygotic twins would advance understanding of the nature, course and management of this condition.
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