Williams syndrome (WS) is a rare genetic disorder involving a characteristic cardiac defect, typical facial appearance, and an uneven profile of cognitive strengths and weaknesses. WS is caused by a hemizygous deletion in chromosome band 7q11.23, including the gene for elastin (ELN). Typically, individuals with WS seem driven to greet and interact with strangers. The goal of the present study was to investigate age-related changes in the expression of hypersociability in WS. Parents of 64 children with WS, 31 children with Down syndrome (DS), and 27 normal controls (NC) provided data concerning specific aspects of their children's social behavior using the Salk Institute Sociability Questionnaire (SISQ). Children ranged in age from 1 year, 1 month to 12 years, 10 months. Consistent with earlier findings, whole group analyses showed the WS group to be significantly higher on all aspects of sociability studied. Comparisons among the groups at different ages revealed that hypersociability is evident even among very young children with WS, and, significantly, children with WS exceed children with DS with respect to Global Sociability and Approach Strangers in every age group. The findings from children who have the typical deletion for WS are contrasted with data obtained from a young child with WS who has a smaller deletion and many physical features of WS, but who does not demonstrate hypersociability, providing intriguing clues to a genetic basis of social behavior in this syndrome. These data suggest the involvement of a genetic predisposition in the expression of hypersociability in WS. © 2003 Wiley-Liss, Inc.