Although it has been well established that genetic factors are involved in the onset of stuttering, much less is known about the extent to which genes influence the expression and pathogenesis of this complex and variable condition. Using a large population-based sample of Adult Australian twins in two age cohorts, this study will be the first to obtain heritability estimates for specific clinical features of stuttering. Based upon a database search and attrition analysis that has already been performed, it is estimated that 225 complete pairs of twins in which at least one member of the pair self-reported a history of stuttering will be recruited to participate in this study. These 225 pairs and 50 control pairs will complete a telephone interview and a brief monologue that will be audio-taped directly from the phone lines. Using quantitative genetic modeling procedures, these data will be influenced to estimate the extent to which genetic factors influence stuttering characteristics in two clinical domains: symptom expression and extent of affective involvement. For affected cases and their cotwin, data assessing self-reported relapse, recovery, and the presence of co-morbid communication disorders in childhood will also be obtained. To address important concerns about diagnostic classification, the results from the clinical interview will be validated against self-report alone and against a best estimate consensus diagnosis obtained from clinical experts who have access to all relevant diagnostic indicators. As a secondary objective, this study will examine the relationship between personality and speech outcome in the affected cases and will compare personality scores of the affected cases and will compare personality scores of the affected cases with personality scores obtained from the entire twin study sample. In addition to answering the important questions posed here, the present study is likely to generate a number of intriguing hypothesis about the origins of individual difference for this condition that can be tested in future investigations using more accessible, non-twin, clinically-based samples.