Genetic factors in stuttering confirmed.

To the Editor:-Pedigree or family studies of stuttering have shown increased risks among relatives, but they have not distinguished between the importance of genetic and shared environmental influences. Twin studies that are free of ascertainment bias can distinguish between these causes, but previous twin studies on clinical samples have properly not sought to distinguish. We report data from a large nonclinical sample of twins that are consistent with a model in which 71% of the variance in liability to stuttering was attributed to additive genetic variance. An alternative model positing familial aggregation due to individual and shared family environment did not fit the data. Stuttering is a developmental disorder of speech motor control not related to neurosis or emotional disorder in either the parent or child. 1 Ideopathic stuttering begins between ages 2 and 12 years, is more common in males (sex ratio, 2.4:1), and is familiaL 2.3 Two large pedigree studies3.4 (total probands = 724) found that the risk of a first-degree relative's being a stutterer was threefold to sixfold more than the risk in the general population, and these risks varied in ways consistent with polygenic sex-limited inheritance. Studies in which twin pairs are ascertained from clinics or from advertisements show increased pairwise concordance in monozygotic (MZ) (MZ = 49/66 pairs) vs dizygotic (OZ) twins (OZ = 10191 pairs), an MZlDZ concordance ratio of 6.8:1. 5 None of these twin samples was from an unselected population, and thus the ascertainment bias may have distorted these results. We inserted an item about stuttering in an extensive questionnaire mailed to a quasipopulation sample LEITERS TO THE EDITOR