Concordance for stuttering in monozygotic and dizygotic twin pairs.

The role of genetic factors in the etiology of stuttering was investigated in 30 pairs of same-sex twins, each pair containing at least one stutterer. Care was taken to ensure maximum accuracy in zygosity classification and objectivity in stuttering diagnosis. Zygosity classification was made independently of stuttering diagnosis. Pair-wise concordance for stuttering was significantly higher in identical twins (63%) than in fraternal same-sex twins (19%). The estimated risk of stuttering in the monozygotic (MZ) co-twin of a stutterer was .77, and .32 in the dizygotic (DZ) co-twin of a stutterer. The findings strongly suggest the operation of genetic factors in stuttering. However, the presence of as many as six pairs of identical twins who were discordant for stuttering underlines the importance of the interaction of genetic and environmental factors in the etiology of this disorder.