Intrapair similarity in frequency of disfluency in monozygotic and dizygotic twin pairs containing stutterers.

It is known that concordance for stuttering is higher in monozygotic (MZ) than in dizygotic (DZ) twins. Evidence for genetic contribution to the frequency of speech disruption in stutterers was examined in 17 pairs of MZ and 13 pairs of DZ twins, each pair containing at least one stutterer. Intraclass correlations and analyses of variance indicated positive evidence for a genetic contribution to the overall frequency of disfluency in speech and to the frequency of certain types of speech disruption (blocked and prolonged sounds). There was little evidence for genetic influence on other types of speech disruption (various types of repetitions and interjections) In those disfluency types for which genetic predictions were confirmed, evidence of excessive dissimilarity in DZ cotwins suggests that a simple, additive gene-environment model is unlikely to be appropriate for disfluency frequency data.