title: Bivariate genetic analyses of stuttering and nonfluency in a large sample of 5-year-old twins.
abstract: OBJECTIVE
  Behavioral genetic studies of speech fluency have focused on participants who present with clinical stuttering. Knowledge about genetic influences on the development and regulation of normal speech fluency is limited. The primary aims of this study were to identify the heritability of stuttering and high nonfluency and to assess the relative contribution of genetic and environmental factors to the correlation between these 2 fluency phenotypes.
  
  
  METHODS
  Information on 6 specific speech fluency behaviors was obtained by maternal report for over 10,500 5-year-old Dutch twin pairs.
  
  
  RESULTS
  Genetic analyses revealed that both fluency phenotypes were moderately heritable, with heritability estimates of 42% and 45% for probable stuttering and high nonfluency, respectively. Shared environmental factors were also significant, explaining 44% of the individual differences in probable stuttering and 32% in nonfluency. For both phenotypes, the magnitude of the genetic and environmental influences did not differ between boys and girls. The overlap between the 2 traits was substantial (tetrachoric correlation was .72). A bivariate genetic analysis showed that this overlap was due to both overlapping genetic and environmental influences.
  
  
  CONCLUSIONS
  These findings provide a foundation to justify further studies in normal fluency control, a scientific area that has received little cross-disciplinary attention.
authors.affiliation: VU University, Department of Biological Psychology, Van der Boechorststraat 1, 1081 BT Amsterdam, the Netherlands. toos@psy.vu.nl
authors.forename: Catharina Eugenie Maria
authors.initials: CE
authors.lastname: van Beijsterveldt
authors.forename: Susan
authors.initials: S
authors.lastname: Felsenfeld
authors.forename: Dorret Irene
authors.initials: DI
authors.lastname: Boomsma
mesh_terms: D002663:Child Language; D002675:Child, Preschool; D015331:Cohort Studies; D004777:Environment; D005260:Female; D006801:Humans; D008297:Male; D008957:Models, Genetic; D015999:Multivariate Analysis; D009426:Netherlands; D010641:Phenotype; D011336:Probability; D012737:Sex Factors; D013064:Speech Disorders / Q000235:genetics*; D013342:Stuttering / Q000235:genetics*; D011795:Surveys and Questionnaires; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic