Journal of Fluency Disorders 37 (2012) 202–210 Contents lists available at SciVerse ScienceDirect Journal of Fluency Disorders Genetic and environmental effects on stuttering: A twin study from Finland Pirkko Rautakoski * , Therese Hannus, Susanna Simberg, N. Kenneth Sandnabba, Pekka Santtila Center of Behavior Genetics, Abo Akademi University, Finland a r t i c l e i n f o Article history: Received 17 August 2011 Received in revised form 25 December 2011 Accepted 25 December 2011 Available online 28 February 2012 Keywords: Stuttering Prevalence Twin study Behavioral genetics a b s t r a c t The present study explored the prevalence of self-reported stuttering in a Finnish twin population and examined the extent to which the variance in liability to stuttering was attributable to genetic and environmental effects. We analyzed data of 1728 Finnish twins, born between 1961 and 1989. The participants were asked to complete a questionnaire on speech, language, and voice. In two of the questions they were asked to report the occur- rence of childhood and present stuttering of their own and that of their sibling. According to the results, 2.3% (52) of the participants were reported to have stuttered as children and 28.8% of them (15) were reported to continue to stutter in adulthood. There was no signifi- cant gender difference in the prevalence of stuttering in either childhood or adulthood. For childhood stuttering, the tetrachoric correlation was higher for monozygotic pairs (r = .74) than for dizygotic pairs (r = .27). By means of structural equation modeling it was found that 82% of the variance in liability to childhood stuttering was attributable to additive genetic effects, with the remaining 18% due to non-shared environmental effects. In conclusion, the results of the present study confirm findings from prior studies and support a strong genetic and only a moderate non-shared environmental effect on stuttering. Potential small differ- ences in the prevalence of stuttering in different populations are suggested by our data. Educational objectives: The reader will be able to recognize the contribution of genetic and environmental effects on stuttering. © 2012 Elsevier Inc. All rights reserved. 1. Incidence and prevalence of stuttering According to a review by Guitar (2006), the incidence of stuttering is about 5%. However, there is some variation in the incidence of stuttering in different studies depending on the population and the method used. The incidence of stuttering in three year old children, investigated by speech screening, was 5.2% in Månsson’s (2000) study, while the overall incidence of stuttering was 2.2% in a population study carried out by telephone interviews by Craig, Hancock, Tran, Craig, and Peters (2002). In twin studies, the reported incidence and prevalence rates vary to some extent. When the population was of adults and self-reports were used, the incidence was 3.2% for males and 1.2% for females in a study by Andrews, Morris-Yates, Howie, and Martin (1991), 8.8% in a study by Felsenfeld et al. (2000) and 5.6–5.8% in younger groups and 4.5% in the oldest group in the study by Fagnani, Fibiger, Skytthe, and Hjelmborg (2010). * Corresponding author at: Abo Akademi University, Tehtaankatu 2, FI-20500 Turku, Finland. Tel.: +358 2 215 3631; fax: +358 2 215 4534. E-mail address: pirkko.rautakoski@abo.fi (P. Rautakoski). 0094-730X/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jfludis.2011.12.003