Longitudinal Study of Language and Speech of Twins at 4 and 6 Years: Twinning Effects Decrease, Zygosity Effects Disappear, and Heritability Increases Purpose This study investigates the heritability of language, speech, and nonverbal cognitive development of twins at 4 and 6 years of age. Possible confounding effects of twinning and zygosity, evident at 2 years, were investigated among other possible predictors of outcomes. Method The population-based twin sample included 627 ... Research Article
Research Article  |   January 22, 2018
Longitudinal Study of Language and Speech of Twins at 4 and 6 Years: Twinning Effects Decrease, Zygosity Effects Disappear, and Heritability Increases
 
Author Affiliations & Notes
  • Mabel L. Rice
    Child Language Doctoral Program, University of Kansas, Lawrence
  • Stephen R. Zubrick
    Graduate School of Education, University of Western Australia, Perth
    The Telethon Kids Institute, Subiaco, Australia
  • Catherine L. Taylor
    The Telethon Kids Institute, Subiaco, Australia
    Centre for Child Health Research, The University of Western Australia, Perth
  • Lesa Hoffman
    Child Language Doctoral Program, University of Kansas, Lawrence
  • Javier Gayán
    Bioinfosol, Seville, Spain
  • Disclosure: The authors have declared that no competing interests existed at the time of publication.
    Disclosure: The authors have declared that no competing interests existed at the time of publication. ×
  • Correspondence to Mabel L. Rice: mabel@ku.edu
  • Editor: Sean Redmond
    Editor: Sean Redmond×
  • Associate Editor: Emma Hayiou-Thomas
    Associate Editor: Emma Hayiou-Thomas×
Article Information
Development / Special Populations / Genetic & Congenital Disorders / Speech, Voice & Prosody / Language / Research Articles
Research Article   |   January 22, 2018
Longitudinal Study of Language and Speech of Twins at 4 and 6 Years: Twinning Effects Decrease, Zygosity Effects Disappear, and Heritability Increases
Journal of Speech, Language, and Hearing Research, January 2018, Vol. 61, 79-93. doi:10.1044/2017_JSLHR-L-16-0366
History: Received September 17, 2016 , Revised June 26, 2017 , Accepted September 6, 2017
 
Journal of Speech, Language, and Hearing Research, January 2018, Vol. 61, 79-93. doi:10.1044/2017_JSLHR-L-16-0366
History: Received September 17, 2016; Revised June 26, 2017; Accepted September 6, 2017

Purpose This study investigates the heritability of language, speech, and nonverbal cognitive development of twins at 4 and 6 years of age. Possible confounding effects of twinning and zygosity, evident at 2 years, were investigated among other possible predictors of outcomes.

Method The population-based twin sample included 627 twin pairs and 1 twin without a co-twin (197 monozygotic and 431 dizygotic), 610 boys and 645 girls, 1,255 children in total. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using general linear mixed models using maximum likelihood.

Results Twinning effects decreased from 4 to 6 years; zygosity effects disappeared by 6 years. Heritability increased from 4 to 6 years across all 9 phenotypes, and the heritability estimates were higher than reported previously, in the range of .44–.92 at 6 years. The highest estimate, .92, was for the clinical grammar marker.

Conclusions Across multiple dimensions of speech, language, and nonverbal cognition, heritability estimates are robust. A finiteness marker of grammar shows the highest inherited influences in this early period of children's language acquisition.

Acknowledgments
This study was supported by NIH R01DC005226 (Rice, Zubrick, and Taylor as PIs). Preparation of this paper was supported by NIH R01DC001803 (Rice, PI), NIH P30DC005803 (Rice, PI), NIH R42DC013749 (Rice as co-PI), and NIH P30HD002528 (Rice as affiliated researcher). Zubrick and Taylor were supported by a grant from the Australian Research Council (CE140100027).
The authors especially thank the children and families who participated in the study and the following members of the research team: Sarah Beveridge-Pearce, Bradley Calamel, Tanya Dickson, Julie Fedele, Lucy Giggs, Antonietta Grant, Jennifer Hafekost, Erika Hagemann, Jessica Hall, Anna Hunt, Alicia Lant, Stephanie McBeath, Megan McClurg, Alani Morgan, Virginia Muniandy, Elke Scheepers, Elke, Leanne Scott, Michaela Stone, and Kerry Van de Pol. The authors also thank Denise Perpich for data management and preliminary data summaries and the staff at the Western Australian Data Linkage Branch and the Maternal and Child Health Unit.
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