Low Expressive Vocabulary Higher Heritability as a Function of More Severe Cases Research Article
Research Article  |   August 01, 2005
Low Expressive Vocabulary
 
Author Affiliations & Notes
  • Laura S. DeThorne
    Pennsylvania State University, University Park
  • Stephen A. Petrill
    Pennsylvania State University, University Park
  • Marianna E. Hayiou-Thomas
    Institute of Psychiatry, King's College, London, United Kingdom
  • Robert Plomin
    Institute of Psychiatry, King's College, London, United Kingdom
  • Corresponding author: e-mail: lauras@uiuc.edu
Article Information
Special Populations / Genetic & Congenital Disorders / Language Disorders / Attention, Memory & Executive Functions / Language / Research Articles
Research Article   |   August 01, 2005
Low Expressive Vocabulary
Journal of Speech, Language, and Hearing Research, August 2005, Vol. 48, 792-804. doi:10.1044/1092-4388(2005/055)
History: Received April 28, 2004 , Revised August 23, 2004 , Accepted September 28, 2004
 
Journal of Speech, Language, and Hearing Research, August 2005, Vol. 48, 792-804. doi:10.1044/1092-4388(2005/055)
History: Received April 28, 2004; Revised August 23, 2004; Accepted September 28, 2004
Web of Science® Times Cited: 17

This study of 4,274 pairs of 4-year-old twins from the Twins Early Development Study explored the magnitude of genetic and environmental effects on low expressive vocabulary skill, both as a function of general cognitive ability and as a function of the severity of expressive vocabulary impairment. Assessments were conducted through parent report measures. Two types of vocabulary deficit were identified: low vocabulary paired with typical general cognition (i.e., specific expressive vocabulary impairment) and low vocabulary paired with low general cognition (i.e., nonspecific expressive vocabulary impairment). The magnitude of genetic and environmental effects on low expressive vocabulary skill did not differ for these 2 types of expressive vocabulary deficit. By systematically varying the cutoffs used to define vocabulary and general cognitive delay, potential changes in the magnitude of genetic and environmental effects were examined. Results suggested that the severity of vocabulary deficit rather than level of cognitive functioning was a more meaningful etiological distinction: The heritability of low expressive vocabulary was higher and the influence of shared environment lower, as increasingly severe vocabulary deficits were identified. Implications for molecular genetics and the construct of specific language deficits are discussed.

Acknowledgments
The Twins Early Development Study (TEDS) is funded by Program Grant G9424799 from the U.K. Medical Research Council, awarded to Robert Plomin. Thanks to Dean Coward and the College of Health and Human Development at Pennsylvania State University for supporting the international travel that facilitated this study . In addition, we would like to recognize and thank the numerous twins and their families that made TEDS, and the many projects that emerged from it, a reality.
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