Thalia C. Eley¹, & The Twins' Early Development Study²

This paper presents initial findings from the first birth-cohort in the Twins' Early Development Study. This is the first twin study of vocabulary development and mild mental retardation in infancy. Vocabulary scores (L. Fenson, P. S. Dale, J. S. Reznick, E. Bates, D. Thal, & S. J. Pethick, 1994, Monog. Soc. Res. Ch. Dev.,59: Serial No. 242, 1-173) and a parent-administered test of non-verbal cognitive ability (PARCA: K. J. Saudino, P. Dale, B. R. Oliver, S. A. Petrill, V. Richardson, M. Rutter, E. Simonoff, J. Stevenson, R. Plomin, in press, Br. J. Dev. Psych.) were collected from 3033 pairs of two-year-old twins. Model-fitting analyses of the variance-covariance matrices for the full distribution, and extreme group analyses using DF regression (J. C. DeFries, & D. W. Fulker, 1985, Beh. Genet.,15,467-473) for the lowest 5% of the sample were conducted for the vocabulary and non-verbal cognitive measures. Extreme group heritability for vocabulary scores (h²_g = .59, 95% CI .24-.94) was substantially greater than individual differences heritability (h² = ??, 95% CI ??-??). The reverse pattern was found for shared environment (c²_g = .31, 95% CI .00-.92, c² = ??, 95% CI ??-??). In contrast, the results from the PARCA indicate similar levels of genetic and shared environment influence on individual differences in the normal range and extreme group membership (h²_g = .25, 95% CI .05-.46, h² = ??, 95% CI ??-??, c²_g = .54, 95% CI .22-.86, c² = ??, 95% CI). These results suggest that while the genes influencing language disability may differ from those influencing language ability, the genetic influences on mild mental retardation are similar to those on the full range of cognitive ability in infancy.

Address:   Social, Genetic and Developmental Psychiatry Research Centre, Institute of Psychiatry, De'Crespigny Park, London SE5 8AF

¹Social, Genetic and Developmental Psychiatry Research Centre, Institute of Psychiatry, University of London, UK. ²The Twins' Early Development Study is funded by the Medical Research Council, UK. Principal Investigator: Robert Plomin; Co-Investigators: Dorothy Bishop, Philip Dale, Emily Simonoff, Jim Stevenson, Eric Taylor; Project Co-ordinator: Bonamy Oliver

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