Heritability of past tense marking and the grammarvocabulary

Heritability of past tense marking and the grammar-vocabulary distinction: A twin study 1 Ganger , 2 Pinker , 3 Chawla , 2 Birk Jennifer B. Steven Sonia Jeffrey 1 University of Pittsburgh, 2 Harvard University, 3 Boston University BACKGROUND • Distinction between vocabulary and grammar questioned in recent empirical and theoretical work. • Twins (behavior genetics) can address the issue. SUMMARY RESULTS I: UNIVARIATE RESULTS II: BIVARIATE • Phenotypic r = -. 05 (n. s. ) between irregular past tense marking and overregularization. • Bivariate heritability ranges from -. 63 to. 22, depending on which twins are used in single-entry analysis. QUESTIONS • Do vocabulary and grammar have similar profiles of heritability and environmental influence? • Do they have shared genetic variation (bivariate heritability) or genes (genetic correlation)? • Not reliable with this sample size, but appears small • Genetic correlation (rg) ranges from. 00 to. 01 • Previous research: genetic overlap for vocabulary and grammar (Dione et al; Hayiou-Thomas et al) • But checklists and standardized tests may not adequately isolate grammar and vocabulary METHODS Measures: English past tense forms • Vocabulary: rate of correct irregular past tense marking (e. g. , ran) • Grammar: rate of overregularization (e. g. , runned) and correct regular past tense marking (e. g. , walked) Modeling Results (Univariate) Model h 2 c 2 e 2 Χ 2(df) p ACE . 16 . 77 . 06 10. 99(3) . 012 . 90 . 10 22. 84(4) . 07 46. 68(4) CE AE . 93 Procedure • Parents kept journals of twins’ past tense use Statistical Analysis • h 2 and c 2 estimated with De. Fries-Fulker regression with modifications by Rodgers & Kohler • Patterns confirmed using SEM with Mx (Neal & Cardon) ΔΧ 2(df) p . 000 11. 84(1) . 001 . 000 35. 69(1) . 000 Overregularization Model h 2 c 2 e 2 Χ 2(df) p ACE . 40 . 35 . 25 2. 77(3) . 429 . 68 . 32 6. 95(4) . 25 4. 46(4) CE AE . 75 • Regular past tense marking patterned with irregular marking, suggesting memorization plays a role in early learning of these forms • Overregularization (grammar): significant heritability, small nonshared environment, no shared environment • No phenotypic (or genetic) correlation between overregularization and irregular past tense CONCLUSIONS Irregular past tense marking Participants • 90 MZ (40 M, 50 F) & 61 DZ (31 M, 30 F) same-sex twin pairs; M = 2. 29 (. 40) years old • Irregular correct past tense marking (vocabulary): mainly influenced by shared environment ΔΧ 2(df) p . 138 4. 19(1) . 041 . 347 1. 70(1) . 193 Individual differences in the development vocabulary and grammar appear to come from distinct sources. Results are consistent with the hypothesis that vocabulary and grammar are separate components of language. REFERENCES De. Fries, J. C. , & Fulker, D. W. (1985). Multiple regression analysis of twin data. Behavior Genetics, 15, 467 -473. Rodgers, J. L. , & Kohler, H. -P. (2005). Reformulating and simplifying the DF analysis model. Behavior Genetics, 35(2), 211 -217. Dionne, G. , Dale, P. S. , Boivin, M. , & Plomin, R. (2003). Genetic evidence for bidirectional effects of early lexical and grammatical development. Child Development, 74(2), 394 -412 Hayiou-Thomas, M. E. , Kovas, Y. , Harlaar, N. , Plomin, R. , Bishop, D. V. M. , & Dale, P. S. (2006). Common aetiology for diverse language skills in 4 1/2 -year-old twins. Journal of Child Language, 339 -368. Neale, M. C. , & Cardon, L. R. (1992). Methodology for Genetic Studies of twins and families. Dordrecht: Kluwer.