Do vocabulary and grammar have similar profiles of heritability and environmental influence?

1
Heritability of past tense marking and the
grammar-vocabulary distinction A twin
study Jennifer B. Ganger1, Steven Pinker2, Sonia
Chawla3, Jeffrey Birk2 1University of Pittsburgh,
2Harvard University, 3Boston University
SUMMARY
BACKGROUND

• Distinction between vocabulary and grammar
  questioned in recent empirical and theoretical
  work.
• Twins (behavior genetics) can address the issue.

RESULTS I UNIVARIATE
RESULTS II BIVARIATE

• Irregular correct past tense marking
  (vocabulary) mainly influenced by shared
  environment
• Regular past tense marking patterned with
  irregular marking, suggesting memorization plays
  a role in early learning of these forms
• Overregularization (grammar) significant
  heritability, small non-shared environment, no
  shared environment
• No phenotypic (or genetic) correlation between
  overregularization and irregular past tense

• 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.
• Not reliable with this sample size, but appears
  small
• Genetic correlation (rg) ranges from .00 to .01

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)?
• 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

Modeling Results (Univariate)
CONCLUSIONS
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.
Irregular past tense marking Irregular past tense marking Irregular past tense marking Irregular past tense marking Irregular past tense marking Irregular past tense marking Irregular past tense marking Irregular past tense marking
Model h2 c2 e2 ?2(df) p ??2(df) p
ACE .16 .77 .06 10.99(3) .012
CE .90 .10 22.84(4) .000 11.84(1) .001
AE .93 .07 46.68(4) .000 35.69(1) .000
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)
• Participants
• 90 MZ (40 M, 50 F) 61 DZ (31 M, 30 F) same-sex
  twin pairs M 2.29 (.40) years old
• Procedure
• Parents kept journals of twins past tense use
• Statistical Analysis
• h2 and c2 estimated with DeFries-Fulker
  regression with modifications by Rodgers Kohler
  
• Patterns confirmed using SEM with Mx (Neal
  Cardon)

REFERENCES
DeFries, 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, 33, 339-368. Neale, M. C.,
Cardon, L. R. (1992). Methodology for Genetic
Studies of twins and families. Dordrecht Kluwer.
Overregularization Overregularization Overregularization Overregularization Overregularization Overregularization Overregularization Overregularization
Model h2 c2 e2 ?2(df) p ??2(df) p
ACE .40 .35 .25 2.77(3) .429
CE .68 .32 6.95(4) .138 4.19(1) .041
AE .75 .25 4.46(4) .347 1.70(1) .193