Authors:
Rice ML, Zubrick SR, Taylor CL, Gayán J, Bontempo DE
Authors notes:
Journal of Speech, Language, and Hearing Research 57(3): 917-928.
Keywords:
language acquisition, late language emergence, twin early grammar, twin language, twin zygosity effects
Abstract:
This study investigated the etiology of late language emergence (LLE) in 24-month-old twins, considering possible twinning, zygosity, gender, and heritability effects for vocabulary and grammar phenotypes.
A population-based sample of 473 twin pairs participated.
Multilevel modeling estimated means and variances of vocabulary and grammar phenotypes, controlling for familiality.
Heritability was estimated to determine effects of heritability, shared environment, and nonshared environment.
Twins had lower average language scores than norms for single-born children, with lower average performance for monozygotic than dizygotic twins and for boys than girls, although gender and zygosity did not interact.
Gender did not predict LLE.
Significant heritability was detected for vocabulary and grammar phenotypes in the full sample and in the sample selected for LLE.
LLE and the appearance of Word Combinations were also significantly heritable.
The findings revealed an increased likelihood of LLE in twin toddlers compared with single-born children that is modulated by zygosity and gender differences.
Heritability estimates are consistent with previous research for vocabulary and add further suggestion of heritable differences in early grammar acquisition.