J Speech Lang Hear Res. 2014 Jun 1;57(3):917-28. doi: 10.1044/1092-4388(2013/12-0350).
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 with DeFries-Fulker regression and variance components models 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 (0.26) and grammar phenotypes (0.52 and 0.43 for boys and girls, respectively) in the full sample and in the sample selected for LLE (0.42 and 0.44). LLE and the appearance of Word Combinations were also significantly heritable (0.22-0.23).
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.
本研究旨在探讨 24 个月大的双胞胎出现语言发育迟缓（late language emergence，LLE）的病因，考虑到可能的双胞胎、同卵双生子、性别以及词汇和语法表型的遗传性影响。
本研究采用基于人群的样本，共纳入了 473 对双胞胎。采用多层次模型估计词汇和语法表型的均值和方差，同时控制了家族性因素。采用 DeFries-Fulker 回归和方差成分模型估计遗传度，以确定遗传度、共享环境和非共享环境的影响。
与单胎儿童的正常水平相比，双胞胎的平均语言得分较低，同卵双生子的平均表现低于异卵双生子，男孩的平均表现低于女孩，尽管性别和同卵性并未相互作用。性别不能预测 LLE。在全样本和选择 LLE 的样本中，词汇（0.26）和语法表型（男孩和女孩分别为 0.52 和 0.43）均显示出显著的遗传度，而在全样本和选择 LLE 的样本中，LLE 和单词组合的出现也具有显著的遗传度（0.22-0.23）。
研究结果显示，与单胎儿童相比，双胞胎幼儿出现 LLE 的可能性更高，这受到同卵性和性别差异的调节。遗传度估计与先前关于词汇的研究一致，并进一步提示了早期语法习得中可遗传差异的存在。
