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Purpose: This study investigates the heritability of language, speech, and nonverbal cognitive development of twins at 4 and 6 years of age. Possible confounding effects of twinning and zygosity, evident at 2 years, were investigated among other possible predictors of outcomes. Method: The population-based twin sample included 627 twin pairs and 1 twin without a co-twin (197 monozygotic and 431 dizygotic), 610 boys and 645 girls, 1,255 children in total. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using general linear mixed models using maximum likelihood. Results: Twinning effects decreased from 4 to 6 years; zygosity effects disappeared by 6 years. Heritability increased from 4 to 6 years across all 9 phenotypes, and the heritability estimates were higher than reported previously, in the range of 0.44-0.92 at 6 years. The highest estimate, 0.92, was for the clinical grammar marker. Conclusions: Across multiple dimensions of speech, language, and nonverbal cognition, heritability estimates are robust. A finiteness marker of grammar shows the highest inherited influences in this early period of children's language acquisition.

Purpose: This study investigates the heritability of language, speech, and nonverbal cognitive development of twins at 4 and 6 years of age. Possible confounding effects of twinning and zygosity, evident at 2 years, were investigated among other possible predictors of outcomes. Method: The population-based twin sample included 627 twin pairs and 1 twin without a co-twin (197 monozygotic and 431 dizygotic), 610 boys and 645 girls, 1,255 children in total. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using general linear mixed models using maximum likelihood. Results: Twinning effects decreased from 4 to 6 years; zygosity effects disappeared by 6 years. Heritability increased from 4 to 6 years across all 9 phenotypes, and the heritability estimates were higher than reported previously, in the range of .44-.92 at 6 years. The highest estimate, .92, was for the clinical grammar marker. Conclusions: Across multiple dimensions of speech, language, and nonverbal cognition, heritability estimates are robust. A finiteness marker of grammar shows the highest inherited influences in this early period of children's language acquisition. [ABSTRACT FROM AUTHOR]

Purpose: 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. Method: 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. Results: 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). Conclusions: 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. [ABSTRACT FROM AUTHOR]

Children with reading deficits perform more slowly than normally-achieving readers on speed of processing measures, such as rapid naming (RN). Although rapid naming is a well-established correlate of reading performance and both are heritable, few studies have attempted to assess the cause of their covariation. Measures of rapid naming (numbers, colors, objects, and letters subtests), phonological decoding, orthographic choice, and a composite variable (DISCR) derived from the reading recognition, reading comprehension, and spelling subtests of the Peabody Individual Achievement Test were obtained from a total of 550 twin pairs with a positive school history of reading problems. Basic DeFries and Fulker (DF) multiple regression models for the analysis of selected twin data confirmed the heritable nature of phonological decoding, orthographic choice, DISCR, and rapid-naming composites. Bivariate DF models were employed to examine the extent to which deficits in the three reading-related measures covary genetically with rapid naming. Significant bivariate heritability estimates for each of the reading measures with the numbers and letters rapid-naming composite were also obtained. As expected, univariate sib-pair linkage analyses indicated the presence of a quantitative trait locus (QTL) on chromosome 6p21.3 for phonological decoding and orthographic choice deficits. Bivariate linkage analyses were then conducted to test the hypothesis that this QTL for reading difficulties is pleiotropic for slower performance on RN tasks. The results obtained from these analyses did not provide substantial evidence that the 6p QTL for reading difficulties has significant effects on rapid naming; however, larger samples would be required to test this hypothesis more rigorously. [ABSTRACT FROM AUTHOR]