Individual variations and change in general cognitive, language and reading abilities tend to inter-correlate, suggesting possible common etiology in their development (Rhemtulla & Tucker-Drob, 2011). Substantial genetic overlap between these cognitive abilities has been demonstrated, supporting the Generalist Genes Hypothesis, which predicts that the same set of genes largely influences diverse domains of cognitive abilities (Plomin & Kovas, 2005). To date, the Generalist Genes Hypothesis has been tested on alphabetic languages only, mainly English. Whether this hypothesis generalizes to wider populations using different languages remains unknown. To fill in this gap in knowledge, this study investigates the extent to which Generalist Genes could account for individual variation in learning Chinese, which has very different characteristics from English, being a tonal language with a logographic script. Having lexical tones is one of the major unique characteristics of Chinese compared to English. A Chinese syllable in different lexical tones represents different meanings and different characters. Also, the print–sound mappings are ambiguous in Chinese. These unique characteristics make research on Chinese prominent, because many of the research findings on English may not apply in Chinese, and the comparisons of research findings on English and Chinese are ideal in indicating universal or language-specific findings and theories.
In English, the development of language and reading skills is interwoven, and these skills share genetic origins as shown in twin research (e.g. Thompson, Detterman & Plomin, 1991). The genetic overlap between language and reading abilities is substantial, with genetic correlations ranging from .63 to 1.00 (Davis, Haworth & Plomin, 2009; Gayán & Olson, 2003; Haworth, Kovas, Harlaar, Hayiou-Thomas, Petrill, Dale & Plomin, 2009; Hohnen & Stevenson, 1999; Thompson et al., 1991). In other words, 63%–100% of the genetic factors influencing language abilities contributed to reading skills. These genetic links are also evidenced in extreme groups (e.g. Hayiou-Thomas, Harlaar, Dale & Plomin, 2010), with genetic correlations varying from .44 to .64 in a lower performance group and .52 in a higher performance group (Bishop, 2001; Haworth, Kovas et al., 2009; Haworth, Dale & Plomin, 2009). It is noteworthy that these studies tapped language and reading using different assessments that tested different aspects of ability, but still moderate to strong genetic overlap was consistently demonstrated. Moreover, these genetic links contributed not only to concurrent relationships but also longitudinal links between language and reading abilities. Specifically, early language abilities at 2 to 4.5 years of age shared sources of genetic influence with subsequent reading skills at 7 to 10 years of age (Harlaar, Hayiou-Thomas, Dale & Plomin, 2008; Hayiou-Thomas et al., 2010; Hayiou-Thomas, Harlaar, Dale & Plomin, 2006).
The effects of Generalist Genes extend from within verbal skills to across verbal and nonverbal abilities. Research has demonstrated that general cognitive abilities had strong genetic correlations with language skills (over .63) and reading skills (over .61) in unselected samples (e.g. Colledge, Bishop, Koeppen-Schomerus, Price, Happé, Eley, Dale & Plomin, 2002; Davis et al., 2009; Haworth, Kovas et al., 2009; Thompson et al., 1991). General cognitive abilities shared genetic origins with language-related cognitive skills, such as phonological awareness (Hohnen & Stevenson, 1999; Gayán & Olson, 2003). In sum, converging research evidence has shown that nonverbal cognitive, language and reading skills share genetic underpinnings, which supports the Generalist Genes Hypothesis.
This study investigates the etiology of general cognitive, language and reading abilities and their overlap in 312 Chinese twin pairs aged from 3 to 11. The analysis reported here used the same sample as (Chow, Ho, Wong, Waye & Bishop, 2011), who reported results of univariate genetic contributions to various language and reading measures. Here we addressed a further question. Do nonverbal cognitive, language and reading abilities share common sources of genetic influence, supporting the notion of Generalist Genes? If yes, do Generalist Genes apply across nonverbal cognitive, language and reading skills? Or are common genetic underpinnings found for verbal skills only?