How much of language is genetic and how much learned? Although it is rarely prominent in debates about the innateness of language, one of the strong implications

Three main sources of data are considered: studies of twins, adoption studies and linkage studies. The studies of twins focus on comparisons between monozygotic twins (who in principle have identical DNA) and dizygotic twins (who share no more DNA than any two siblings do). In both cases, the two children share a substantially similar environment (raised by the same parents in the same household in the presence of a twin sibling), so differences within the twin pair are likely to reflect genetic differences between the individual twins. Thus, if language ability is genetically coded, monozygotic twins should be more similar to each other linguistically than dizygotic twins are to each other. The data strongly support the genetic basis for some language disorders (SLI and dyslexia). Moreover, there is also support for the genetic basis of a variety of linguistic abilities in normally developing twins, including vocabulary development, phonological awareness, morpho-syntactic development, and even reading development. The adoption studies compare adopted children’s linguistic abilities with those of their adopted and biological relatives. The results here are a bit more mixed, but overall they support a genetic component for language ability. Interestingly, the adoption studies also suggest that genetic factors continue to be a predictive element of language ability into adulthood. Linkage studies consider evidence about which specific genes might underlie language disorders. These studies involve comparing the genomes of close relatives (often sibling pairs) with and without a particular disorder, such as SLI or dyslexia. The results suggest that language disorders often have heterogeneous genetic links, and although a few specific genes do seem to be implicated in several studies (e.g. 7q31, 15q21), we are still quite far from understanding which part of the genome codes for language, or how genetic differences lead to different language abilities (i.e. phenotypes). Over 100 published studies were included in this review and the resulting paper is a dense one, with full details of the meta-analysis and of the statistics necessary to understand the genetic relationships. The author is forthright about the difficulties inherent in trying to tease out a genetic component from environmental ones in a domain as complex as language; given the potential for radical claims in this area (‘grammar gene found!’) her conservative outlook is greatly applauded. The results so far would probably not convince a sceptic – the linkage results in particular would need to be strengthened – but they are extremely encouraging. We might yet discover just how, and to what extent, language is wired in our DNA.