A large UK study of two-year-old twins suggests that delay in learning to speak (known as specific language impairment or SLI) is a largely genetic trait [Dale, P.S. et al. (1998) Nature Neurosci. 1, 324-328; see also Editorial on p. 259 of the August issue of the journal]. Although for 95% of the population genetics makes a relatively small contribution (about 25%) to vocabulary acquisition at this age, a much higher heritability (73%) was found for the 5% of children who are slowest to begin speaking. In other words, these children do not represent the tail of a normal distribution, but have a distinct genetic condition.
Comment: The discovery of the genes underlying SLI is likely to be some way off, and it may well be a genetically heterogeneous condition. Even if the genes can be found, it seems very unlikely that there would ever be justification for genetic testing or screening for the variants associated with SLI. The value of uncovering the genetic basis of SLI may lie rather in what it could reveal about how the brain functions in language acquisition. In public health terms, if SLI is indeed largely genetic, what are the practical consequences? Firstly, do children with genetically-based SLI go on to have further language and/or behavioural difficulties, or are they just late starters who eventually catch up with their peers? If the former is the case, can anything be done about SLI? Rather than landing us in the realms of genetic determinism, the recognition that SLI is a genetic condition might on the contrary lead to the discovery that these children respond a quite different range of interventions from those that have been found to improve language acquisition in the 'normal' population.