Genes may be to blame for language delay in identical twins, according to a new study which suggests language acquisition depends on genetics rather than upbringing.
A study of 473 sets of twins followed since birth found that compared to single-born children, 47 per cent of 24-month-old identical twins had language delay compared to 31 per cent of non-identical twins.
Overall, twins had twice the rate of late language emergence of single-born children. None of the children had disabilities affecting language acquisition.
University of Kansas Distinguished Professor Mabel Rice, lead author, said that all of the language traits analysed in the study - vocabulary, combining words and grammar - were significantly heritable with genes accounting for about 43 per cent of the overall twins' deficit.
The "twinning effect" - a lower level of language performance for twins than single-born children - was expected to be comparable for both kinds of twins, but was greater for identical twins, said Rice, strengthening the case for the heritability of language development.
"This finding disputes hypotheses that attribute delays in early language acquisition of twins to mothers whose attention is reduced due to the demands of caring for two toddlers," said Rice.
"This should reassure busy parents who worry about giving sufficient individual attention to each child," Rice said.
However, said Rice, prematurity and birth complications, more common in identical twins, could also affect their higher rates of language delay.
Late language emergence means that a child's language is below age and gender expectations in the number of words they speak and combining two or more words into sentences.
In the study 71 per cent of two-year-old twins were not combining words compared to 17 per cent of single-born children.
The study was published in the Journal of Speech, Language, and Hearing Research.
