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.
"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.
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"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.
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.