Contrary to the popular tendency to attribute delays in early language acquisition of twins to mothers, researchers have found that genes play a significant role in their overall language deficit.
All of the language traits analysed in the study - vocabulary, combining words and grammar - were significantly heritable with genes accounting for about 43 percent of the overall twins' deficit.
Overall, twins had twice the rate of late language emergence of single-born children. None of the children had disabilities affecting language acquisition.
"This should reassure busy parents who worry about giving sufficient individual attention to each child," said professor Mabel Rice from University of Kansas in the US.
The study of 473 sets of twins followed since their births found that compared to single-born children, 47 percent of 24-month-old identical twins had language delay compared to 31 percent of non-identical twins.
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, Rice said while strengthening the case for the heritability of language development.
However, prematurity and birth complications, more common in identical twins, could also affect their higher rates of language delay, Rice noted.
The study appeared in the Journal of Speech, Language, and Hearing Research.
