A study of 473 sets of twins followed since birth found that compared to single-born children, 47 percent of 24-month-old identical twins（同卵双胞胎） had language delay compared to 31 percent 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. The results of the study were published in the June 2014 Journal of Speech, Language, and Hearing Research.

University of Kansas Distinguished Professor Mabel Rice, lead author, said that all of the language traits analyzed in the study -- vocabulary, combining words and grammar -- were significantly heritable with genes accounting for about 43 percent 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."

However, said Rice, prematurity and birth complications, more common in identical twins, could also affect their higher rates of language delay. A study of pregnancy and birth risks for late talking in twins is currently underway by the study authors.

Further, the study will continue at least until 2017 to continue to follow the twins through preschool and school years up to adolescence to answer the question of whether late-talking twins do catch up to their peers.

"Twin studies provide unique opportunities to study inherited and environmental contributions to language acquisition," said Rice. "The outcomes inform our understanding of how these influences contribute to language acquisition in single born children, as well."