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 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.
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.
(Photo: Telethon Kids Institute, Perth, Western Australia)