A world first study of language development in toddler twins confirms the widely held belief that twins start to talk later than single-born children.
The results from the LOOKING at Language study, based at The University of Western Australia affiliated Perth's Telethon Kids Institute, also showed that language delay is more common in identical twins than their non-identical counterparts.
The results have been published online in the Journal of Speech, Language and Hearing Research.
Principal Investigator UWA Professor Cate Taylor said the findings are based on a population-based sample of 473 sets of Western Australian twins who have been followed from birth.
"Overall, twins have double the rate of late language emergence than single-born children, 38% for twins compared to 19% for single-borns," Professor Taylor said.
"When we looked further at the twins, and split them into identical or non-identical twin pairs, we found the rate of language delay in identical twins was 47% compared to 31% in non-identical twins."
Late language emergence is when a child's language is below age and gender expectations, that is, they speak few words and do not join words together to form sentences. In this study, 71% of two-year-old twins were not combining words, compared to only 17% of single-born children.
Professor Taylor said the findings challenge existing views on why twins may have language delay.
"For years, researchers have been fascinated by language development in twins with the main theory that mothers speak less to twins due to the double demands of caring for two children of the same age," Professor Taylor said.
"This does not explain why language delay was more common in identical and non-identical twins. The explanation lies in factors other than growing up as two. Being two, affects twins in other ways. For example, twins in general are exposed to more pregnancy and birth complications than single-borns and identical twins more so than non-identical twins."
Professor Taylor said the differences seen between identical and non-identical twins could be attributed to pregnancy and birth factors. A study of pregnancy and birth risks for late talking in twins is currently underway.
"The answer to the question, "Do the twins catch-up?", is ahead of us. We are currently investigating the twin's language development in the preschool and school years," Professor Taylor said.
"It is vital to know if and when late-talking twins catch-up to their peers or whether twin-single-born language differences persist through childhood and into adolescence."
The project, which began in 2002, is an international collaboration between the UWA affiliated Telethon Kids Institute, University of Kansas and the University of Nebraska Medical Center.