The results support inclusion policies in schools that aim to have students with disabilities in the same classrooms alongside their typically developing peers, said Laura Justice, co-author of the study and professor of teaching and learning at The Ohio State University.
"Students with disabilities are the ones who are affected most by the language skills of the other children in their class," Justice said.
"We found that children with disabilities get a big boost in their language scores over the course of a year when they can interact with other children who have good language skills."
In fact, after one year of preschool, children with disabilities had language skills comparable to children without disabilities when surrounded by highly skilled peers in their classroom.
"The biggest problem comes when we have a classroom of children with disabilities with no highly skilled peers among them," Justice said. "In that case, they have limited opportunity to improve their use of language."
Justice conducted the study with Jessica Logan, a senior researcher in education, and Tzu-Jung Lin, assistant professor of educational studies, both at Ohio State; and Joan Kaderavek, a professor of education at the University of Toledo.
Their study will appear in the journal Psychological Science .
The study involved 670 preschool-aged children enrolled in 83 early childhood special education classrooms in Ohio. About half of the children had an Individualized Education Plan, signaling presence of a disability. Between 25 and 100 percent of children in each classroom had a disability.
All children's language skills were measured in the fall and spring of the academic year with a commonly used test called the Descriptive Pragmatics Profile.
The average score of all children in an individual classroom was used to determine each child's relative status in terms of language development, and whether their classmates were more highly skilled, less skilled or average.
While all children's language skills were affected somewhat by the skill levels of their classmates, the effect was strongest for those with disabilities, the study found.
For those children with disabilities who were in classrooms with the most highly skilled peers, language scores in the spring were about 40 percent better than those of children with disabilities who were placed with the lowest-ranked peers.
Students who had no disabilities showed about a 27 percent difference in scores between those with the highest-ranked peers and the lowest-ranked peers.
This study, like others, finds that the most highly skilled students are the ones whose language improvement is least affected by the skill of their classmates, Justice said.
"The highly skilled children aren't hurt by being in classrooms with children who have disabilities," she said.
"But children with disabilities are vulnerable if they aren't placed with more highly skilled peers."
Justice said she and her colleagues are currently doing research that directly compares the effects teachers have on language development versus the effect of peers.
Early results suggest teachers matter most, "but peers definitely have an impact on language development," she said.
Peers help because they spend more time one-on-one with their fellow classmates than teachers can. Children with disabilities have the opportunity to observe, imitate and model the language use of their peers who do not have disabilities.
"In a sense, the typically developing children act as experts who can help their classmates who have disabilities," Justice said.
Statistics from the U.S. Department of Education show that more than half of preschoolers with disabilities are enrolled in early childhood classrooms with typically developing peers.
Justice said these results suggest that all preschoolers with disabilities would benefit from inclusion policies.
"We have to give serious thought to how we organize our classrooms to give students with disabilities the best chance to succeed," she said.