The study, published in Developmental Medicine & Child Neurology, revisited 38 adults under the age of 31 diagnosed with developmental language disorders as children who had attended special schools or classes for children with language impairments. The researchers found about one-quarter of them met current diagnostic criteria for autistic spectrum disorders.

"Our study shows pretty direct evidence to support the theory that changes in diagnosis may contribute towards the rise in autism," study leader Dorothy Bishop of the University of Oxford said in a statement. "These were children that people were saying were not autistic in the 1980s, but when we talk to their parents now about what they were like as children, it's clear that they would be classified as autistic now."

Bishop noted the criteria for diagnosing autism "were much more stringent in the 1980s than nowadays and a child wouldn't be classed as autistic unless he or she was very severe. Now, children are being identified who have more subtle characteristics and who could in the past easily have been missed."