Study author Paul Shattuck of the University of Wisconsin at Madison said, there may be as yet unknown environmental triggers behind autism. Still his research suggested the past decade's rise in autism cases was more of a labelling issue.
As he wrote in Paediatric journal, autism was fully recognized in 1994 by all U.S. states as a behavioural classification for schoolchildren, who receive individualized attention whatever their diagnosis.
Subsequent increases in the number of autism cases have varied widely by state but the average prevalence among 6- to 11-year-olds enrolled in special education programs increased from 0.6 per 1,000 pupils in 1994 to 3.1 per 1,000 in 2003.
During the same period, diagnoses of mental retardation fell by 2.8 per 1,000 students and diagnoses of learning disabilities dropped by 8.3 per 1,000 students.
Craig Newschaffer of Johns Hopkins Bloomberg School of Public Health in Baltimore said, "We do not know whether individual children have switched classifications, and of course we can never know whether a given child in a particular birth cohort would have been classified differently had they been born either earlier or later. At best, analyses of this type are merely trying to determine if trends in one classification have the potential to offset those in another."