According to new research, by the age of 8 as many as one in every 150 children in the United States develops autism or a related disorder such as Asperger's syndrome.
The study by the Centers for Disease Control and Prevention (CDC) examined cases of autism spectrum disorders (ASD) in 14 states between 2000 and 2002 in order to establish the prevalence of the disorder in the U.S.
The rigorous and comprehensive analysis has served to confirm recent estimates, which suggest the number to be at around one in 160 children.
The analysis also found that delays in diagnosis were quite common with an average of at least a year and a half from the time parents first reported odd speech problems or other social deficits, typically at around the age of 3.
For children with autism, problems arise in communicating, forming relationships and adjusting to change, and they often have unusual behaviours and interests, which can be very disabling; those with other disorders on the spectrum generally have social difficulties which are less severe, and in some cases mild.
The study suggests as many as 560,000 Americans age 21 or under struggle to cope with such disorders and the researchers say that both genetic variation and developmental factors combine to cause the disorders, but little is known about the actual cause of the disorder.
The new numbers unlikely to settle the continuing debate about whether there has been a true rise in autism and, if so, what is the underlying causes of that increase.
Dr. Julie L. Gerberding, director of the CDC says although estimates are better and more consistent, it is unclear whether there is an increase in autism spectrum disorders or whether the increase is the result of better information.
Gerberding says however, what is known is that the disorders are affecting too many children.
The researchers calculated their estimates on the basis of a detailed review of records from schools and health clinics in six states in 2000 and 2002, and from an additional eight states in 2002 alone.
The analysis involved Alabama, Arizona, Arkansas, Colorado, Georgia, Maryland, Missouri, New Jersey, North Carolina, Pennsylvania, South Carolina, Utah, West Virginia and Wisconsin.
They looked at the behaviour problems in 8-year-olds, which is when the disorders would usually become apparent.
The investigators found similar rates in most of the states surveyed, with two exceptions. The prevalence was lowest in Alabama, at one in 300, and highest in New Jersey, at some one in 100.
Dr. Catherine Rice, the CDC researcher who led the study, said it was likely that the Alabama number was an underestimate, because the researchers did not have access to school records in that state.
The higher rate in New Jersey may reflect other differences, experts said, including a higher level of awareness and wider availability of services in communities and schools.
The purpose of CDC's project was to develop a system for better understanding of the size and characteristics of the population of children with an ASD.
The most commonly documented concerns were in language development, followed by social development.
While the studies did not investigate the causes of ASDs, CDC's Centers for Autism and Development Disabilities Research and Epidemiology (CADDRE) Network is currently conducting a multi-state study to help identify factors that may put children at risk for ASDs and other developmental disabilities.