Autistic Spectrum Disorders.  Changes in the California Caseload.  An Update: 1999 Through 2002.  California Department of Developmental Services, April 2003.  PDF of report.

The California Department of Developmental Services (DDS) has just released an update of their 1999 report, which is an accounting of the number of state residents registered with the agency who have a classification of autism.  They document continuing increases in individuals with autism, with the number increasing 97 percent from 1998 through 2002. 

Commentary by Dr. Newschaffer:

Although the California DDS update underscores the undeniable fact that many more individuals have autism spectrum disorders (ASDs) than previously believed, in my opinion it is still very difficult to determine to what extent these increases are due to changes in approaches to diagnosis and classification. 

Because only those individuals with an autistic disorder classification, and not those with other ASDs, are eligible to receive DDS services in California, there may be pressure on those with other ASDs to obtain an autistic disorder classification.  The proportion of individuals in the California DDS system with an autism classification and who did not have any indication of mental retardation has increased from 20 percent in 1987 to over 50 percent in December of 2002.

Lingam R, Simmons A, Andrews N, Miller E, Stowe J, Taylor B.  Prevalence of autism and parentally reported triggers in a north east London population.  Archives of Disease in Childhood 2003; 88(8):666-670.  PubMed abstract. 

Also see:  O'Neil.  Patterns: Autism Cases Level Off in Britain.  New York Times, July 29, 2003.

Children born from 1979-1998 in five northeast London districts were identified.  The prevalance of autism (including autistic disorder and pervasive developmental disorder - not otherwise specified) increased for each year of birth until 1991, then leveled off to about 2.6 per 1,000 live births.  The average age at diagnosis decreased during this same time.

Commentary by Dr. Newschaffer:

This is the first epidemiologic report suggesting that the prevalence of autism may be leveling off after having increased for several years.  The findings are provocative, but are based on a statistical model that uses past data on children's ages at diagnosis to predict numbers of children in more recent birth cohorts likely to be diagnosed at older ages.  There is no a priori reason to believe that this type of model would artificially suggest a leveling off of prevalence; nonetheless, another 5 to 10 years is needed to reveal whether reality conforms to the model findings of Dr. Lingam and colleagues.