Autism Incidence and Prevalence in California, 2000-2010

Background: Both the incidence and prevalence of autism in the United States have risen steadily over the past few decades. The most recent widely cited estimate of the prevalence rate of Autism Spectrum Disorders is from 2006 and is one in every 110 children aged eight years. Whether the prevalence of autism in the United States has continued to increase, remained stable, or even decreased since 2006 is currently not known. 

Objectives: In this paper, we estimate the incidence and prevalence rates for autism in the state of California for the years 2000 through 2010 using the largest administrative dataset available. We then examine the demographic and socioeconomic composition as well as the developmental functioning of those who were diagnosed during the study period.

Methods: To calculate incidence and prevalence rates of autism in California, we link the state’s Birth Master Files to the Department of Developmental Services’ (DDS) annual autism caseload records. We then explore factors that may influence observed trends by determining whether the groups of children diagnosed in each year vary by sex, age, race/ethnicity, Medi-Cal status, maternal education, maternal age at birth, or presence of intellectual disability.

Results: The incidence of autism for children between the ages of three and nine in California rose between 2000 and 2008 but then declined from 2008 to 2010. This most recent dip in autism incidence could be the result of changes within the DDS’ reporting system that were made in 2008 or cuts that were made to the DDS’ budget due to California’s budget deficit. Between 2000 and 2010: the sex ratio of those diagnosed with autism remained relatively stable; age of diagnosis decreased; the proportion of Hispanic people being diagnosed with autism increased; the maternal level of education of those being diagnosed remained relatively stable; and the percentage of people diagnosed with autism who also had DDS record of intellectual disability decreased.

Conclusions: Overall, autism incidence rose between 2000 and 2008 and then declined between 2008 and 2010. Not only are our results the most recent estimate of the incidence of autism in the United States to date, but we also used a statewide dataset, the largest dataset of its kind available.