How prevalent is autism?
The worldwide prevalence of Autism Spectrum Disorder (ASD) is estimated to be about 6 per 1000 children. In the United States, about one in 68 children is estimated to have ASD .
A summary of autism prevalence studies published from 1966 to 2004  indicated that autism prevalence has increased with time and varies remarkably with geographic location (Figures 1 and 2). However, a significant portion of these variations is likely to be attributable to differences in diagnostic criteria, screening methods, and age of children screened. Therefore, it is not clear what portion of these variations reflects true differences in autism incidence.
Figure 1. Estimated prevalence of autism (per 1000 children) between 1980 and 2010, plotted for seven regions of the world and in 5-year intervals
Figure 2. World map of autism prevalence (per 1000 children). Color-coded maps are available for 5-year intervals between 1980 and 2010. Countries shown in light blue have no data available on autism prevalence for the specified time period.
The apparent increase in autism prevalence in the past decades has become a major concern. To obtain an accurate estimate of changes in ASD prevalence, the Centers for Disease Control and Prevention (CDC) launched the Autism and Developmental Disabilities Monitoring (ADDM) Network in the United States in 2000. The sites participate in this program, identifies children with ASD by screening children's health and education records, which included professional assessments of their developmental progress, at health-care or education facilities. Figures 3 and 4 show the estimated autism prevalence in children aged 8 years in 2002, 2006 and 2008, across different states in the US .
Figure 3. Estimated prevalence of Autism Spectrum Disorders (ASDs) among children aged 8 years at 11 sites in the United States, from the CDC's ADDM Network, for 2002, 2006, and 2008.
Figure 4. Map of estimated prevalence of Autism Spectrum Disorders (ASDs) among children aged 8 years at 11 sites in the United States, from the CDC's ADDM Network, for 2002, 2006, and 2008.
These data demonstrate that the estimated prevalence of ASDs in the ADDM network surveillance populations varies widely between states and increases with time. In addition, estimated ASD prevalence fluctuates by sex and race/ethnicity. Importantly, because these sites do not make up a nationally representative sample, these combined prevalence estimates should not be generalized to the United States as a whole. The team concluded that the extent to which these increases reflect better case ascertainment as a result of increases in awareness and access to services or true increases in prevalence of ASD symptoms is not known. Therefore ongoing public health surveillance is needed to quantify and understand these changes over time and to evaluate the factors affecting ASD prevalence over time.
1. Williams, J. G., J. P. Higgins, et al. (2006) Systematic review of prevalence studies of autism spectrum disorders. Arch Dis Child 91(1): 8-15. PMID: 15863467.
2. CDC. (2012) Prevalence of autism spectrum disorders - autism and developmental disabilities monitoring network, 14 sites, United States, 2008. MMWR Surveill Summ 61(3):1-19. PMID: 22456193.
3. CDC (2014) Prevalence of Autism Spectrum Disorder Among Children Aged 8 Years — Autism and Developmental Disabilities Monitoring Network, 11 Sites, United States, 2010. http://www.cdc.gov/mmwr/preview/mmwrhtml/ss6302a1.htm?s_cid=ss6302a1_w