Increasing Socioeconomic Disparity In the Prevalence of Autism Spectrum Disorder In Wisconsin

Background:  Although developmental disabilities as a whole are most prevalent in children of low socioeconomic status (SES), recent epidemiologic studies in the United States suggest this may not be true for autism spectrum disorder (ASD) and that the prevalence of ASD appears to increase with increasing SES. A study in Wisconsin based on surveillance data from the year 2002, for example, found ASD prevalence to increase in a stepwise manner from a low of 2.5 (95% confidence interval 1.6, 4.0) per 1,000 children in the lowest SES quintile to a high of 6.8 (95% confidence interval 5.1, 8.9) in the highest SES quintile.

Objectives: To replicate the Wisconsin 2002 study using comparable data for the year 2006 and evaluate the hypothesis that the SES disparity in ASD prevalence would be reduced in 2006. Support for this hypothesis might suggest that awareness and identification of ASD among children of low SES have improved over time.

Methods: A cross-sectional study was implemented with data from the Wisconsin Surveillance of Autism and Other Developmental Disabilities System, a multiple source surveillance system that incorporates data from health care and developmental disabilities service providers to determine the number of 8-year-old children with ASD in southeastern Wisconsin. In 2006, there were 257 children with ASD among a population of 34,058 8-year-old children residing in the surveillance area. Area-level census SES indicators were used to compute ASD prevalence by SES quintiles of the population.

Results: The prevalence of ASD in this population in 2006 was 7.6 per 1,000 children, a 46% increase relative to 2002. In addition, as in 2002, the prevalence increased stepwise with increasing SES, from a low of 2.4 (95% confidence interval 1.5, 3.8) per 1,000 children in the lowest SES quintile to a high of 11.5 (95% confidence interval 9.3, 14.2) in the highest SES quintile. However, because the increase in prevalence between 2002 and 2006 occurred differentially in the higher SES quintiles, the SES disparity in ASD prevalence also increased between the two study years. The prevalence ratios for the highest versus lowest SES quintiles increased from 2.7 (95% confidence interval 1.6, 4.5) in 2002 to 4.8 (95% confidence interval 2.9, 7.9) in 2006.

Conclusions: The SES gradient in ASD prevalence in Wisconsin has become steeper rather than diminished during a time period of increasing overall prevalence of ASD. These results point to possible under-ascertainment and lack of access to services for children with autism who are socioeconomically disadvantaged. Further research is needed to monitor and understand the sources of the SES disparity in ASD prevalence.