When I first started working in the field of autism in the 1990's, the prevalence was 1:2500, the latest estimate from 2009, reported by the Centers for Disease Control, is 1 in 110 children!
Since then, there has been much research to investigate the cause(s) of this dramatic increase in autism prevalence over time.
1. Some research suggests that increases are due to changes in diagnosis. King, M. & Bearman, P. (2009) found that approximately 26% of children in California who were previously diagnosed with MR had revisions to a diagnosis of autism from 1992 to 2005. In other words, 1 in 4 children diagnosed with autism in California today would not have been diagnosed using older diagnostic criteria.
2. Other research suggests an increase in the awareness of autism. Also using California data, Liu, King & Bearman (2010) found that children living very close to a child previously diagnosed with autism are more likely to be diagnosed with autism due to more awareness of the symptoms from parents talking to and educating other parents about autism resulting in an increased likelihood of their children being diagnosed. It is estimated that 16% of the increase in autism prevalence between 2000–2005 in California was due to social influence and increased awareness.
3. Societal factor are also estimated to account for some of the increase in autism cases, including advanced parental age. King, Fountain, Dakhlallah & Bearman (2009) looked at changes in risk factors associated with autism across successive birth cohorts. Along with others they are investigating the relationship between parental age and autism, demonstrating that older parents are at increased risk for having a child with autism. However, the underlying mechanism behind the relationship between increased parental age and risk for autism continues to be studied.
While this research is beginning to help us understand the increase in autism prevalence, half of the increase is still unexplained and not due to better diagnosis, greater awareness, and/or social factors alone.
1. King, M. & Bearman, P. (2009). Diagnostic Change and Increased Prevalence of Autism. International Journal of Epidemiology, Volume 38, Issue 5:1224-1234.
2. Liu, K., King, M., & Bearman, P. (2010). Social Influence and the Autism Epidemic. American Journal of Sociology, Volume 115, Number 5:1387-1434.
3. King, M., Fountain, C., Dakhlallah, D., & Bearman, P. (2009) Estimated Autism Risk and Older Reproductive Age. American Journal of Public Health, Volume 99, Issue 9:1673-1679.
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