What is Causing the Increase in Autism Prevalence?
“Got Questions?” is a new weekly feature on our blog to address the desire for scientific understanding in our community. We received over 3000 responses when we asked what science questions were on your mind. We answered a few here and will address the other themes we received in this weekly post.
In the last two decades, autism prevalence as reported in the scientific literature has increased by more than 600%. In 2009, the latest estimate of autism prevalence in the United States, as reported by the Centers for Disease Control was 1 in 110 children. Since then, a number of studies have sought to investigate the cause(s) of this dramatic increase in autism prevalence over time. The most recent findings suggest that at least a portion of the increase in prevalence can be attributed to changes in diagnostic practices and increased awareness of autism over time, as well as other social factors such as advanced parental age. However, converging evidence also suggests that while these factors account for a portion of the increase, they cannot alone explain the dramatic rise in autism prevalence.
The criteria for assessing autism has changed over the last 20 years resulting in a broadening of autism diagnoses and the identification of cases that would not have been diagnosed as such using older criteria. Looking at a population of children in California, researchers from Columbia University reported that approximately 26% of the rise in autism caseload between 1992 and 2005 could be directly attributed to changes in diagnostic criteria, specifically the shift from mental retardation diagnoses to autism diagnoses. In other words, 1 in 4 children diagnosed with autism in California today would not have been diagnosed using older diagnostic criteria.
Another aspect of the autism landscape that has changed over the past 20 years is an increase in the awareness of autism among the general public as well as healthcare professionals. In 2010, the Columbia University research team again examined the California sample in an effort to quantify the influence of autism awareness on the rise in autism prevalence. The investigators found that children living in close proximity to another child that had been previously diagnosed with autism had a better chance of being diagnosed with autism themselves. The proposed explanation is information diffusion, or 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 over time in California was due to social influence and increased awareness.
An additional social factor that has been implicated in contributing to the increase in autism prevalence is advanced parental age. A number of recent publications investigating the relationship between parental age and autism have demonstrated that older parents are at increased risk for having a child with autism. This is not surprising since increased parental age is associated with a slightly increased risk for other developmental disorders as well. Researchers from the California Department of Public Health reported that parental age and particularly maternal age is a significant risk factor for autism, with a 10-year increase in maternal age increasing the odds of having a child with autism by 38% and mothers over the age of 40 at highest risk. Researchers at Columbia University similarly found, that older mothers and fathers were at increased risk of having a child with autism and estimated that the increase in parental age over time can account for 11% of the increase in prevalence over the same time period. The underlying mechanism behind the relationship between increased parental age and risk for autism is currently under investigation.
Based on the abovementioned research, approximately 53% percent of the increase in autism prevalence over time may be explained by changes in diagnosis (26%), greater awareness (16%), and an increase in parental age (11%). 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 social factors alone. Environmental factors, and their interactions with genetic susceptibilities, are likely contributors to increase in prevalence and are the subject of numerous research projects currently supported by Autism Speaks.
The increase in autism prevalence is real and the public health crisis is growing. More families are affected by autism today then ever before. While Autism Speaks has cast a wide net to explore the role of genetic and environmental factors in increasing the risk for autism, the research community requires additional funding support to increase the pace of discovery. Never before has the need for research into the causes of autism and effective treatments for autism been greater.
 Prevalence of Autism Spectrum Disorders – Autism and Developmental Disabilities Monitoring Network, United States, 2006. Morbidity and Mortality Weekly Report. 2009;58(SS-10). http://www.ncbi.nlm.nih.gov/pubmed/20023608
 King, M. & Bearman, P. (2009). Diagnostic Change and Increased Prevalence of Autism. International Journal of Epidemiology, Volume 38, Issue 5:1224-1234.
 Grether, et al. Risk of Autism and Increasing Maternal and Paternal Age in a Large North American Population. American Journal of Epidemiology. 2009;170(9):1118-26. http://www.ncbi.nlm.nih.gov/pubmed/19783586