Researchers May Have Answers for the Rise in Autism

A study published in the journal Epidemiology this month by Irva Hertz-Picciotto and Lora Delwiche offers some definitive answers to a decades-long debate concerning the rise in autism.

Over the past 20 years there have been increasing reports of a rise in cases of autism. Many considered these reports evidence of an epidemic and pointed to various factors such as environmental toxins to explain the rise in autism. However, there were many other factors that could explain this rise. Among the most frequently cited explanations to discount an actual rise in autism were the methods used to diagnose new cases of autism, inclusion of milder cases of autism, earlier age of diagnosis, and poor methods for estimating prevalence.

Many in the autism community felt these explanations were insufficient and offered counterarguments and reanalyses of datasets. And thus the debate continued with few clear answers.

The study by Hertz-Picciotto and Delwiche evaluated all clients of the Department of Developmental Services Regional Centers in California aged 3 years and older for the period of 1990 through 2006. For children under 3, records from the Early Start Report were used for a similar time period.

The authors controlled for a number of confounds that previous studies failed to do. The interested reader is encouraged to read their study as the description of their methods was very thorough (The Rise in Autism and the Role of Age at Diagnosis. Epidemiology, 20, 84-90). All in all, the study represents the most rigorous analysis of the rise in autism that has been published to date.

Their study found that since the early 1990’s the incidence of autism rose 7 to 8 fold. They discuss that “…changes in diagnostic criteria, the inclusion of milder cases, and an earlier age at diagnosis, during this period suggests that these factors probably contribute 2.2-, 1.5-, and 1.24 – fold increases in autism, respectively, and hence cannot fully explain the magnitude of the rise in autism”. This leaves a 2- to 3-fold increase in the incidence of autism.

The authors conclude “…the current occurrence of autism, a seriously disabling disorder in young children, at rates of greater than 30 per 10,000 individuals – and still rising in California – is a major public health and educational concern”.

We here at CARD would certainly agree.

Dennis Dixon, Ph.D.
Research and Development Manager
Center for Autism and Related Disorders, Inc.


  1. Exactly what do we agree with? Well, the statement was specifically in regards to the final quote from their article. More generally though, that there is a significant rise in autism diagnosis and that this rise can not entirely be explained by the things noted above. There are other unaccounted for factors that are influencing the rise in diagnosis. What are these factors? I don't think anyone has sufficient evidence to say definitively. Unfortunately the truism applies that where data are lacking opinions prevail.

  2. Thank you for the clarification/elaboration.