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UC Davis Study Examines Rise in Autism Rates

By Eddy Ball
February 2009

A new analysis of data on autism performed by NIEHS grantees at the University of California Davis (UCD) undermines arguments that California's dramatic 7- to 8-fold increase in autism cases may be largely due to changing diagnostic practices of physicians. The researchers, who are affiliated with the UCD Department of Public Health Sciences and the UCD Medical Investigation of Neurodevelopmental Disorders (M.I.N.D.) Institute, found that including milder cases, performing earlier diagnosis and demographic factors can explain only a small part of the rise in the number of children with the disorder.

In their study (;jsessionid=J1shJSPL60MWwzG1QRCQZd9yYP7SwcyB2Dfgpn3lNj0nbh2v1qvC!-348297060!181195629!8091!-1) Exit NIEHS, which was published in the January issue of Epidemiology, epidemiologist Irva Hertz-Picciotto, Ph.D., and programmer analyst Lora Delwiche examined data on autism cases from 1990 through 2006 gathered by the California Department of Developmental Services, along with data from the United States Census Bureau and the California Department of Public Health's Office of Vital Statistics. Their findings offer new insight into the alarming increase in prevalence and point to environmental causes as the most productive direction for future research.

According to the researchers, the increase was not simply a result of California's continuing influx of new residents from elsewhere, earlier detection or the inclusion of milder cases. Taken together, these factors accounted for less than one-seventh of the increase, which the authors described as "a major public health and educational concern" that shows no sign of abating.

An increasingly diverse population was also not to blame, observed lead author Hertz-Picciotto in interviews following release of the study, since the disorder occurs among different ethnic groups at similar rates. It is also unlikely that families with autistic children moving to California had a significant impact on the rise.

For Hertz-Picciotto, the increase underscores the need for greater emphasis on investigating further the role of environmental causes of the disorder. "Right now, about 10 to 20 times more research dollars are spent on studies of the genetic causes of autism than environmental ones," she said. "We need to even out the funding."

Hertz-Picciotto is involved in two large NIEHS-funded epidemiology studies currently underway at the M.I.N.D. Institute ( Exit NIEHS aimed at discovering the causes of autism. The Childhood Autism Risk from Genetics and the Environment (CHARGE) ( study is a major investigation of environmental factors and gene-environment interactions in the disorder involving 2,000 children, including 700 children with autism. The Markers of Autism Risk in Babies-Learning Early Signs (MARBLES) ( will follow mothers of a child with autism through a subsequent pregnancy to uncover early markers that may predict outcome of the pregnancy.

Citation: Hertz-Picciotto I, Delwiche L. 2009. The Rise in Autism and the Role of Age at Diagnosis. Epidemiology 20(1): 84-90.

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