New Study on California's Rise in Autism

Since 1990, autism prevalence in California has increased seven to eight times. A new study in the January issue of Epidemiology concludes that (1) the incidence of autism in California "shows no sign yet of plateauing" and that (2) a number of factors which have been said to also contribute to the autism rate---"[y]ounger ages at diagnosis, differential migration, changes in diagnostic criteria, and inclusion of milder cases" do not fully account for the observed increases. 

A review of the study in today's Science Daily says that "results from the study also suggest that research should shift from genetics to the host of chemicals and infectious microbes in the environment that are likely at the root of changes in the neurodevelopment of California's children." The study itself is not so much focused on arguing for environmental factors as causing autism, as it is a review of the data of autism rates in California, to determine how the above-noted factors (changes in diagnostic criteria) have contributed to the increase. The authors write that "other artifacts [not specified] have yet to be quantified."

Epidemiology is the journal for the International Society for Environmental Epidemiology (ISEE) and the lead author for the study is Irva Hertz-Picciotto of the U.C. Davis M.I.N.D. Institute, which is conducting the MARBLES study. MARBLES stands for "Markers of Autism Risk in Babies—Learning Early Signs" and the study specifically looks for “biological and environmental triggers that children are exposed to prenatally and post-partum." Professor Hertz-Picciotto, an epidemiologist, is the principal investigator of the study.

The abstract:

Background: Autism prevalence in California, based on individuals eligible for state-funded services, rose throughout the 1990s. The extent to which this trend is explained by changes in age at diagnosis or inclusion of milder cases has not been previously evaluated.

Methods: Autism cases were identified from 1990 through 2006 in databases of the California Department of Developmental Services, which coordinates services for individuals with specific developmental disorders. The main outcomes were population incident cases younger than age 10 years for each quarter, cumulative incidence by age and birth year, age-specific incidence rates stratified by birth year, and proportions of diagnoses by age across birth years.

Results: Autism incidence in children rose throughout the period. Cumulative incidence to 5 years of age per 10,000 births rose consistently from 6.2 for 1990 births to 42.5 for 2001 births. Age-specific incidence rates increased most steeply for 2- and 3-year olds. The proportion diagnosed by age 5 years increased only slightly, from 54% for 1990 births to 61% for 1996 births. Changing age at diagnosis can explain a 12% increase, and inclusion of milder cases, a 56% increase.

Conclusions: Autism incidence in California shows no sign yet of plateauing. Younger ages at diagnosis, differential migration, changes in diagnostic criteria, and inclusion of milder cases do not fully explain the observed increases. Other artifacts have yet to be quantified, and as a result, the extent to which the continued rise represents a true increase in the occurrence of autism remains unclear.