The Year of Autism Epidemiology

In 2009, two major studies using different research methodologies yielded strikingly similar and eye-opening results showing that ASD affects approximately 1% of children in the United States. Based on data collected just four years earlier, it was found that ASD affected 1 out of every 150 children in the U.S. This represents a 57% increase in ASD prevalence in a relatively short period of time. Both studies also found that ASD continues to be four times more common in boys than girls.

In the first study, published in Pediatrics¹, authors from the U.S. Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC) collected data through the National Survey of Children's Health (NSCH) on parent-reported diagnosis of ASD. Among a nationally representative sample of 78,000 children aged 3 to 17 years, the investigators found that 1 in 91, or an estimated 673,000 children in the U.S. had an ASD. While concerns lingered over the parent-reported nature of the data, this large-scale study set the stage for another major publication on ASD prevalence with similar results.

In December² researchers at the CDC released new prevalence data collected by the Autism and Developmental Disabilities Monitoring Network (ADDM), a series of surveillance sites throughout the U.S. that maintain medical and service records on children with autism. By abstracting data and subjecting those records to stringent clinical evaluation, the authors found that approximately 1 in 110 children, 1 in 70 boys, met the criteria for ASD. This 1 in 110 statistic, based on data collected in 2006, represents a 57% increase from the 1 in 150 statistic which was based on data collected by the ADDM network in 2002 using identical research methods to the current study.

In a year that was jam-packed with publications on autism epidemiology, a number of other studies sought to investigate the reasons for the dramatic increase witnessed in autism prevalence. Researchers from Columbia University reported³ that approximately 25% of the rise in autism caseload in California between 1992 and 2005 could be directly attributed to changes in diagnostic criteria that resulted in a shift from mental retardation diagnosis to autism diagnosis. Therefore, converging evidence from this study and others around the world suggests that while changes in diagnostic practice may account for a portion of the increase, they cannot alone explain the rise in autism prevalence, and other factors, including environmental factors, likely play a role. One environmental factor that continues to be implicated in the increase in autism prevalence is parental age. Researchers from the California Department of Public Health reported in 2009⁴ 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.

The prevalence studies of 2009 helped shed additional light on the immense nature of the autism public health crisis. With 1% of the U.S. population affected by ASD, and emerging data suggesting that 1% of the global population may be affected by ASD, never has the need for funding to support research into the causes and treatments of ASD been greater. In addition, these findings call attention to the necessity for more accessible diagnostic and intervention services for the growing population of those affected. In the CDC's ADDM report and a separate study published in the Journal of the American Academy of Child and Adolescent Psychiatry⁵, it was reported that while we can now reliably diagnosis autism spectrum disorders at two years of age, children on average are still being diagnosed at close to 6 years of age. This means that there is a large gap between the time that children can effectively be diagnosed at age 2, and the time they are actually receiving a diagnosis – valuable time lost where early intervention services can dramatically improve outcomes [see 2009 Early Intervention for Toddlers with ASD].

1. Kogan, et al. The prevalence of parent-reported diagnosis of autism spectrum disorder among children in the United States, 2007. Pediatrics. 2009 Nov;124(5):1395-403. Epub 2009 Oct 5. [PubMed link: 19805460]
2. Prevalence of Autism Spectrum Disorders – Autism and Developmental Disabilities Monitoring Network, United States, 2006. MMWR Surveill Summ. 2009 Dec 18;58(10):1-20. [PubMed link: 20023608]
3. King et al. Diagnostic change and the increased prevalence of autism. Int J Epidemiol. 2009 Oct;38(5):1224-34. Epub 2009 Sep 7. [PubMed link: 19737791]
4. Grether, et al. Risk of autism and increasing maternal and paternal age in a large north American population. Am J Epidemiol. 2009 Nov 1;170(9):1118-26. Epub 2009 Sep 25.[PubMed link: 19783586]
5. Shattuck et al. Timing of identification among children with an autism spectrum disorder: findings from a population-based surveillance study. J Am Acad Child Adolesc Psychiatry. 2009 May;48(5):474-83. [PubMed link: 19318992]