Determining the cause requires investigating multiple factors—from cold, hard statistics to social, biological, and environmental influences.

Autism spectrum disorder (ASD) is a common disorder, affecting 1 in 44 U.S. children per the Centers for Disease Control and Prevention. Common or not, the road to recognition has been grueling. The term autism was first used by psychiatrist Eugen Bleuler in 1911 to describe schizophrenic patients who were considered self-absorbed and withdrawn. Today, it is an umbrella term for neurodevelopmental disorders that encompass a broad collection of characteristics and physical issues.

This is a true testament to the dramatic evolution of ASD over the last century.

Lost in this ambiguous historical translation is the human element, that each person with autism displays completely unique characteristics—different social challenges, different physical manifestations, even different strengths. It’s part of the undeniable nature of ASD, as well as what makes the disorder particularly difficult to diagnose.

With the diagnosis spike over the last decade—jumping from 1 in 155 cases in 2007 to 1 in 59 as of 2018—there is a question on everyone’s mind; is the spike due to an actual increase, or more awareness and testing, or is it attributed to inaccurate diagnoses?

Stacker has created a list of 10 possible explanations for the recent spike in ASD diagnoses based on scientific and governmental reports, including research from Scientific American and the National Institutes of Health.


  • Increased awareness is a big contributor

    Increased awareness of autism has led to more reported diagnoses in medical records and school files. The CDC uses these sources for data collection, causing an increase of ASD prevalence. With that said, per the Autism Science Foundation, a single factor can’t be attributed to the years of increase. This is seen in how the CDC samples—exclusively 8-year-old children, only every two years, and from a selection of 11 states.

  • Policy for autism diagnosis begins requiring screening for all children, revealing more cases

    The American Academy of Pediatrics introduced a policy recommending pediatricians screen for autism beginning at 18 months and again at 24 months. This earlier screening, happening more frequently, adds to ASD prevalence. The AAP policy is only a recommendation with the final call relegated to states and pediatricians. The AAP also doesn’t require a specific tool for ASD screening. These two factors reveal vulnerabilities in the accuracy of ASD diagnosis data.

  • Correcting misdiagnosis and allowing multiple diagnoses increase cases

    Policy changes in regards to correcting ASD misdiagnosis and allowing for multiple diagnoses has increased ASD prevalence. Historically, ASD has been grossly misdiagnosed, generally as an intellectual disability (ID) or attention deficit hyperactivity disorder (ADHD). Compounding the misdiagnosis factor, medical professionals were restricted to single diagnoses, most opting for an alternative to ASD given the lack of support and services. Both of these policies changed in 2013 with the updated version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), after which ASD prevalence continued to spike.

  • Changing autism spectrum disorder criteria broadens the range of diagnoses possibilities

    Broadening the criteria for ASD has resulted in an increase in diagnoses. In the 1940s, autism was diagnosed based on characteristics seen in severe cases, yet the 2013 publication of the DSM-5 provided a condensed, single-fit ASD diagnosis. This new criteria included partnering conditions such as high functioning autism, Asperger’s syndrome, and pervasive developmental disorder (PPD-NOS). More children fit under the ASD umbrella.

  • Difference in state-by-state diagnosing of ASD increases prevalence

    While ASD diagnosis practices are recommended federally, direct policy implementation is largely state-decided. State-by-state policy demonstrates the fluidity of ASD data. Per Scientific American, Colorado reports a 1 in 93 prevalence of autism, while New Jersey reports a 1 in 41. With that said, this statistical difference isn’t necessarily due to actual prevalence rates, but is most likely due to how the states diagnose ASD and support awareness campaigns. If the CDC samples data from states that provide more ASD services, then the numbers will be higher and vice versa.

  • Biological, social, and environmental factors may increase prevalence

    While the cause is unknown, studies have found that genetic, biological, and environmental factors increase risk of ASD. For example, paternal age, maternal physical and mental health during pregnancy, and exposure to air pollution and pesticides are a few of these factors. As norms change—more couples having children later in life, higher rates of metabolic syndrome, and poor outdoor air quality—so could the numbers of ASD diagnoses. The studies are limited regarding the relation of these factors and the most recent prevalence spike, so, at best, these external outliers are looked upon as theoretical.

  • A change in school-based special education reporting policy causes a cascade of prevalence

    Two 1990s policy changes played a significant role in the reporting of ASD. In 1990, the California Department of Developmental Services (CA DDS)—in conjunction with the U.S. Department of Education’s Office of Special Education—recorded an uptick of school-based ASD service requests. At the same time, the 1990 Individuals with Disabilities Education Act (IDEA) began requiring a tally of children enrolled in special education services, autism being included in 1991 a year later. CA DDS pulls data not just for autistic individuals, but also for epilepsy, cerebral palsy, intellectual disability, and intellectual disability-related conditions, which could inflate the data.

  • Clinicians lean into ASD diagnoses for access to specialized services

    As awareness of ASD grows, so do specialized services. Prevalence of ASD is affected by this growing scaffolding of ASD-specific support, accessed via a professional’s reported diagnosis. It’s been found that clinicians are more likely to give an ASD diagnosis in order to access specialized services for families in need. Previously undiagnosed borderline cases now tip into an ASD diagnosis, therefore boosting prevalence. It can be argued that, as of 2013, these borderline cases were already included in the broader ASD definition laid out in the DSM and therefore would have been diagnosed anyway.

  • Widespread screening reaches children of color, increasing diagnoses

    Mostly due to lack of services, Black and Hispanic children are reported to have lower ASD diagnoses. For instance, a study published in August of 2020 found that Black children were diagnosed six months later than white children, even when parents voiced concerns for years, highlighting the racial disparities of ASD services in communities of color. On the other hand, with changes in the DSM and AAP policy, widespread screening has seen a boost in ASD diagnoses for both Black and Hispanic groups. In fact, the cases of autism in Hispanic children almost tripled between 2000 and 2006.

  • CDC collection relies on medical and school reporting, providing an efficient data intake with massive potential vulnerabilities

    The CDC tracks autism using a curated sample—medical and school records for 8-year-old children, every two years, from 11 states. This sampling reveals undiagnosed and diagnosed children, which could contribute to increased prevalence. With that said, this selective sampling allows for missing cases and inaccuracy. Assessing an individual in person is far more accurate than using third-party records. Plus, children who don’t have records—those that are home-schooled children or those living in isolated and rural areas, for instance—won’t be counted. Both of these factors may skew the data.