Ask SciMoms: Are there more cases of autism?

We were recently asked if there are more autistic children today than in the past. This is a common question, as the claimed increase in the prevalence of autism is used to scare us about many products in our environment, ranging from WiFi to pesticides. In this post, we will examine whether the increase in diagnosis actually reflects an increase in autism and what factors are thought to contribute to autism.

What is the autism spectrum?

The autism spectrum or Autism Spectrum Disorders (ASD) are a set of complex neurological disorders. The spectrum of behaviors classified as ASD is very broad in terms of the types of behaviors and phenotypes it describes. According to the CDC:

Autism spectrum disorder (ASD) is a developmental disability that can cause significant social, communication and behavioral challenges. There is often nothing about how people with ASD look that sets them apart from other people, but people with ASD may communicate, interact, behave, and learn in ways that are different from most other people. The learning, thinking, and problem-solving abilities of people with ASD can range from gifted to severely challenged. Some people with ASD need a lot of help in their daily lives; others need less.

A diagnosis of ASD now includes several conditions that used to be diagnosed separately: autistic disorder, pervasive developmental disorder not otherwise specified (PDD-NOS), and Asperger syndrome. These conditions are now all called autism spectrum disorder.

Some children diagnosed with ASD also have intellectual disabilities, language impairment, or other neurodevelopmental disorders. Behaviors also change over time. Autism is not any one thing: this diagnostic label represents a diversity of behaviors and varies greatly between individuals.

Do more children have autism today?

You may have encountered across the term “autism epidemic” in articles that try to convince you that substances in your house, vaccines, or certain foods can account for the seeming rise in autism. This offensive idea implies that autism is a plague of some sort.

Setting aside this extreme view that autism can be compared to an epidemic, are there actually more cases of autism?

How do we monitor the number of cases of ASDs?

The CDC tracks the prevalence of ASD via the Autism and Developmental Disabilities Monitoring (ADDM) Network. The network tracks diagnoses in over 11 communities across the US and estimates the prevalence of ASD in the US. The numbers provided are based on diagnoses in 8-year old children. This age was chosen based on the assumption that by this age, there would be a diagnosis of ASD if there are symptoms. It’s important to keep in mind that these communities do not represent the entire United States: even among these 11 there are differences in the number of ASD diagnoses.

What do the numbers say?

In the year 2000, the prevalence of ASD was estimated to be 1 in 150 of 8-year old children. The latest numbers from 2014 estimate the prevalence at 1 in 59 children. There is clearly an increase in the number of children diagnosed with ASDs.

However, this increase in diagnosis does not necessarily mean that there are more autistic children. Researchers have identified a number of factors that could cause the increase in diagnosis, including but not limited to:

Better services for underserved communities

The CDC numbers show significant socioeconomic and racial disparities in diagnosis rates. For example, studies have shown that populations with greater access to services have higher rates of autism. As services are expanded, diagnosis in underserved populations may be increasing. In recent years, CDC’s monitoring shows that more Latino and black children have been diagnosed with ASD which the CDC attributed to better community outreach. At the same time, more girls have been diagnosed, suggesting that girls have also been underdiagnosed historically.

These numbers suggest that there were populations where autism was being under-diagnosed, so more children in vulnerable populations may be getting access to help and care that they were not receiving in the past. This improvement in equal care should not be overlooked or ignored.

Diagnostic substitution

The definition of what we identify as ASD has also changed dramatically. This has led to something called diagnostic replacement where children who now receive an ASD diagnosis would have received a different diagnosis 10 or 20 years ago. This 2014 CDC report on rates of ASD diagnosis found that prevalence of all development disorders was unchanged, while diagnosis of ASDs went up and diagnosis of other developmental disorders (besides ASD and intellectual disabilities) went down.

Increased awareness of autism

Support for autistic children and funding for research on ASD has increased, and with this, there has been increased awareness of autism and associated behaviours. Stories in social media and broadcasting may have contributed to the fact that we are all more familiar with autism. Medical institutions such as the CDC have implemented programs to better educate parents on childhood development milestones and on the importance of early intervention. Additionally, teachers and early childhood educators may be better trained to recognize signs of ASD. These factors, among others, may have contributed to increased awareness of autism and parents discussing concerns with their doctors, in cases that may have otherwise been ignored. This increased awareness may also be important in the increases seen in underserved communities that we mentioned above.

Routine pediatric screening at well-child visits

The development of new standards for pediatricians has also contributed to increased diagnosis. Pediatricians check for developmental delays at regular well-child visits as early as 9 months. Doctors specifically look for signs of ASD at 18 and 24 month check-ups, and even more frequently under specific circumstances, such as an autistic sibling or cousin. Studies have shown that there are still significant barriers to the adoption of these practices: for example, many doctors state that they do not have enough time to conduct these checks. However, in general, these best practices are identifying more children with signs of ASD and ensuring that they receive intervention.

So what causes autism?

We know that autism risk has a very strong genetic component, with siblings of autistic kids more likely to also be autistic (a number that is higher for fraternal twins and even higher for identical twins).

As Alison has written previously on Exposing Autism One:

While autism is primarily genetic in origin, there is evidence for some environmental factors that affect risk. From the National Institute of Environmental Health Sciences (NIEHS) website on autism:

The clearest evidence for environmental risk factors in autism involves events before and during birth. They may include:

  • Advanced parental age at time of conception
  • Prenatal exposure to air pollution
  • Maternal obesity or diabetes
  • Extreme prematurity and very low birth weight
  • Any birth difficulty leading to periods of prenatal oxygen deprivation to the baby’s brain
  • Prenatal exposure to certain pesticides

Again, however, these factors alone are unlikely to cause ASD. Rather, they appear to increase a child’s chances for developing ASD, when combined with the aforementioned genetic factors.

There are several important points here:

  1. Existing evidence for environmental factors is primarily for prenatal exposures and  exposures soon after birth. Changing these exposures after diagnosis is unlikely to change these neurodevelopmental pathways, just as exposure to them after birth is unlikely to cause autism. As previously noted, children cannot “get autism”.
  2. These risk factors are not causative and are unlikely to cause ASDs on their own. Like any individual gene, these environmental factors very slightly alter the risk of ASDs and probably only in a subset of people. (For a detailed discussion, see this article  – also by Alison – about autism and maternal antidepressant use from Grounded Parents, Don’t Flush Your Meds: Antidepressants, Pregnancy and Autism.)
  3. Only one of these items listed by NIEHS, prenatal exposure to certain pesticides, relates to what people on the internet are talking about (synthetic chemicals) and it is not at all a blanket statement about all synthetic chemicals or even all pesticides. This highlights the large disconnect between what the public thinks and what scientists think about environmental risks for autism.
  4. It is important to note that the individuals most at risk for these synthetic chemicals/pesticides are farmers and their families. We need to do a better job at ensuring that the farming community has the resources and equipment they need to stay safe if these chemicals cannot be replaced, or that their chemical replacements present a lower risk to these vulnerable populations. The risk of these pesticides to consumers via our food is low.
  5. The fact that air pollution is a possible risk factor for ASD highlights that climate change and the factors that contribute towards it impact many areas of life. Populations that will be most impacted are also underserved ones.

While we are talking about environmental factors, it is also worth reiterating that vaccines do not cause autism. There are few questions that have been examined as thoroughly and comprehensively as the lack of any link between autism and vaccines (a brief list of papers can be found here and an extensive list here).

Using ASD to Market Fear

Marketing campaigns and articles that use autism to instill fear should be regarded with caution (see our comic series on this topic). As outlined in this article, there have been more diagnoses of autism over the years. This can be correlated with nearly anything whose use or consumption has also increased over the years: GMOs, WiFi, iPhone sales, quinoa and kale consumption, organic food sales, etc. When reading such articles, focus on the data beyond the correlation: is there any evidence to support that one actually causes the other? Who are they interviewing? Is there a reputable medical organization that is supporting the arguments in the article?


There are clearly more children being diagnosed with autism today. This is due to many factors, largely driven by better resources and education for families and doctors as well as improved diagnostic criteria. It is possible that a small part of the increase in diagnosis reflects an actual increase in prevalence. However, this would only account for the small portion of the overall increase not accounted for by these other factors.