The rise in autism diagnoses has been steady and striking. In the 1960s, roughly 1 in 10,000 people was diagnosed with autism. Today, 1 in 54 children has the condition, according to the Centers for Disease Control and Prevention. And the rise in the U.S. is mirrored in countries around the world.
What’s responsible for this surge? Scientists have vigorously debated the role of genetics, the environment, and changes in how the condition is diagnosed. In a recent effort to disentangle these threads, researchers determined that the stability of genetic and environmental influences implicates changes in diagnostic practices and increased awareness as likely forces of change.
“The proportion of autism that’s genetic and environmental is consistent over time,” says Mark Taylor, a senior researcher at the Karolinska Institutet in Sweden and lead author of the study. “Although the prevalence of autism has increased a lot, this study doesn’t provide evidence that it’s because there has also been some change to the environment.”
Taylor and his colleagues analyzed two sets of data from twins: the Swedish Twin Registry, which tracked diagnoses of autism spectrum disorder from 1982 to 2008, and the Child and Adolescent Twin Study in Sweden, which measured parental ratings of autistic traits from 1992 to 2008. Together the data encompassed nearly 38,000 twin pairs.
The researchers assessed the difference between identical twins (who share 100 percent of their DNA) and fraternal twins (who share 50 percent of their DNA) to understand if and how much the genetic and environmental roots of autism have changed over time. And genetics play a critical role in autism—some estimates place the heritability at 80 percent.
As the scientists reported in the journal JAMA Psychiatry, genetic and environmental contributions did not significantly shift over time. Researchers continue to investigate environmental factors that may be implicated in autism, such as maternal infection during pregnancy, diabetes, and high blood pressure. The present study doesn’t render specific factors invalid but rather shows they are not responsible for the surge in diagnoses.
The findings echo previous studies that arrived at a similar conclusion through different methods. One 2011 study, for example, assessed adults with standardized surveys and determined that there was no significant difference in autism prevalence between children and adults.
Paternal age is often discussed as a risk factor for autism. A father’s age increases the likelihood of spontaneous genetic mutations, called de novo or germline mutations, which can contribute to autism. And the age when men become fathers has increased over time: In the U.S., for example, average paternal age rose from 27.4 to 30.9 between 1972 and 2015. But spontaneous mutations only account for a tiny sliver of the rise in autism diagnosis rates, explains John Constantino, a professor of psychiatry and pediatrics and co-director of the Intellectual and Developmental Disabilities Research Center at the Washington University School of Medicine in Saint Louis.
“We’re diagnosing autism 10 to 50 times more now than we were 25 years ago. The advance in paternal age is only responsible for around 1 percent of that whole effect,” Constantino says. The influence of parental age on developmental disabilities should be taken seriously, given that a small change is still meaningful in the context of the global population, he notes. It just doesn’t account for the trend overall.
If genetic and environmental factors have remained steady over time, cultural and diagnostic shifts must be responsible for the spike in prevalence, Taylor says. Both families and clinicians today are likely more aware of autism and its symptoms than in past decades, making diagnosis more likely.
Changes in diagnostic criteria also play a role. Clinicians diagnose mental health conditions based on criteria delineated in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The pre-2013 version, the DSM-IV, contained three categories: autistic disorder, Asperger’s disorder, and pervasive developmental disorder not otherwise specified. The current iteration, the DSM-5, replaces those categories with one overarching diagnosis: autism spectrum disorder.
Creating a label to encompass previously discrete conditions necessitates more expansive language, explains Laurent Mottron, a professor of psychiatry at the University of Montreal. Such changes in the criteria may have resulted in additional people receiving an autism diagnosis.
This change positions autism closer to the way science and medicine perceive many other conditions, Constantino says. “If you survey a whole population for the characteristics of autism, they fall onto a bell curve, just like height or weight or blood pressure,” Constantino says. The current definition of autism is no longer reserved for the most extreme cases; it embraces subtler ones as well.