The rise in the number of cases of attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) always gets quite a bit of attention from both the public and the media. This has led to the rise of several misinformed theories about the cause. On the other hand, the increase in the number of people diagnosed has also led to crucial scientific investigations.
A multinational study published in JAMA Psychiatry examined whether rising diagnosis rates of ASD and ADHD have been accompanied by changes in the genetic risk profile of diagnosed individuals.
After looking at more than 37,000 individuals from Denmark who were diagnosed with either ASD or ADHD between 1994 and 2016, the researchers found that the genetic risks of autism and ADHD have decreased over time among people with diagnoses.
People diagnosed with ASD or ADHD today still carry a higher genetic risk than the general population, but that genetic burden is not as pronounced as it was among patients diagnosed two decades ago. The sharp rise in cases might be driven by changes in how doctors diagnose these conditions and greater awareness of milder symptoms.
What changed over the years?
Diagnoses of ADHD and ASD have seen a fourfold to tenfold jump in some populations. This trend has fueled debates, questions and speculation about what is driving the increase. Some studies have pointed to possible environmental influences, while others have highlighted changes in diagnostic practices and growing public awareness.
Over the past 40 years, the definitions of both of these conditions have been broadened. They are no longer defined as childhood conditions but are now recognized as lifelong disorders that can appear in many different ways and often occur alongside other mental health conditions. While these changes might be the major force behind the rise in diagnoses, researchers still need to understand whether there is an underlying biological risk factor in patients.
In this study, the researchers zoomed in on the possible genetic aspects associated with the conditions. Humans share nearly identical DNA, but small inherited differences make each of us unique. Some of these differences influence traits such as height or hair color, while others can affect a person’s risk of developing certain health conditions.
To identify the genetic factors, the researchers calculated the polygenic risk scores (PRSs) of every participant. PRS is a tool that estimates a person’s genetic predisposition to a trait or condition by combining the effects of many genetic variants.
By combining PRS data with statistical models, the team tracked a simple pattern: Did the genetic risk profile of people receiving a diagnosis change over time?
For both ASD and ADHD, the researchers found that for every 10 years that passed, the average genetic risk score of the people being diagnosed dropped significantly. The drop wasn’t just in the specific disorder being diagnosed.
People diagnosed with ADHD or ASD more recently also showed lower genetic risk for other conditions, such as schizophrenia and bipolar disorder. Also, for any given age, more people were being diagnosed with ADHD or ASD in later years than in earlier years.
The researchers proposed three possible explanations for the rise in diagnoses. The first was that diagnoses are increasingly capturing people with milder forms of ADHD or ASD who may not have met the threshold in the past. This matched the data, with genetic risk scores for both conditions declining over time.
The second was that shifting diagnostic boundaries were bringing in people whose symptoms were more consistent with another disorder. The data, however, showed the opposite. Instead of higher genetic risk scores for other disorders, the researchers found declines across almost all psychiatric risk profiles.
The third was that improved detection identified people who had always had the condition but had previously been undiagnosed. Here, too, the findings did not fit, as genetic risk scores fell significantly over time.
Based on the findings, the researchers noted that broadening diagnostic criteria may be a key driver of rising ADHD and ASD rates. The unique perspective on the diagnosis trends challenges existing narratives that blame a single environmental factor or vaccines for the increase.
Instead, the findings raise more nuanced questions about overdiagnosis versus long-overdue recognition, and how society views disorder versus normal variation in behavior and development.
