Recent research sheds light on the factors driving the surge in ADHD and autism diagnoses. A comprehensive study involving 140,000 individuals in Denmark found that those recently diagnosed with ADHD or autism possess fewer genetic variants than those diagnosed a decade ago. This indicates that the broadening of diagnostic criteria may be responsible for the increasing numbers, rather than an overdiagnosis of these conditions.

Over the last 20 years, global autism and ADHD diagnoses have surged, with rates increasing up to tenfold, particularly among girls and adults. This trend could be attributed to heightened awareness, the expansion of diagnostic criteria, and the influence of commercial interests among pharmaceutical firms and diagnostic clinics. Additionally, shifts in environmental factors, such as rising screen time and unverified claims regarding childhood vaccinations and prenatal paracetamol exposure, may have a role.

Genetics is another critical factor influencing these conditions. Research suggests that ADHD and autism are highly heritable, with thousands of common genetic variants identified that slightly increase the likelihood of developing core traits associated with these conditions.

Advances in DNA analysis allow researchers to evaluate a person’s “polygenic risk score” for specific traits. High scores for ADHD or autism are “not diagnostic by themselves,” asserts Dr. Sonya LaBianca at Copenhagen University Hospital. Environmental, cultural, and social factors, as well as rare variants, are not included in these scores. However, due to the stable nature of the population’s DNA over a decade, these scores serve as a benchmark to examine other influences.

LaBianca and her team used genome-wide association studies from 2018 and 2019 to develop polygenic risk scores for ADHD and autism, applying these to the iPSYCH Cohort Study of 140,000 people, of whom over 37,000 were diagnosed with ADHD or autism from 1994 to 2016. The researchers also assessed polygenic risk scores for various mental health disorders, including depression and specific personality traits like risk-taking.

They explored three possible reasons for the increasing rates of ADHD and autism. The first reason suggests that diagnostic thresholds have lowered due to the expansion of diagnostic categories. The second relates to individuals previously diagnosed with other mental health issues now being recognized as neurodivergent. Finally, improved understanding enables the identification of individuals who previously went unnoticed.

Findings indicated that recently diagnosed individuals had significantly lower polygenic risk scores for ADHD and autism compared to earlier diagnoses. Moreover, they also exhibited lower risk scores for other mental health issues and traits related to ADHD, such as impulsivity. This suggests that the current diagnostic practices are now reaching a broader range of individuals, with the primary explanation for rising case numbers being the expansion of diagnostic criteria, according to LaBianca. While increased awareness and diminished stigma may also play a part, there is less data to substantiate this claim.

While this might imply overdiagnosis, LaBianca emphasizes that even those with the lowest polygenic scores still had significantly more risk variants than neurotypical controls. “This supports our finding that we are not overdiagnosing,” she notes. “I would only use the term ‘overdiagnosis’ if diagnosing individuals with similar polygenic levels as the general population.”

Professor Tinka Polderman from Vrije Universiteit Amsterdam also posits that the expansion of diagnostic criteria is the main reason for the rising rates of diagnoses. However, she cautions against viewing genetics and environment as completely separate entities. “Polygenic risk scores only provide one piece of the puzzle,” she explains.

“As more individuals seek support for these conditions, we must also consider other risk factors if one has a low genetic predisposition,” LaBianca advises.