As rates of ASD and ADHD rise, genetic contributions fall: Evidence for widening diagnostic criteria

ImportanceThe incidence of ADHD and autism spectrum disorder (ASD) has increased markedly over recent decades, raising concerns about the emergence of new risk factors. Current literature typically attributes increased rates to changes in diagnostic practice, stigmatization, and awareness, but critically few studies have explored changes in underlying risk factors. ObjectiveTo assess changes in the genetic risk profile of individuals diagnosed with ASD or ADHD according to year of incident diagnosis. DesignWe used the iPSYCH2015 study, a population-based case-cohort with complete ascertainment of incident diagnoses for ASD and ADHD made from 1994 to 2016. SettingDenmark. ParticipantsASD (N=17,071) and ADHD (N=20,111). ExposureYear of incident diagnosis. Regression models tested changes in the mean genetic risk profile of individuals diagnosed in each consecutive year (1994-2016), adjusting for age, sex, and ancestry. Main OutcomesWe used polygenic scores for psychiatric (ADHD, ASD, depression, bipolar, schizophrenia) and cognitive-behavioral (addiction, educational attainment, IQ, neuroticism, risk-taking) outcomes to capture the genetic risk profiles of diagnosed individuals. ResultsA more recent ADHD diagnosis was associated (p<0.001) with less genetic risk for ADHD ({beta}=-0.06 SD per 10 years) and other disorders (ASD, bipolar, schizophrenia). Similarly, a more recent ASD diagnosis was associated with less genetic risk for ASD ({beta}=-0.07) and other disorders/traits (bipolar, schizophrenia, educational attainment). Conclusions and RelevanceOur novel approach suggests that over recent decades diagnostic practice around ADHD and ASD has evolved to capture a different profile of genetic risk. These findings support broadening diagnostic criteria as the explanation for the rise in incidence, with implications for understanding prevalence trends in relation to changes in risk factors and clinical practice. KEY POINTSO_ST_ABSQuestionC_ST_ABSHas the genetic risk profile of individuals diagnosed with autism spectrum disorder (ASD) and ADHD changed as diagnostic rates have risen? FindingsIn the iPSYCH case-cohort study, we observed the genetic contributions to ASD and ADHD diagnoses have weakened over the past two decades. MeaningOur results suggest that recent increase in ASD and ADHD diagnoses coincide with a broadening of diagnostic criteria.