Trends in neurodevelopmental disorder diagnoses and co-occurrence by gender and household income

Abstract Background Neurodevelopmental disorders (NDDs) contribute substantially to childhood morbidity. NDDs often co-occur, and they are more common among boys and in families with low socioeconomic position. While NDDs are increasingly diagnosed, limited evidence exists on how their incidence-and associated gender and social disparities-has evolved across cohorts, both for individual disorders and their co-occurrence. Methods We use Finnish register data on full birth cohorts (1997-2017) to assess cohort differences in cumulative incidence and co-occurrence of attention-deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and developmental learning disorder (DLD) by gender and household disposable income. Diagnoses were identified from data on healthcare visits and ADHD medication purchases. Results Among boys, cumulative ADHD incidence at age 16 nearly doubled between cohorts 1997-1999 and 2003-2005, reaching 10.2%, while ASD rose to 2.7%, and DLD to 4.4%. Among girls, ADHD incidence nearly tripled to 4.1%, while ASD reached 1.1% and DLD 2.5%. Diagnosis co-occurrence also increased, with ADHD-DLD the most frequent pairing. ADHD and ASD diagnoses increased in all income groups, but disparities persisted over time. Among boys in the 2003-2005 cohort, ADHD cumulative incidence by age 16 reached 14.5% in the lowest income quartile, compared to 6.8% in the highest. Cumulative DLD incidence only increased in lower-income families. Conclusions The findings underscore the growing public health challenge posed by rising NDD diagnoses and highlight persisting inequalities in the distribution of diagnoses across social strata. Future research should further investigate the mechanisms driving these trends, including diagnostic practices and healthcare accessibility. Key messages • Individual and co-occurring ADHD, ASD, and DLD diagnoses have increased across cohorts among boys and girls, but gender disparities have persisted. • Children from lower income households remain substantially more likely to be diagnosed with NDDs than those whose parents have higher income.