Abstract
Background: No prior twin study has explored the heritability of clinically diagnosed ADHD. Such studies are needed to resolve conflicting results regarding the importance of genetic effects for ADHD in adults. We aimed to estimate the relative contribution of genetic and environmental influences for clinically diagnosed ADHD across the life-span with a specific focus on ADHD in adults.
Method: Information about zygosity and sex was obtained from 59,514 twins born between 1959 and 2001 included in the nation-wide population-based Swedish Twin Register. Clinical data of ADHD diagnosis was obtained from the Prescribed Drug (i.e., stimulant or non-stimulant medication for ADHD) and National Patient Registers (i.e., ICD-10 diagnosis of ADHD). Twin methods were applied to clinical data of ADHD diagnosis using structural equation modeling with monozygotic and dizygotic twins.
Results: The best-fitting model revealed a high heritability of ADHD (0.88; 95% CI, 0.83-0.92) for the entire sample. Shared environmental effects, on the other hand, were non-significant and of minimal importance. The heritability of ADHD in adults was also substantial (0.72; 95% CI, 0.56-0.84).
Conclusion: This study showed that the heritability of clinically diagnosed ADHD is high across the life span. Our finding of high heritability for clinically diagnosed ADHD in adults indicate that the previous reports of low heritability is best explained by rater effects and that gene-identification studies of ADHD in adults needs to consider pervasiveness (e.g., multiple raters) and developmentally (e.g., childhood onset criteria) informative data.