Previous studies have demonstrated that the common variant risk, also referred to as the single nucleotide polymorphism (SNP) heritability, of ADHD is also associated with depression25, conduct problems26, schizophrenia27, continuous measures of ADHD symptoms28,29 and other neurodevelopmental traits29 in the population. Genetic studies of quantitative ADHD symptom scores in children further support the hypothesis that ADHD is the extreme of a quantitative trait30.
