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Background: Attention deficit hyperactivity disorder (ADHD) frequently persists into adulthood. Family and twin studies delineate a disorder with strong genetic influences among children and adolescents based on parent- and teacher-reported data but little is known about the genetic and environmental contribution to DSM-IV ADHD symptoms in adulthood. We therefore aimed to investigate the impact of genetic and environmental influences on the inattentive and hyperactive-impulsive symptoms of ADHD in adults., Method: Twin methods were applied to self-reported assessments of ADHD symptoms from a large population-based Swedish twin study that included data from 15 198 Swedish male and female twins aged 20 to 46 years., Results: The broad heritability [i.e., A + D, where A is an additive genetic factor and D (dominance) a non-additive genetic factor] was 37% (A = 11%, D = 26%) for inattention and 38% (A = 18%, D = 20%) for hyperactivity-impulsivity. The results also indicate that 52% of the phenotypic correlation between inattention and hyperactivity-impulsivity (r = 0.43) was explained by genetic influences whereas the remaining part of the covariance was explained by non-shared environmental influences. These results were replicated across age strata., Conclusions: Our findings of moderate broad heritability estimates are consistent with previous literature on self-rated ADHD symptoms in older children, adolescents and adults and retrospective reports of self-rated childhood ADHD by adults but differ from studies of younger children with informant ratings. Future research needs to clarify whether our data indicate a true decrease in the heritability of ADHD in adults compared to children, or whether this relates to the use of self-ratings in contrast to informant data.

Objective: To explore the associations of subtypes of adult ADHD with other psychiatric problems, stressful life events, and sex differences., Method: Odds ratios were calculated using information from 17,899 participants from a population-based survey of adult twins born in Sweden between 1959 and 1985., Results: Symptoms of attention deficit hyperactivity disorder (ADHD) were associated with an increased risk for symptoms of (odds ratio [95% confidence interval]): generalized anxiety disorder (5.6 [4.3; 6.5]), major depression (2.8 [2.4; 3.2]), bipolar disorder (8.0 [5.1; 12.6]), obsessive-compulsive disorder (3.9 [3.1; 4.9]), and alcohol dependence (2.6 [2.2; 3.1]). Symptoms of ADHD were found to be associated with an increased risk for stressful life events (1.8 [1.3; 2.4]). No significant difference in comorbidity was observed between the two sexes., Conclusion: Both women and men with ADHD are at increased risk for symptoms of other psychiatric disorders. They are also at increased risk for stressful life events.