![]() 2) suggest that the heritability of ADHD is stable during the transition from childhood into adulthood. Another study found the heritability of clinically diagnosed ADHD in adults to be 72%. In contrast, one study estimated heritability to be 80% after combining self and parent ratings into a composite index of ADHD. ), which is substantially lower than the heritability among children and adolescents. The first twin studies of ADHD in adults used self-reports and estimated heritability at 30–40% (Fig. ![]() These effects could explain why heritability estimates are lower in studies relying on different informants for each twin in a pair compared with studies using the same raters. Using different informants for ADHD symptom ratings of each twin in a pair introduces rater effects (i.e., each rater experiences and reports different ADHD symptoms) or rater bias (i.e., a rater consistently over- or underestimates ADHD symptoms or similarities between twins). Low reliability of self-reports may explain why heritability estimates are lower in studies of self-rated ADHD symptoms. They showed that self-ratings, as well as different-parent and different-teacher ratings within twin pairs, were associated with lower heritability estimates (~30–40%) compared with heritabilities based on same-parent and same-teacher ratings (~70–80%). Two twin studies examined these rater effects. In contrast, studies using self-ratings in adolescence and adulthood show lower heritabilities (<50%). Parent and teacher ratings of ADHD symptoms result in high heritability estimates (70–80%). ADHD’s clinical features and course Reporter effects ![]() These data are consistent with clinical studies showing the clinical implications of subthreshold ADHD. Both studies suggest that the diagnosis of ADHD is the extreme of a continuous distribution of ADHD symptoms in the population and that the etiologic factors involved in the disorder also account for the full range of symptoms. This study confirmed an early study of 583 same-sexed twin pairs using ADHD-III-R symptoms. A study of 16,366 Swedish twins found a strong genetic link between the extreme and the sub-threshold variation of DSM-IV ADHD symptoms. Twin studies have explored whether ADHD is best viewed as a categorical disorder or as an extreme of a continuous trait. Their heritability estimates range from 77 to 88%, which is consistent with the larger number of studies using symptom count measures of ADHD. The heritability is similar in males and females and for the inattentive and hyperactive-impulsive components of ADHD. A similar heritability estimate of around 80% was seen in a study of MZ and DZ twins, full siblings, and maternal and paternal half-siblings. The mean heritability across 37 twin studies of ADHD or measures of inattentiveness and hyperactivity is 74% (Fig. Twin studies rely on the difference between the within-pair similarities of monozygotic (MZ) twin pairs, who are genetically identical, and dizygotic (DZ) twin pairs, who share, on average, 50% of their segregating genes. The adoptive relatives had a risk for ADHD like the risk in relatives of control children. Adoption studies suggest that the familial factors of ADHD are attributable to genetic factors rather than shared environmental factors with the most recent one reporting rates of ADHD to be greater among biological relatives of non-adopted ADHD children than adoptive relatives of adopted ADHD children. Although many details of ADHD’s pathophysiology are unknown, neuropsychological and neuroimaging studies implicate brain circuits regulating executive functioning, reward processing, timing, and temporal information processing.Įvidence for heritability from family, adoption, and twin studiesĪ study of 894 ADHD probands and 1135 of their siblings aged 5–17 years old found a ninefold increased risk of ADHD in siblings of ADHD probands compared with siblings of controls. People with ADHD are at risk for a wide range of functional impairments: school failure, peer rejection, injuries due to accidents, criminal behavior, occupational failure, divorce, suicide, and premature death. Longitudinal studies show that two-thirds of ADHD youth will continue to have impairing symptoms of ADHD in adulthood. It occurs in about 5% of children with little geographic or cross-cultural variation in prevalence and often co-occurs with other conditions, including mood, anxiety, conduct, learning, and substance use disorders. Decades of research have documented and replicated key facts about the disorder (for a review, see ref. Attention deficit hyperactivity disorder (ADHD) is a childhood-onset condition with impairing symptoms of inattention, impulsivity, and hyperactivity.
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