Epidemiology

Prevalence: ADHD is one of the most common disorders of childhood. One high quality estimate of global prevalence in children gives a figure of around 5.29%.[12][13]​​​​​ In the US, a large national survey of parents during 2016-2019, found an overall prevalence of 2% among children aged 3-5 years, of 10% in children aged 6-11 years, and of 13% in children aged 12-17 years.​​[14]​ Although prevalence appears to have increased substantially in recent decades, follow-up meta-regression analyses suggest that this difference is due to methodological differences between studies and that, in fact, prevalence has remained stable worldwide (and within specific populations) since the 1980s.[12][15][16]

Presentations: the combined-type presentation accounts for 50% to 75% of all people with ADHD, the inattentive-type presentation accounts for 20% to 30%, and the hyperactive-impulsive-type presentation accounts for 15%. Over time, inattentive symptoms tend to persist and hyperactive-impulsive symptoms tend to diminish.[10]

Sex disparities: community studies show male-to-female prevalence rates of around 2.1 to 1.0, while clinic populations show the ratio as high as 10 to 1.[14]​​[17]​ This sex difference has been explained by the fact that boys present more often with disruptive behaviour that prompts referral, whereas girls more commonly have the inattentive presentation and have lower comorbidity with oppositional defiant disorder (ODD) and conduct disorder.[10][17]​ It is therefore likely that ADHD is under-recognised and underdiagnosed in girls.[18]

Ethnic differences: several large studies suggest that Hispanic and Asian children have a lower prevalence of ADHD than white or black American children.[14]​​[19]​ According to the Multimodal Treatment Study of AD/HD (MTA), there were higher reported levels of ADHD in the classroom for African-American than for white children.[20] The prevalence of ADHD in black adolescents under 18 years old in the US was reported as 14% in one large meta-analysis, a substantially higher rate than the general US population.[21]​ It is unclear if these findings represent actual prevalence differences or whether they may be related to confounding variables such as access to care.

Class and income differences: ADHD has been associated with poverty, lower family income, and lower social class in the US, the UK, and other countries.[22]

Comorbidity with other mental, emotional, or behavioural disorders is common. According to a 2016 national parent survey in the US, 6 in 10 children with ADHD had at least one co-existing disorder. In about half of children, this was a behaviour or conduct problem. About 3 in 10 children had anxiety. Other comorbidities include depression, autism spectrum disorder, Tourette's syndrome, and learning and language disabilities.[14][23] Children with epilepsy are at increased risk for cognitive and behavioural disorders including ADHD. While the reported prevalence rates vary depending on study population and methods, clinical-based studies commonly report ADHD in 25% to 40% of children with epilepsy.[24]​ Adolescents with ADHD are at increased risk of eating disorders and substance use disorders, including alcohol use disorder, cannabis use disorder, and other drug use disorders.[25][26]​​[27] There is some evidence that pharmacological treatment for ADHD decreases the risk for substance use disorders.[28][29]

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