ADHD Prevalence, Heritability, and Persistence Over Time
According to the Centers for Disease Control and Prevention (CDC), ADHD affects approximately 9.8% of children aged 3-17 years in the United States. Boys are more likely to be diagnosed (13%), than girls (6%). There has been an upward trend in prevalence from 1997 to the most recent measurement period.
According to the National Institute of Mental Health (NIMH), the prevalence of ADHD in adults in the United States is estimated to be around 4.4%, and was slightly higher in men (5.4%) compared to women (3.2%). Rates found in the New Zealand Health Survey are substantially lower for children at around 3%, and the survey does not include adults. The NZ and US data for children are based on parent-reported diagnosis and does not include screening or assessment data.
Most children and adolescents continue to have ADHD as adults - 60%-75%, or more. Our understanding of this has changed a lot in the last 20 years. In my clinical psychology training in the late 2000s we were still taught that most kids outgrew ADHD, and I remember being a bit doubtful of ADHD existing in adults (how times have changed!) There are reported higher rates of ADHD in boys than girls, with rates for men and women being much closer together. General consensus around this is that girls with ADHD are not detected as often as boys, possibly due to presenting with fewer hyperactive and impulsive symptoms.
Certainly, in my time assessing adults I can see this pattern in school reports. Boys are generally rated as more disruptive than girls. However, this does not mean that all these girls have inattentive presentations, which I think is how this is sometimes interpreted. Even girls with the combined presentation in adulthood (including some who are very hyperactive or impulsive) appear better able to suppress or moderate their disruptive symptoms in school settings. I believe this is likely due to gendered expectations about behaviour, as well as higher social understanding and desire to fit in with other children.
A meta-analysis in the Journal of Attention Disorders found that 15% of adults attending psychiatric outpatient clinics (1593 males and 1809 females) met clinical criteria for ADHD, mostly undiagnosed at that point. This data suggests that ADHD is quite prevalent in people seeking mental health services, which makes sense given the high comorbidity of other mental health disorders and ADHD, and that those other disorders appear to be diagnosed much more readily than ADHD. So many of the adults I see for ADHD diagnosis have already been diagnosed and treated for anxiety and depression, sometimes for decades, without ADHD being detected.
ADHD is a highly heritable condition, meaning that it tends to run in families. Research suggests that genetics may account for approximately 70-80% of the variance in ADHD risk, with environmental factors accounting for the remaining variance. A twin study found that concordance rates for ADHD were 88%, meaning that if one twin was diagnosed with ADHD, the other twin met criteria in 88% of cases. A few dozen genes have been identified that are associated with ADHD, including genes involved in dopamine signaling and regulation, which play a key role in attention and reward processing. Environmental factors that increase the likelihood of ADHD include maternal exposure to some toxins (lead, mercury, pesticides), maternal smoking or alcohol use during pregnancy, premature birth, low birth weight, traumatic brain injury or encephalitis, and a few others.
Because of the high genetic loading of ADHD, it is common to find a first-degree relative of a person with ADHD who also shows clinical or sub-clinical signs of ADHD themselves. A number of times during the process of my adult ADHD assessments, possible ADHD symptoms in a child have been discussed. Several of my clients have had one or more children diagnosed and treated for ADHD. I have also spoken to many parents of my clients as part of the assessment process who mention that they also notice ADHD symptoms in themselves. I have assessed a parent of my original clients on a few occasions. I see this raising of awareness, diagnosis and treatment as a prime motivator to do this work. The more people who are diagnosed and treated, the more that the visibility of ADHD is increased, which will hopefully lead to further assessments, more awareness, and more acceptance of ADHD within families and the wider community.