• Epidemiology
• Etiology
• Onset and Course
• Comorbidity
• Associated Features
• Associated Impairments and Functional Outcomes

Skyrocketing diagnoses, widespread media coverage, and pharmaceutical marketing have converged to make ADHD a ubiquitous and seemingly familiar disorder. However, what comprises the average picture of ADHD among the general public and, perhaps, many clinicians? Likely, the image that comes to mind is of an elementary school-aged boy, struggling to stay seated, interrupting teachers with inappropriate remarks, disregarding an array of parental requests and rules, and struggling to find what he needs amidst the disorganized mess of his bedroom or backpack. Although stereotypic and applicable to only a portion of those diagnosed with ADHD, this perception does indeed capture common ways in which the disorder can manifest in youth. But how well does this image fit the half or more of childhood cases whose ADHD persists into adulthood?

Broadly speaking, the core symptoms of ADHD—high levels of inattention, impulsivity, and/or hyperactivity—are similar in adults and children, despite age-related changes in their expression and consequences. Accordingly, the symptom criteria for diagnosing ADHD in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR; American Psychiatric Association, 2022) are constant across the age range. The only difference is one fewer symptom (namely, five instead of six) required for those aged 17 years or older to accommodate the normative decline of many ADHD features with age among those both with and without the disorder. Specifically, diagnosing the disorder in adults requires the onset of some symptoms before age 12 and the current presence of at least five developmentally inappropriate inattentive and/or hyperactive/impulsive symptoms that have persisted for at least six months, have been present in at least two settings, and have interfered with functioning in one or more domains¹. Depending on whether symptom count thresholds are met in only the Inattentive domain, only in the Hyperactive/Impulsive domain, or in both domains, a diagnostic specifier of Predominantly inattentive presentation, Predominantly hyperactive/impulsive presentation, or Combined presentation is assigned.

Features of inattention in adults with ADHD (American Psychiatric Association, 2022) include often struggling to sustain their focus; becoming distracted by extraneous stimuli; neglecting to attend to details (leading to careless mistakes on academic, occupational, or personal tasks); forgetting important dates, appointments, and tasks; losing track of conversations; and appearing not to be listening when others speak to them. Disorganization can contribute to difficulties with losing things, working efficiently, and completing tasks promptly, if at all. Self-regulation, task completion, and goal attainment are often compromised by a diverse array of deficits in executive function, including, but not limited to, working memory, planning, time management, self-monitoring, and mental flexibility (e.g., shifting problem-solving approaches as needed; Barkley, 1997; Christiansen et al., 2019; Retz et al., 2012; Shiels & Hawk, 2010).

The excessive running, climbing, and inability to sit still for even brief periods that often characterize hyperactivity in children with ADHD are frequently absent among adults with the disorder (Harpin, 2005; Wender et al., 2001). Instead, when difficulties in this domain persist into adulthood, they typically manifest in persistent feelings of restlessness, fidgeting, intolerance for inactivity, trouble relaxing, a need to be constantly active, and talking too much, too quickly, and too loudly.

Impulsivity in adults with ADHD can manifest verbally, socially, physically, and emotionally, as well as in a general sense of impatience (American Psychiatric Association, 2022; White, 1999; Winstanley et al., 2006). Expressions of verbal impulsivity include interrupting others, blurting out answers or unsolicited comments, and sharing views or information without consideration of the potential emotional impact on others or the damaging social or professional consequences for themselves. Poor inhibition is also frequently reflected in difficulty waiting (e.g., in lines, in traffic, for instructions to be completed), hasty decision-making, a propensity to take short-cuts or to rush through tasks, and, in some cases, a willingness to take unnecessary risks (e.g., related to driving, investing, drinking). Adults with ADHD often struggle with delaying gratification and will tend to opt for an immediate reward even if it means sacrificing a larger payoff in the future. Some experts maintain that an underrepresented yet central aspect of adult ADHD, related to both disinhibition and executive function deficits, is emotional dysregulation, which may manifest as affective lability, emotional over-reactivity, and the inability to inhibit emotional expression (Adler et al., 2017; Drechsler et al., 2020).

Despite the inherently categorical nature of diagnosis based on classification systems like the DSM, ADHD is best thought of in dimensional terms; as representing individuals who fall at the extreme ends of continuums related to attentional capacity, activity level, and inhibitory control (Heidbreder, 2015; Katzman et al., 2017; Swanson et al., 2012). As discussed later in this chapter, it is critical to differentiate the persistent, pervasive, and impairing nature of ADHD from occasional or normative levels of inattention or disinhibition. Additionally, as is the case with youth, the expression of ADHD in adults is notably diverse. Variability is a hallmark feature of ADHD, not only with respect to differences across individuals in terms of the patterning and severity of core symptoms, associated symptoms, and impairments but also within a given adult across time and context. The fluctuation in ADHD symptoms as a function of the nature of prevailing tasks and situations is often profound. Typically, symptoms will be most pronounced in contexts that are sedentary, demanding sustained attention and/or self-control, bereft of immediately available reinforcers, and experienced by the individual as dull, routine, or effortful.

Epidemiology

Estimating the prevalence of ADHD in adults is complicated by the lack of objective markers of the disorder, risks for both under- and over-reporting, changes in the phenomenology of ADHD over the life-span, the questionable applicability for adults of symptom lists and diagnostic thresholds based on the expression of the disorder in youth, and the outdated assumption that ADHD rarely persisted into adulthood. Nonetheless, global prevalence estimates of adult ADHD emerging from published studies range from approximately 2.5% to 6.8% (APA, 2013; Drechsler et al., 2020; Fayyad et al., 2016; Kessler et al., 2006; Posner et al., 2020; Song et al. 2021; Willcutt, 2012)². In the U.S., approximately 10 million adults are believed to have ADHD and the rate of ADHD diagnosis is increasing more among adults than any other segment of the population (Hinshaw & Ellison, 2016). The commonly observed gender disparity among children with ADHD (with boys outnumbering girls by a ratio of about 2.5:1) appears to decline over time to the point that, in adulthood, men and women are close to equally affected (1.5:1 or lower; Hinshaw et al., 2021; Owens et al., 2015)³. ADHD appears to be a universal disorder, occurring in all ethnic and socioeconomic groups, and at largely similar (though not equivalent) rates across the wide range of developed countries studied. Importantly, the rates at which ADHD is diagnosed (as opposed to its true prevalence) do differ by ethnicity, socioeconomic class, country, and, within the U.S., by state, likely attributable to disproportionate referrals and service-seeking rates, among other factors.

Etiology

ADHD is widely recognized as a neurobiological disorder. This scientific consensus is based, in part, on established differences between the brains of those with and without ADHD with respect to neurochemistry (e.g., fewer dopamine receptors in ADHD-associated neural pathways), structure (e.g., developmental lags in normative cortical thickening, smaller sized cortical and subcortical structures), and function (e.g., lower activity levels across various prefrontal regions). Multiple factors, alone or in combination, account for these neurobiological differences associated with ADHD. However, genetic factors represent, by far, the single largest contributor, accounting for approximately 74% of the variance in core ADHD symptoms. As is the case for other psychiatric disorders, multiple genes contribute to the expression of ADHD, with implicated genes impacting neural pathways related to the regulation of neurotransmitters (such as dopamine, serotonin, and norepinephrine), neurite outgrowth, and axon guidance (Faraone & Larsson, 2019).

The high heritability of ADHD does not preclude the involvement of environmental factors in its etiology. Indeed, prenatal, perinatal, and/or early childhood exposure to a host of neuro-compromising environmental factors has been established to increase the risk for ADHD (Froehlich et al., 2011; Thapar et al., 2013). These risk factors include (but are not limited to) gestational exposure to alcohol and tobacco, maternal stress during pregnancy, low birth weight and prematurity, and exposure to environmental toxins such as lead, organophosphates (such as those used in pesticides and fertilizers), and phthalates and bisphenol A chemicals (BPA, which can be found in plastic bottles, receipts, plastic wrap, and soda cans, among other common products).

Multiple pathways lead to the symptom profiles that are diagnosed as ADHD. Some cases may reflect the impact of potent single factors (e.g., prenatal exposure to high levels of heavy metals, ADHD-risk genes) but the causal models receiving the most attention are those that consider the intersection among genetic and environmental factors. These include gene-environment interaction models (wherein genes moderate the effects of environmental factors or environmental factors moderate the effects of genes; Nigg et al., 2020) and epigenetics (wherein exposure to certain environmental factors during critical developmental periods alters the expression of genes, impacting outcomes for both the individual and future generations; Walton et al., 2016).

Onset and Course

As a neurodevelopmental disorder, ADHD is defined as having a childhood onset, although for some, detection and diagnosis do not occur until adolescence or adulthood. Diagnosis, according to the DSM-5-TR, requires evidence of several ADHD symptoms before age 12, whereas the more stringent age of onset criteria set forth in the ICD-11 (World Health Organization, 2018) require that symptoms emerge by 6 years of age. The course of ADHD, like its clinical presentation, is variable. Although symptoms often decrease in frequency and intensity as people enter adulthood (Biederman, 2011), ADHD is typically a chronic condition that tends to be associated with functional impairments across the lifespan. The long-held false belief that ADHD was a self-remitting childhood disorder “outgrown” during the teenage years was likely due to the fact that its most conspicuous feature, physical overactivity, often fades or disappears by adolescence. However, mental restlessness tends to persist through the teen years and into adulthood, along with distressing and impairing problems of impulsivity, inattention, disorganization, planning, and emotional dysregulation (APA, 2013; Barkley, 2015; Kooij et al., 2019). The chronicity of ADHD has been substantiated by multiple prospective longitudinal studies (extending as long as 33 years), the results of which suggest that ADHD persists into adolescence in up to 80% of cases and into adulthood for 50-65% of cases (Barkley et al., 2006; Faraone et al., 2006; Klein et al., 2012; Roy et al., 2016; Weiss & Hechtman, 1993).

Comorbidity

As is the case with their child and adolescent counterparts, adults with ADHD are highly likely to have comorbid psychiatric conditions. Eighty percent or more of adults with ADHD have at least one other mental health disorder, whereas more than half have at least two other disorders (Barkley, 2015; Katzman et al., 2017). These comorbid disorders may appear before or coincident with ADHD but they often emerge later, perhaps as secondary diagnoses reflecting the stressors and challenges associated with living with ADHD (Hinshaw & Ellison, 2016). The most common disorders accompanying ADHD in adults are major depression, dysthymia, bipolar disorder, anxiety disorders, substance use disorders, and personality disorders (Katzman et al., 2017). A widely cited U.S. epidemiologic survey found that among adults with ADHD, 38% had a comorbid mood disorder, 47% had a comorbid anxiety disorder, and 15% had a substance use disorder⁴ (Kessler et al., 2006). Specific learning disabilities also occur at higher rates in adults with ADHD relative to the general population (Murphy et al., 2002). Clinical consideration of comorbidity in adults with ADHD is of critical importance, not only because it tends to be associated with a greater disease burden (with respect to severity and impairment) but also because it can complicate the recognition, diagnosis, and treatment of ADHD (Newcorn, 2008).

Associated Features

In addition to diagnosable co-occurring disorders, adults with ADHD are more likely to experience a wider range of other problems than adults without ADHD (Asherson, 2013). These associated features span multiple domains (e.g., cognitive, occupational, emotional, behavioral, social, medical), add to the phenomenological heterogeneity of adult ADHD, and often reflect or contribute to the impairments associated with the disorder.

Although far from an exhaustive list, these associated features include difficulties with mood and emotional regulation (e.g., irritability, volatility, angry outbursts, anxiety, dysphoria). Anxious and/or depressive features that accompany ADHD symptoms are important to consider in clinical formulation and treatment planning as they can lead to significant impairment and negative functional outcomes.

In addition, compared to the general population, adults with ADHD have higher rates of suicidal ideation, suicide attempts, completed suicides, and various forms of non-suicidal self-injury (e.g., cutting, burning; Allely, 2014; Chronis-Tuscano et al., 2010; Huang et al., 2018; Hurtig et al., 2012; Impey & Heun, 2012; Ljung et al., 2014; Nock, 2010; O’Connor et al., 2013; Septier et al., 2019; Simioni et al., 2018). Alarmingly high rates of self-injurious behavior and suicide attempts have been reported in studies of young women with ADHD, whereas completed suicides occur more often in men with the disorder (Hinshaw et al., 2012; Nigg, 2013; Septier et al., 2019). These concerning associations between adult ADHD, self-injurious behavior, and suicidality are believed to result from complex interactions between the impulsivity and emotional dysregulation that are central features of the disorder, high rates of comorbid depression, anxiety, and substance abuse, and commonly associated features such as poor self-esteem and strained social relationships (Brezo et al., 2006; Hu et al; 2016; Moran et al; 2012; Sheftall et al. 2016; Sodano et al. 2021).

Associated Impairments and Functional Outcomes

Among the more sobering findings from the considerable body of research addressing adult ADHD (including the aforementioned longitudinal studies) is the degree to which the disorder elevates one’s risk for serious and widespread impairments. Indeed, the increased responsibilities and expectations of adulthood, the loss of school-based structure, and the cumulative effect of untreated or undertreated ADHD symptoms can increase the severity and span of impairments in adults as opposed to youth with ADHD.

Having ADHD as a child is a risk factor for a myriad of educational, occupational, social, financial, behavioral, and health problems in adulthood (Barkley, 2015; Sodano et al., 2021; Stein, 2008). Academically, adults with ADHD, as compared to those without the disorder, are less likely to attend college, less likely to complete college (or require more time to do so), have more absences, course withdrawals, and failing grades, and obtain lower grade point averages and academic achievement scores. Occupationally, they tend, on average, to have lower vocational achievement, work performance, and pay alongside higher unemployment, disciplinary actions, discord with supervisors, absenteeism, and firings. Higher rates of social dysfunction are reflected in having fewer friends, more marital dissatisfaction, and increased divorce rates. Personal finances are more likely to be mismanaged, with impulsive spending, overuse of credit, late payment penalties, and lower credit ratings all being more common among adults with ADHD. Behavioral impairments that are more likely to afflict those with ADHD span sexual activity (e.g., less contraception use, more partners, sexually transmitted diseases, and unplanned pregnancies), driving (e.g., speeding, suspended or revoked licenses, aggressive driving, increased number and severity of crashes), legal problems (e.g., encounters with the police, arrests, and incarcerations), and behavioral manifestations of emotional dysregulation. ADHD is also associated with an increased risk for several health-related impairments, including more likely and frequent tobacco use, accidental injuries, being overweight, hypertension, coronary heart disease, dental cavities, impaired sleep, generally poorer health, and higher medical utilization rates. Not surprisingly, many adults with ADHD experience demoralization and low self-esteem, likely in response to the cumulative effects of these and other impairments, as well as disappointments, rejections, and unfulfilled potential (Hinshaw & Ellison, 2016).

Other impairment and functional outcomes include sleep disturbance (e.g., initial insomnia, frequent night waking, restlessness during sleep), excessive internet use/gaming, earlier and heavier tobacco, alcohol, and cannabis use, and deficits in multiple aspects of performance that rely upon executive functions (e.g., working memory, organization, planning ahead, monitoring and correcting errors, set-shifting, delaying gratification, problem-solving, self-motivation, and time management). The latter difficulties often result in an erratic and disorganized style that can wreak havoc on one’s personal and professional lives (e.g., missing or arriving late to appointments and meetings, incomplete projects, neglected chores, forgotten birthdays, unpaid bills, excessive debt).

Although ADHD raises the risk for impairments across a wide range of functional domains, it is important to note that these adverse outcomes are not inevitable for adults with the disorder. About half of youth with ADHD show substantially improved functioning by mid-adulthood, some to the point of being indistinguishable from their peers (Klein et al., 2012; Weiss & Hechtman, 1993). Moreover, some adults with ADHD enjoy significant success and achievements, often enabled by social support, effective treatments, adaptive routines, and strategies to compensate for their residual symptoms, and the insight to identify pursuits for which their energy, curiosity, and tolerance for risk are advantageous rather than impeding.