Attention Deficit Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder with an onset typically before 12 years of age. The symptoms include difficulties with attention and/or hyperactivity and impulsivity which are incongruent with a person’s age and interfere with activities and participation. Symptoms include:

• inattention, including difficulty sustaining attention on tasks which do not provide significant stimulation or frequent rewards, distractibility or disorganisation
• hyperactivity, including excessive motor activity and difficulties being still, particularly in structured situations that require self-control
• impulsivity, including a tendency to act in response to immediate stimuli, without consideration of the risks and consequences.

A diagnosis of ADHD is suggested when these symptoms occur often and negatively impact functioning in several areas including psychological, social, academic, occupational, and activities of daily living and leisure.

The two main diagnostic systems used internationally and in Australia to diagnose ADHD are the Diagnostic and Statistical Manual of Mental Disorders, currently in its fifth Edition (DSM-5) (American Psychiatric Association, 2013) and the International Classification of Diseases 11th Edition (ICD-11) (World Health Organization, 2018).

Both the DSM-5 and ICD-11 classifications include three presentations (or subtypes) of ADHD with different combinations of symptoms:

• inattentive presentation, allocated when the symptom threshold for inattention is met
• hyperactive-impulsive presentation, allocated when the symptom threshold for hyperactivity-impulsivity is met
• combined presentation, allocated when the symptom thresholds for both the inattentive and hyperactive-impulsive presentation are met.

DSM-5 provides a list of 9 inattentive and 9 hyperactive-impulsive symptoms. For children, 6 of the 9 symptoms must be present to reach the threshold for diagnosis; for people aged over 17 years, only 5 symptoms are required. Adult-specific descriptions of symptoms are provided in the DSM-5 (American Psychiatric Association, 2013). The ICD-11 provides fewer specific requirements regarding symptom thresholds allowing for more flexibility and clinical judgement.

DSM-5 and ICD-11 both require difficulties to have been present for at least 6 months and to have occurred in more than one setting (such as home, school, work, with friends or relatives), with onset before age 12 years, but both note that some individuals may not come to clinical attention until after this age, and often this is not until adulthood or later in adulthood for some.

ADHD is the most common neurodevelopmental disorder in children and adolescents. The prevalence of ADHD in children and adolescents internationally is 5–8% (Polanczyk, De Lima, Horta, Biederman, & Rohde, 2007; Thomas, Sanders, Doust, Beller, & Glasziou, 2015; Willcutt, 2012), and in Australia is between 6% and 10% (Graetz et al., 2001; Lawrence et al., 2015).

There are no Australian adult prevalence studies using current DSM-5 diagnostic criteria, which specify a reduced symptom count of five (rather than 6) symptoms of inattention and hyperactivity/impulsivity for adults. The prevalence of adult ADHD in Australia is likely to be similar to that found internationally, which is between 2% and 6% of the population (Simon, Czobor, Bálint, Mészáros, & Bitter, 2009; Song et al., 2021; Willcutt, 2012). The prevalence of ADHD is higher in boys than girls, with this disparity reducing somewhat in adulthood. The inattentive presentation is the most prevalent.

The best estimates of the prevalence of ADHD in Australia come from the first and second Australian Child and Adolescent Surveys of Mental Health and Wellbeing. The first study conducted in 1998 included 3,597 children aged 6 to 17 years. It reported prevalence figures using DSM-IV of 7.5% overall, 9.4% in those aged 7–12 years, and 6.8% in those aged 12–14 years (Graetz et al., 2001). Subtype analysis found that the inattentive type (3.7%) was more common than the combined (1.9%) and hyperactive-impulsive types (1.9%).

The second survey (Young Minds Matter), conducted in 2013/14 through interviews with 6,300 parents/carers, reported that ADHD was the most comment mental health disorder in Australian children aged 4–17 years (Lawrence et al., 2015). ADHD occurred in 8.2% of children aged 4–11 years (10.9% boys, 5.4% girls), and 6.3% in children aged 12–17 years (9.8% boys, 2.7% girls) (Lawrence et al., 2015). Thus, ADHD prevalence in Australian children and adolescents is estimated to be between 6% and 10%. It is more common in boys than girls, and the inattentive presentation is the most common.

Only one Australian study of adults with ADHD was identified, which explored ADHD prevalence in identical twins using telephone interviews. However, the study did not use clinical diagnoses or clinician assessment of ADHD, drew from a small sample size and used DSM-III and DSM-IV criteria via telephone interviews with researchers (Ebejer et al., 2012).

In most cases ADHD can be considered a multifactorial disorder, where multiple biological and environmental risk factors, cumulatively increase the likelihood of developing the disorder. ADHD is highly heritable. Disruption to dopamine and noradrenaline, particularly lowered synaptic levels, is thought to be a key to the pathophysiology of ADHD (Arnsten & Pliszka, 2011; Levy, 1991; Pliszka, McCracken, & Maas, 1996).

Meta-analysis of brain imaging data has revealed that individuals with ADHD show less activation in regions of the brain that are associated with executive functions such as inhibitory control (Hart, Radua, Nakao, Mataix-Cols, & Rubia, 2013). Several environmental factors contributing risk towards the development of ADHD have emerged. As with genetic risk factors, these environmental exposures are not specific to ADHD, but may contribute to the general risk of developmental pathology across clinical syndromes. In most children with ADHD, no environmental risk factors are identified.

Genetics

ADHD is highly heritable in both children and in adults, with heritability estimated at 70–80% (Faraone et al., 2021; Faraone & Larsson, 2019; Larsson, Chang, D’Onofrio, & Lichtenstein, 2014; Levy, Hay, McStephen, Wood, & Waldman, 1997). It has been considered as both a continuous trait that varies in the general population and as a discrete diagnostic category. A recent genome-wide association meta-analysis identified 12 independent genomic loci that increase susceptibility to ADHD (Demontis et al., 2019).
Notably, significant genetic correlations were observed between ADHD and 43 other phenotypes, including educational outcomes, major depressive disorder, smoking, obesity-related phenotypes and mortality (Demontis et al., 2019). These findings explain the well-recognised clinical phenomenon whereby individuals with a similar genetic risk burden (for example, full biological siblings) may present with different developmental or mental health disorders such as ADHD, intellectual disability, autism spectrum disorder, or mood disorders; a concept in developmental psychopathology known as multifinality. The molecular pathways by which genes confer risk for ADHD and related disorders are not yet known.

Neurotransmitter differences

The clinical effectiveness of psychostimulants in treating ADHD has led to the hypothesis that disruption to dopamine and noradrenaline – particularly lowered synaptic levels – is a key to the pathophysiology of ADHD (Arnsten & Pliszka, 2011; Levy, 1991; Pliszka et al., 1996). For instance, methylphenidate, which is used to treat ADHD, raises extracellular levels of dopamine and noradrenaline (Gamo, Wang, & Arnsten, 2010). Amphetamine (another stimulant treatment) also raises levels of dopamine and noradrenaline, but also interacts with other neurochemicals including acetylcholine, serotonin, opioids and glutamate (Cortese, 2020). The non-stimulant medications atomoxetine also raises levels of both noradrenaline and dopamine in the prefrontal cortex (Gamo et al., 2010), whereas other non-stimulants such as clonidine or guanfacine act more specifically to affect noradrenaline levels (Cortese, 2020).

Support for disruption to monoamine signalling (noradrenaline and dopamine are monoamines) has also arisen from the neurochemistry of animal models of ADHD (Gainetdinov et al., 1999; Giros, Jaber, Jones, Wightman, & Caron, 1996; Rahi & Kumar, 2021; Russell, Allie, & Wiggins, 2000). Although molecular imaging studies focusing on transporter and receptor densities of the dopamine system in individuals with ADHD showed initial promise, subsequent studies have proven equivocal (Fusar-Poli, Rubia, Rossi, Sartori, & Balottin, 2012).

Cognitive differences

Neuropsychological studies show that ADHD is associated with difficulties with executive functions such as working memory, planning, sustained attention and inhibitory control, and maintaining consistent performance over time (Faraone et al., 2021). People with ADHD may also show a preference for smaller immediate rewards over larger delayed rewards and may display impulsive decision making. There is marked heterogeneity among people with ADHD in terms of neuropsychological performance; some people with ADHD may experience few difficulties across these domains whereas others may experience many more (Nigg, Willcutt, Doyle, & Sonuga-Barke, 2005). This neuropsychological heterogeneity likely reflects multiple pathways in the brain that are relevant to the aetiology of ADHD. Neuropsychological difficulties may impact people with ADHD across a broad range of settings, including educational and occupational settings, and may impact their ability to engage with treatment.

Brain Imaging

Large-scale brain imaging consortia, such as ENIGMA (http://enigma.ini.usc.edu), have significantly enhanced our understanding of the structural brain correlates of ADHD. Hoogman et al. (2017) performed a cross-sectional mega-analysis of subcortical structural brain differences between individuals with and without ADHD across ages. They reported smaller volumes of the nucleus accumbens, amygdala, caudate, hippocampus and putamen, and overall intracranial volumes, with effect sizes generally higher in children than adults (Hoogman et al., 2017). A subsequent analysis by the same group examined the structure of cortical areas and found lower surface area values for frontal, cingulate and temporal regions in children but not in adolescents or adults (Hoogman et al., 2019) (see also Faraone et al., 2021 for review). Further, using computational neuroanatomic techniques, Shaw et al. (2007) found a delay in cortical maturation, particularly in the prefrontal regions that play a critical role in the control of cognitive processes such as attention.

Studies of functional brain imaging are typically performed at rest or under cognitive challenge. Meta-analysis has revealed that individuals with ADHD show less activation in regions of the brain that are associated with inhibitory control, such as the inferior frontal cortex, supplementary motor areas and basal ganglia, as well as dorsolateral prefrontal, parietal and cerebellar areas important for attention, compared to those without ADHD (Hart et al., 2013).

In resting-state functional MRI the subject is not required to perform a task, but rather is asked to lie quietly in the MRI scanner, thus permitting ease of scanning across a wide-age range. Typically, investigators are interested in patterns of correlated activity across the brain. Such analyses have identified several distinct networks across the brain. One such network, known as the default mode network, is active during wakeful rest. It has been proposed that individuals with ADHD are less able to suppress default-mode activity that may break through to intrude during task-active scenarios, and may contribute to fluctuating performance and inattention (Kelly, Uddin, Biswal, Castellanos, & Milham, 2008), although recent studies have provided conflicting evidence of this (Cortese, Aoki, Itahashi, Castellanos, & Eickhoff, 2021; Sutcubasi et al., 2020). Although brain imaging offers the potential to reveal novel biological insights, the reliability of findings on ADHD is compromised by heterogeneity within and between studies and the effects of age and medication history.

Environmental risk factors

Several environmental factors that may contribute towards the risk of developing ADHD have emerged. These were recently comprehensively reviewed in the World Federation of ADHD International Consensus Statement (Faraone et al., 2021) and include exposure to toxicants such as lead, phthalate, organophosphate pesticides, long-term maternal use of paracetamol during pregnancy, and prenatal exposure to the anti-epileptic drug valproate. Prenatal exposure to maternal smoking has also been linked to an increased incidence of ADHD, but this effect is significantly diminished when adjusting for family history of ADHD, suggesting a link to an underlying genetic predisposition rather than a pure environmental risk per se (Faraone et al., 2021).

Research has focused on prenatal and birth complication events as potential risk factors for ADHD. Marked preterm birth (gestational age less than 32 weeks) and very low birth weight (birth weight less than 1.5 kg) have emerged as risk factors for ADHD from meta-analyses of large datasets. Maternal obesity, hypertension, preeclampsia, and hypothyroidism during pregnancy have also been associated with an increased risk of ADHD in offspring (Faraone et al., 2021).

A number of large-scale studies and meta-analyses of cohort studies have linked the risk for ADHD to nutrient deficiencies (Faraone et al., 2021). These include lower overall blood levels of ferritin, and omega-3 polyunsaturated fatty acids in individuals with ADHD, compared with non-ADHD controls and the association of lower maternal vitamin D levels with increased risk of ADHD in offspring (Faraone et al., 2021).

There is also a range of situational/environmental factors that can substantially increase the risk for the development of ADHD. These factors include intrauterine exposure to maternal stress (for example, death of a close relative during pregnancy), trauma (for example, sexual abuse), physical neglect (particularly for ADHD inattentive type), and psychosocial adversity (lowered family income, out-of-home care, paternal criminality, or maternal mental disorder) (Faraone et al., 2021). As with genetic risk factors, these environmental exposures are not specific to ADHD. Rather they may contribute to the general risk of developmental pathology across clinical syndromes.

Gene-environment interactions are also important to consider. Relevant parental characteristics such as smoking and parenting style are likely influenced by genetic factors (Rutter, 2005). Furthermore, these risks may be epigenetically transmitted across generations (Nigg, 2018). Cross-disciplinary research integrating genetic, neurobiological, environmental, and social data is needed to further advance our understanding of the aetiological pathways leading to ADHD.

Much of the existing research focuses on average outcomes for individuals with ADHD, with less focus on how outcomes, including positive outcomes, may vary. Little is known about the outcomes associated with adults with ADHD in Australia. Additionally, little is known about outcomes for older adults.

On average, children with ADHD have poorer outcomes across multiple domains compared with children without ADHD. There is substantial literature now demonstrating that ADHD affects numerous areas of functioning for children with ADHD, including social and academic functioning, increased family conflict, peer rejection, conduct difficulties and reduced self-esteem (Faraone et al., 2015).

Many individuals with ADHD will go on to complete school and attend university, but the factors associated with positive outcomes are less well understood (Dvorsky & Langberg, 2016). Factors that may promote positive outcomes in children with ADHD include social acceptance by peers, positive parenting approaches, and positive self-perceptions (Dvorsky & Langberg, 2016).

It is well established that ADHD is a long-term disorder, persisting in most and associated with a broad range of poorer outcomes in late adolescence and adulthood (Cherkasova et al., 2021; Di Lorenzo et al., 2021). A recent systematic review examined the long-term adult outcomes associated with ADHD across seven prospective ADHD cohort studies in the United States (10- to 30-year follow-up, mean age range 22–41 years).

Across these studies, symptoms of ADHD persisted for 60–86% of individuals with ADHD, although there was substantial variation in the percentage who continued to meet the full criteria for ADHD (5.7% to 77%) due to differences in diagnostic classification systems used and variation in informants (Cherkasova et al., 2021). Mental health disorders such as disruptive behaviour disorders, including conduct disorder and oppositional defiant disorder, anti-social personality disorder, and substance misuse were commonly reported outcomes (Cherkasova et al., 2021).

Beyond mental health outcomes, individuals with ADHD have poorer educational and future employment outcomes in adulthood (Cherkasova et al., 2021; Christiansen, Labriola, Kirkeskov, & Lund, 2021). One meta-analysis found that individuals with ADHD were nearly four times more likely not to complete school: odds ratio (OR) 3.7; 95% confidence index (CI) 1.96–6.99) (Erskine et al., 2016). A recent systematic review identified 6 prospective studies (1380 with ADHD, 888 without ADHD) examining employment outcomes and found that individuals with a childhood history of ADHD had poorer employment quality including reduced income, and were more likely to receive public assistance (Christiansen et al., 2021). Individuals with ADHD had reduced educational attainment and lower occupational achievement (Christiansen et al., 2021).

Additionally, individuals with childhood ADHD have been reported to have poorer physical health in adulthood, including increased mortality and reductions in life expectancy, risky driving, including accidents and infringements, obesity, and sleep problems (Cherkasova et al., 2021; Cortese et al., 2016; Diaz-Roman, Mitchell, & Cortese, 2018; Faraone et al., 2015; Li, Xie, Lei, Li, & Lei, 2020; Lugo et al., 2020). Health-related quality of life is also poorer in children with ADHD compared to peers across multiple domains including physical, psychosocial, achievement, and family life (Danckaerts et al., 2010; Faraone et al., 2015; Lee et al., 2016).

There is a growing number of Australian studies documenting the outcomes associated with ADHD with results generally consistent with the systematic reviews and meta-analyses reviewed above. One community-based cohort study tracking children with ADHD (n=179) from age 7 to age 10, found that ADHD was associated with poorer academic functioning, poorer emotional and behavioural functioning, poorer social functioning, and higher rates of co-occurring internalising and externalising mental health disorders, compared to children without ADHD (n=212) (Efron et al., 2020; Zendarski et al., 2022).

This study found that best predictors of outcomes at age 10 were age 7 measures of working memory (academic functioning), severity of ADHD symptoms (parent- and teacher-reported emotional and behavioural functioning) and autism symptom severity (parent-reported emotional functioning and parent-reported social functioning) (Efron et al., 2020).

Another prospective cohort study conducted in Victoria, which examined outcomes for adolescents with ADHD (n=130) in the early years of high school, found they had poorer academic performance across multiple domains, poorer school engagement and increased school suspensions compared with state averages (Zendarski, Sciberras, Mensah, & Hiscock, 2017a, 2017b). Depression, lower adolescent supervision and devaluing education were associated with poorer school attitudes (Zendarski et al., 2017b).

Higher cognitive ability, higher neighbourhood socio-economic status and attending an independent school was associated with lower risk of school suspension, while higher levels of conduct and ADHD symptoms were associated with increased risk of suspension. Increased inattention symptoms, bullying, lower adolescent supervision, male sex, and lower school neighbourhood socio-economic status were associated with poorer performance on one or more academic domains (Zendarski et al., 2017a).

A large population-based data linkage study conducted in Western Australia found adverse effects of ADHD on academic performance, with 23% of boys and 28% of girls with ADHD having numeracy scores below benchmarks in the third year of school (11% for boys and girls without ADHD) (Silva et al., 2020). Linked hospital data showed that children with ADHD also had increased risk of early hospitalisations before the age of 4 (Silva, Colvin, Hagemann, Stanley, & Bower, 2014).

There was also an increased odds of having a community-correction (OR = 2.48, 95% CI 2.22-2.76) or an incarceration record (OR = 2.63, 95% CI 2.01-3.44) compared to boys without ADHD (Silva, Colvin, Glauert, & Bower, 2014). Odds of having a community-correction (OR = 2.86, 95% CI 2.03-4.03) or incarceration record (OR = 7.27, 95% CI 2.29-23.08) were even higher for girls with ADHD compared to girls without ADHD. The most common reason for the first justice record was for the offences of burglary and breaking and entering (Silva, Colvin, Glauert, et al., 2014).

There is high prevalence of co-occurring conditions in individuals with ADHD. These conditions may result in higher rates of daily difficulties and can require treatment. The prevalence of co-occurring conditions in ADHD changes over age and with development. In children and adolescents with ADHD, around two-thirds will have a co-occurring mental health condition (Gnanavel, Sharma, Kaushal, & Hussain, 2019; Reale et al., 2017).

This includes other neurodevelopmental disorders: specific learning disorders, intellectual disability, language disorders, tic disorders, autism spectrum disorder, developmental coordination disorder; disruptive, impulse-control and conduct disorders: oppositional defiant disorder, conduct disorder, intermittent explosive disorder; anxiety disorders; depressive disorders: disruptive mood dysregulation disorder, major depressive disorder, persistent depressive disorder; and substance use disorders in adolescence (American Psychiatric Association, 2013; Gnanavel et al., 2019; Reale et al., 2017).

The most common co-occurring conditions in childhood are specific learning disorders, oppositional defiant disorder, language disorders, autism spectrum disorders and anxiety disorders, with depressive disorders and substance use disorders emerging in adolescence.

Adults with ADHD also have a high prevalence of co-occurring disorders, with up to 80% having at least one additional mental health disorder (Katzman, Bilkey, Chokka, Fallu, & Klassen, 2017; Kessler et al., 2006). The highest rates of co-occurring mental health disorders in adults with ADHD are depressive disorders, bipolar disorders, anxiety disorders and substance use disorders (Kessler et al., 2006).

In addition to those conditions outlined for children and adolescents, adults with ADHD may also experience higher prevalence than the general population of substance use disorders; bipolar disorders; obsessive-compulsive disorder; cluster B and C personality disorders (American Psychiatric Association, 2013; Canadian ADHD Resource Alliance (CADDRA), 2018; Gnanavel et al., 2019; Katzman et al., 2017; Kessler et al., 2006; Reale et al., 2017; Schiweck et al., 2021).

ADHD is a disorder that occurs across the lifespan, although it can present in different ways and in combination with different disorders at different ages. Little is known about the presentation of ADHD in older age. The symptoms of ADHD are present before the age of 12 years, but a diagnosis may not occur until later when functional impact may become more obvious as demands for independence increase.

Young children

It is developmentally appropriate for pre-schoolers to be active, impulsive and unable to sit still and concentrate for long periods of time, and therefore educational settings for pre-schoolers vary substantially from school for older children (Halperin & Marks, 2019; Wigal et al., 2020). This can make identifying symptoms of ADHD that exceed what is developmentally appropriate for this age group quite a challenge (Halperin & Marks, 2019).

Pre-schoolers who do have ADHD can exhibit a very high level of overactivity, impulsivity and/or attention difficulties that can cause significant impairment in daily life. Hyperactivity and impulsivity symptoms are the most evident symptoms of ADHD in pre-schoolers (Franke et al., 2018; Halperin & Marks, 2019; Willcutt, 2012), and the DSM-5 items assessing hyperactivity/impulsivity clearly distinguish between pre-schoolers with and without ADHD (Halperin & Marks, 2019).
Co-occurring disorders are common in pre-schoolers with ADHD, with up to 70% meeting criteria for one or more co-occurring disorders (Wigal et al., 2020), most commonly oppositional defiant disorder, communication disorders and anxiety (Wigal et al., 2020). ADHD in pre-schoolers tends to persist into childhood and adolescence (Halperin & Marks, 2019; Wigal et al., 2020).

Children and adolescents

The ADHD inattentive type is the most common presentation of ADHD, although ADHD combined type is more likely to present to clinical services (Willcutt, 2012). This is because in primary school-aged children, hyperactivity and impulsivity symptoms are usually the most overt symptoms of ADHD; inattention symptoms become more evident as children progress through school (Franke et al., 2018; Willcutt, 2012) and academic and cognitive demands increase.

Commonly observed impairments in the school environment include academic underachievement and peer relationship difficulties (American Psychiatric Association, 2013). Children and adolescents tend to also have more strained relationships with parents and siblings (Young et al., 2020). The nature of impairments associated with ADHD vary somewhat based on developmental age. For example, common difficulties in peer relationships experienced by younger children with ADHD may include peer rejection and having fewer friends. As social relationships become more complex in adolescence, these difficulties may increase and be associated with increases in loneliness and use of maladaptive coping strategies (Young et al., 2020). As adolescents with ADHD transition into adulthood, risk taking may increase (including earlier sexual activity, risky driving, early pregnancy, delinquency, criminality and substance misuse) (Franke et al., 2018; Young et al., 2020).

Co-occurring disorders during childhood are common, and can include disruptive behaviour disorders, anxiety and mood disorders, learning and language disorders, intellectual disabilities, sleep difficulties and tics (Faraone et al., 2015; Franke et al., 2018). Emotion regulation difficulties (Shaw, Stringaris, Nigg, & Leibenluft, 2014) affect up to 40-50% of children with ADHD (Faraone et al., 2019), and autism spectrum disorder co-occurs with ADHD in 20–50% of cases (Franke et al., 2018).

Adults

DSM-5 (American Psychiatric Association, 2013) includes examples alongside each ADHD symptom to enable better identification of symptoms in adults (see Table 12 for examples).

Table 12. DSM-5 ADHD symptoms with examples for adolescents/adults

The rate of persistence of ADHD into adulthood varies across studies. A review of 7 North American controlled prospective follow-up studies found high rates of symptomatic persistence (60–86%) (Cherkasova et al., 2021). Some suggest that adults are more likely to continue to present with inattention symptoms relative to overt symptoms of hyperactivity (Franke et al., 2018). However, it can be difficult to distinguish between the presence or absence of symptoms given the different strategies and coping mechanisms that adults may have acquired to manage or mask their symptoms.

ADHD may be easier to identify in women during adulthood, where women may become aware of their symptoms and self-refer for assessment (Franke et al., 2018; Young et al., 2020). Furthermore, an exacerbation of ADHD symptoms and impairments may be seen during transition periods, such as transitioning to living away from the family and commencing university/employment (Young et al., 2020).

Inattention symptoms in adulthood may be noticed when individuals appear distractible, slower to present and formulate ideas, or have difficulty following conversations (Franke et al., 2018; Kooij, Bijlenga, et al., 2019). Some adults with ADHD may experience ‘hyperfocus’ and focus on specific activities for many hours when it is of high interest (Kooij, Bijlenga, et al., 2019).

Mind wandering and mental restlessness may also be present (Kooij, Bijlenga, et al., 2019). Inattentive symptoms can be problematic in the work context if they cause organisational difficulties, or problems prioritising and starting work, and shifting between tasks (Kooij, Bijlenga, et al., 2019). There are many differences in the expression of hyperactivity between childhood and adulthood, many of which are less overt in adulthood (Franke et al., 2018). Adults can present with more subtle hyperactivity, such as feeling restless and not being able to relax (Kooij, Bijlenga, et al., 2019). Impulsivity in adults can manifest in excessive spending, binge eating, interpersonal conflict, risk taking, addictions, and talking excessively or interrupting others (Kooij, Bijlenga, et al., 2019).

Adults with ADHD are equally likely to meet criteria for one or more co-occurring disorder and experience significant impairments in daily life including occupational and relationship functioning, such as difficulties in romantic relationships (Faraone et al., 2015; Franke et al., 2018). Emotional regulation difficulties are also common (Beheshti, Chavanon, & Christiansen, 2020; Franke et al., 2018; Shaw et al., 2014; Young et al., 2020).

The concept of ‘adult-onset ADHD’ has sometimes been cited in clinical literature, referring to adults whose ADHD symptoms commenced in adulthood (Franke et al., 2018; Taylor, Kaplan-Kahn, Lighthall, & Antshel, 2021). A recent systematic review suggested three reasons for the perceived onset of ADHD in adulthood: 1) symptoms not previously being sufficiently elevated or impairing due to lower environmental demands, the presence of supports in the environment or other protective factors such as high IQ; 2) failure to identify ADHD in the presence of other conditions or falsely considering ADHD when another condition is a better explanation of the symptoms; and 3) that ADHD symptoms may actually have been present in childhood but were not identified (Taylor et al., 2021).
Older adults

Very little research has examined the presentation of ADHD in older adults (Franke et al., 2018). One study of 296 adults with ADHD (mean age 69.55 years) reported that the negative impairments associated with ADHD across family, social, financial and organisational difficulties were stable over time, based on individuals’ retrospective reports (Philipp-Wiegmann, Retz-Junginger, Retz, & Roesler, 2016).