Attention deficit hyperactivity disorder (ADHD) is one of the most common mental disorders in childhood [1,2]. ADHD is characterized by the three core symptoms of inattention and/or hyperactivity and impulsivity. In short, these symptoms are present at an abnormal level (in relation to age and developmental stage), occur across situations, and cause significant distress and/or limitations in social, educational, or occupational functioning [3,4]:

• Inattention is defined by forgetfulness, distractibility, and careless mistakes. Moreover, concerned persons commonly seem to have difficulties listening when they are spoken to, finishing what they started, and organizing themselves as expected. They also often avoid or refuse to do their homework and frequently lose their toys and belongings;
• Hyperactivity is characterized as fidgeting and not being able to sit still, respectively, by running and climbing around in inadequate situations. Moreover, hyperactive children may not be able to play in a quiet way or, at least not over an extended time span. Moreover, excessive motor activity often cannot be managed by the social environment and direct orders;
• Impulsivity is regarded as interrupting others or not being able to hold back answers, thus bursting into other people’s conversations. Impulsive children talk excessively and often change the conversation subject. Consequently, these children are very impatient, and they find it difficult to wait their turn.

The prevalence of ADHD is age-dependent and varies with the diagnostic criteria and sources of information used to assess symptomatology. In childhood and adolescence, the prevalence found in international epidemiological studies is around 5.3% worldwide, with no significant differences internationally [5]. For instance, in Germany, the frequency of parent-reported diagnoses of ADHD is about 5%, according to the KiGGS study of the Robert Koch Institute [6]. For adulthood, a prevalence of 2.5% was found in a meta-analysis of six studies based on the DSM-IV criteria [7]. Moreover, Polanczyk et al. [8] could illustrate that the prevalence has not changed over the last three decades on a worldwide basis.

In population samples, the prevalence ratio between boys and girls varies around 3:1, whereas in clinical samples, the prevalence ratio between boys and girls varies around 6:1 [9]. Scahill and Schwab-Stone [10] found, between the ages of 4 to 16 years, a prevalence rate of 9% for boys and 3% for girls. In the same way, girls are less likely to have comorbid aggressive symptoms [9]. In addition, Schlack et al. [6] showed that higher prevalence rates of ADHD are correlated with lower socioeconomic status and that living in big cities is linked with higher prevalence rates of ADHD compared to living in the countryside.

Moreover, prevalence estimates for ADHD types vary considerably across studies, and no consensus has yet been found on which type is more prevalent. Depending on the study, the prevalence rates for the hyperactive and impulsive type range from 0.7% to 2.77%, while the prevalence rates for the inattentive type range from 0.3% to 3.2% [11,12,13,14]. Concerning comorbidities, 30–50% of all ADHD cases meet the criteria for oppositional defiant disorder, 20–30% for learning disabilities, 20% for anxiety disorders, 15% for depressive disorder, and 10–20% for tic disorder [9].

The conditions for the development of ADHD are extremely diverse as multiple interacting factors are involved in the pathogenesis of ADHD. Genetic predispositions and pre-, peri-, and early-postnatal environmental influences that affect structural and functional brain development play a central role [15].

Research on molecular genetics has shown that particularly dopamine and serotonin seem to be involved in the pathogenesis of ADHD. Heightened dopamine transporter concentration and reduced dopamine concentration in the synaptic gap have been found to be associated with ADHD symptoms [16]. Furthermore, findings from both behavioral and molecular genetic studies indicate that genetic as well as non-genetic factors contribute to the development of ADHD. Faraone et al. [17] provide a review of family, twin, and adoption studies and found convincing evidence that genes play an important role in transmitting vulnerability to ADHD. This is most apparent in twin studies, which value the heritability of ADHD at 0.76. Moreover, Fortier et al. [18] present genetic evidence for the implication of the hypothalamic–pituitary–adrenal axis in ADHD, which may serve as an important mediator in the pathogenesis of the disorder. Furthermore, the severity of ADHD was found to be higher in children with parents affected by ADHD than in children whose parents were non-affected [19].

In addition, parental ADHD was linked to a higher likelihood of being diagnosed with the combined ADHD type and a lower chance of being diagnosed with the inattentive type alone [19]. Apart from this finding, there is only scarce evidence on the different backgrounds of the ADHD types [4].

Intriguingly, Tully et al. [20] have found that early births or low birthweight significantly increase the risk for later development of ADHD. They also found that maternal warmth served as a protective factor, counteracting birthweight or birth time as risk factors. Furthermore, infections, toxins, head traumas, and diet need to be further studied in the multifactorial pathogenesis of ADHD [4,9].

Moreover, ADHD is frequently paired with cognitive and executive deficits that are related to diffuse brain abnormalities. The anterior cingulate gyrus and the dorsolateral prefrontal cortex are significantly smaller in ADHD patients. These changes are thought to explain deficits in purposeful behavior. In addition, activity in the frontostriatal region is similarly impaired in these individuals, as shown by fMRI measurements. An understanding of these pathophysiological mechanisms is crucial so that interventions can be targeted to them [21,22].

Additionally, a range of psychosocial factors are commonly discussed in the literature, which have been shown to moderate the intensity of children’s hyperactive and impulsive behaviors. In general, a genetic disposition is considered the main causative variable of ADHD. However, unfavorable conditions in the family and school can lead to an increase in negative interactions in the environment, which, in turn, can amplify symptoms of ADHD. Moreover, the family is the first environment in which children learn rules and boundaries. In families where this process is not successful, the adoption of rules becomes progressively more problematic throughout life and leads to increasingly harsher consequences [4]. In this regard, Fenesy et al. [23] examined childhood ADHD symptoms and negative parenting styles as predictors of prospective changes in social problems over a four-year period. Initially, families of children with and without ADHD were closely examined, encompassing observed positive and negative parenting styles, youth ADHD symptoms, and multiple modal ratings of youth social problems. Along with other research, their findings suggest that psychosocial and educational factors (e.g., authoritarian parenting style, reinforcement through inconsequent parenting, absence of consistent rules, shortage of concrete action plans for parents and children, lack of school interventions, inadequate social skills, poor cognitive stimulations, imbalance of reward and punishment, etc.) are highly implicated in the underlying vicious circle that feeds hyperactive and impulsive symptoms in particular and moderates the severity of the disorder over the developmental period in children [3,4,23,24,25,26,27,28,29,30].

The criteria of the ICD-11 or the DSM-5 classification systems must be fulfilled to diagnose ADHD. Both classification systems specify three types, namely the predominantly inattentive type, the predominantly hyperactive/impulsive type, and the combined type. According to the American Psychiatric Association [1], diagnosis of the inattentive type is made if the sub-points for the hyperactive and impulsive characteristics are not fulfilled; diagnosis of the hyperactive/impulsive type is made if all criteria are met except the sub-points for the inattentive type. Diagnosis of the combined type is given if all diagnostic criteria are met.

The diagnosis also requires the presence of at least moderate functional impairments. The symptoms of inattention or hyperactivity and impulsivity and the resulting functional impairments should occur in several areas of life, usually before the age of 12 [21]. If the symptoms only occur in one area of life (e.g., only at school/work, only in the family), this may be an indication of another mental disorder (e.g., adjustment disorders due to stress in the family or at school) that should be clarified by differential diagnosis [4]. Thus, ADHD diagnostics should always be multidimensional. This means that the affected persons themselves, relatives, or close persons should be interviewed. It also means that a variety of examination methods should be used, such as clinical interviews, questionnaires, behavioral observations, and objective tests [4]. An anamnesis should help to understand the history of the person and to consider critical life events. Differential diagnostics should consider especially oppositional defiant disorder and learning disabilities [9]. In addition, an intelligence test can further clarify the strengths and weaknesses of the person, and intervention methods can be adapted to the present intelligence profile, especially since 50% of ADHD patients have noticeable executive dysfunctions [31,32]. To complete the diagnostic evaluation, medical and neurological examinations are necessary to assess the underlying brain activity and functioning [33]. ADHD may take different manifestations depending on the person’s age, which can reflect differences in brain regulatory systems [34,35,36]. Therefore, an overarching assessment of the child’s functional maturity is needed to correctly diagnose ADHD and define a treatment plan [37].

The prognosis of ADHD varies according to the age of the person concerned. ADHD symptoms are found to persist into adolescence and can affect social and academic areas of life. A total of 40% of patients continue to show symptoms into adolescence, while 25% are diagnosed with an anti-social disorder at the same time, and recent research points to an apparent trend in the increase in the prevalence of ADHD among adults [38,39]. Nevertheless, there is also an important long-term trend in which ADHD patients’ symptoms decrease by about 50% in adulthood. Typically, 50% of patients “outgrow” ADHD, including through treatment, and 25% do not require treatment until they reach adulthood. Commonly, two theories are discussed to explain these developments. First, stimulants help improve frontal lobe development over time, and second, adults with ADHD often choose occupations that do not require sustained attention. As adults, these patients are then able to achieve their educational and occupational goals [40].

ADHD treatment, on the one hand, has been found to be effective in improving the symptoms of oppositional defiant disorder and conduct disorder. Likewise, the risk of substance use has been shown to be considerably reduced. Untreated ADHD, on the other hand, can lead to persistent dysfunction and devastating consequences, including long-term disability, a heightened risk of substance abuse, and an increase in car accidents [21,41,42,43,44]. In this regard, the symptom of inattentiveness, which can hinder the full understanding of the consequences of numerous actions, is hypothesized to be partly responsible for various crimes, such as theft, drug use and sale, robbery, and even burglary [45]. In contrast, individuals suffering from the hyperactive/impulsive type of ADHD tend to disrupt their routines, lose touch with their friends and families, and engage in risky behaviors even if they understand the consequences of their actions better than those suffering purely from inattentiveness. Those with the hyperactive/impulsive type, conversely, commit reckless crimes that lead to arrests and convictions to a higher degree than those with the inattentive type. These crimes relate to theft and especially assault, but not drug trafficking, as the latter, presumably, requires forethought [46]. Thus, there are clear predictors of the differences in the prognosis, which become apparent by evaluating the crimes committed by people suffering from the different types of ADHD alone [46].