Attention-deficit/hyperactivity disorder (ADHD) affects an estimated 5 % of school-aged children [1]. ADHD is conceptualized as a neurodevelopmental disorder, characterized by symptoms falling into three categories: inattention, hyperactivity, and impulsivity, according to the Diagnostic and Statistical Manual of Mental Disorders [2]. Research on the etiology of ADHD indicates that ADHD likely results from some combination of genetic, neurobiological, psychological, and contextual risk factors [3–6]. In addition, ADHD is associated with negative correlates and outcomes among children, such as academic difficulties, peer relationship problems, and family conflict [7–11]. Children with ADHD also require specialized educational resources, and an estimated 20–40 % of children with ADHD also present with learning disabilities [12–14]. Youth with ADHD perform less well on standardized achievement tests [12, 15, 16] and have more physical injuries and automobile accidents [17, 18] than youth without ADHD. Additionally, adults with ADHD as children are more likely to exhibit antisocial behaviors, substance abuse, and difficulties in their occupational functioning [16, 19–22]. Thus, individuals with ADHD experience negative correlates and are at risk for a variety of impairing and negative sequelae.

Research also has demonstrated high rates of comorbidity or co-occurring conditions with ADHD, including mood, anxiety, oppositional defiant, and conduct disorders, with up to 44 % of children with ADHD having at least one other disorder and 43 % having two or more additional disorders in community-based samples [13, 23–26]. Although much research has focused on comorbidity of ADHD with oppositional defiant disorder (ODD) and conduct disorder (CD) [2, 13, 27–29], ADHD also is associated with internalizing conditions, such as anxiety (e.g., social anxiety, generalized anxiety) and mood (e.g., major depressive disorder, bipolar disorder) disorders [26, 30–34]. Given the heterogeneity in symptom presentation, etiology, impairment, and course among youth with ADHD, there are likely different developmental pathways linked to comorbid disorders based on child-specific and contextual factors [35]. However, less is known about (a) which children with ADHD are more likely to develop comorbid conditions, and (b) which comorbid conditions youth may be likely to develop. Further, little research has investigated mechanisms of developing ADHD and comorbidities. The present review first describes common comorbidities of ADHD. Then we examine emotion regulation deficits, a correlate of ADHD and comorbid disorders, which may confer risk for the development of comorbid conditions among youth with ADHD. The interplay between emotion regulation and parenting behaviors is explored among youth with ADHD. Finally, we present a conceptual model that describes developmental relations between ADHD and commonly co-occurring conditions.

ADHD and Common Comorbidities

Children who have comorbid conditions often exhibit more severe symptoms, a more enduring course of the disorders, and more negative correlates and sequelae (e.g., substance use, delinquency, antisocial personality disorder, academic and occupational problems, and interpersonal relationship problems) than children who do not have comorbid conditions [13, 36–38]. Thus, given the potential negative outcomes associated with ADHD, understanding the nature and development of comorbid conditions among youth with ADHD may be especially important [39, 40].

A developmental psychopathology framework is especially relevant for conceptualizing the development of comorbid conditions. First, this framework allows the examination of multiple levels of analysis and multiple domains of risk and resilience [41–43]. We propose child-specific and contextual factors that promote risk or resilience for the development of comorbid conditions among youth with ADHD [39]. Specifically, emotion regulation skills and parenting style are two potential inter-related candidate processes that may lead children with ADHD to develop comorbid conditions [8, 44–46]. Second, in terms of developmental pathways, we consider both equifinality, which refers to the idea that diverse pathways may lead to the same outcome, and multifinality, which indicates that one pathway may result in multiple outcomes depending on other relevant factors (e.g., contextual influences) [47]. Although ADHD is comorbid with many other psychiatric conditions [13, 23, 24], consistent with multifinality, there is less known regarding how these various pathways to comorbid conditions develop in the context of ADHD and which children are more susceptible to comorbidities. Further, some children with ADHD evidence desistance of symptoms over time, although the factors that contribute to this desistance are also less understood [48]. Indeed, ADHD may manifest differently among children over time, as suggested by the concepts of heterotypic and homotypic continuity [23, 48–51]. Whereas homotypic continuity refers to stability in symptoms or presentation of a disorder, heterotypic continuity refers to changes in behaviors or symptoms over time and/or over the developmental course [50, 51].

Researchers have provided various models and explanations that help to conceptualize comorbidity among children [23, 52–54]. The present review will focus on three potential explanations as it is likely that multiple explanations account for comorbidity with ADHD. First, ADHD may confer risk for additional disorders given that ADHD typically develops early in childhood. Second, shared risk factors among conditions may account for the co-occurrence of ADHD and comorbidities. A third explanation, which has not been examined systematically but warrants further attention, considers the idea that correlates or sequelae of ADHD confer risk for additional conditions [39].

Children with ADHD are approximately 11 times more likely to develop comorbid disruptive behavior disorders than children without ADHD [23] and ADHD co-occurs with ODD and CD in 30–50 % of cases in epidemiologic samples [23, 26, 55]. Children with comorbid ADHD and ODD or CD demonstrate more severe and frequent symptoms and overall impairment, as well as more difficult temperament; increased aggressive, inattentive, and destructive behaviors; and problematic parent–child relations compared to children with ADHD alone [8, 56]. Further, children with ADHD who exhibit symptoms of ODD and aggressiveness between the ages of 7 and 10 years old are more likely to develop serious CD, criminal offending, and substance abuse [57–61]. ODD typically develops by the age of 8, whereas ADHD is typically evident by age 12 according to DSM-5, though symptoms may develop before age 12. Thus, the age of onset of ADHD and ODD may be similar. Although symptoms of ADHD may be present before ODD, consistent with the explanation that ADHD confers risk for ODD, the similar timing of their onset suggests that shared risk factors may precede and contribute to the co-occurrence of ADHD and ODD among children who experience this comorbid condition during childhood [62]. CD typically has a later age of onset (i.e., after age 10 and during adolescence, though childhood-onset CD requires onset of at least one symptom before age 10). Although ADHD and CD may share certain temperamental features, the later onset of CD suggests that correlates of ADHD may predispose to or confer risk for CD. Indeed, some research suggests that ADHD that persists into adolescence and aggression among children with ADHD are associated with greater risk of developing comorbid CD [60, 63, 64]. Thus, in terms of developmental timing, CD typically onsets after ADHD, suggesting that ADHD may predispose children to CD or that the two disorders may share certain risk factors in addition to these negative correlates [62, 65].

Though research consistently has shown high associations between ADHD and ODD/CD (e.g., [23, 26]), internalizing disorders also commonly co-occur with ADHD (for reviews, see [28, 66–68]). It has been estimated that up to one-third of children with ADHD have comorbid anxiety disorders [30, 56, 69], with median odds-ratio estimates of 3.0 (95 % CI 2.1–4.3) [23]. Results from the NIMH Collaborative Multisite Multimodal Treatment Study of Children with ADHD (MTA) demonstrated that children with ADHD and comorbid anxiety disorders responded worse to treatment and had lower scores on measures of academic achievement before treatment than children without comorbid anxiety [70]. The onset of anxiety disorders varies based on disorder, with separation anxiety disorder typically developing by the age of 7.5 years and occurring among younger children, generalized anxiety disorder occurring between ages 10 and 14, social anxiety disorder developing around age 11, and specific phobias developing across childhood and adolescence [2]. Given these different ages of onset, multiple explanations might account for the co-occurrence of ADHD and anxiety disorders: ADHD might predict later developing anxiety disorders (e.g., social anxiety or generalized anxiety disorders), or symptoms or correlates of ADHD may confer risk for anxiety disorders (e.g., inattention, academic or interpersonal difficulties may contribute to anxiety among youth with ADHD) [39, 66].

ADHD also commonly co-occurs with mood disorders [23, 26, 33, 56]. Based on a meta-analysis of 21 epidemiological studies, the median odds ratio of co-occurring ADHD and depression is 5.5 (95 % CI 3.5–8.4) [23]. Children with ADHD have higher rates of continuous depressive symptoms (as opposed to isolated major depressive episodes) [71–73] and are more likely to develop depressive disorders later in childhood and adolescence [62, 74, 75] than children without ADHD. Although there may be familial risk factors that predispose children to both ADHD and depression [76], there is little research regarding mediators of the link between depression and ADHD [77]. Some research suggests that shared risk factors (e.g., emotion regulation difficulties) contribute to the co-occurrence of ADHD and depression [75]. However, depression tends to have a later onset than ADHD, with prevalence rates increasing with age [77, 78]. Because ADHD typically precedes the developmental onset of major depressive disorder among children and adolescents when these disorders co-occur, there also may be correlates of ADHD that contribute to the development of depression. For example, children with ADHD often experience problems with academic functioning, social relationships, and parental interactions, which may contribute to increased depressive symptoms and episodes [39, 77, 79, 80]. Thus, similar to the other comorbid conditions considered thus far, negative correlates of ADHD may be risk factors or mechanisms contributing to the development of depression. Although a variety of risk factors may contribute to the development of comorbidity among youth with ADHD, we consider the role of child emotion regulation and its interactions with parenting behaviors in the development of comorbid conditions among youth with ADHD. Before considering these processes in the context of ADHD and comorbid conditions, however, it is necessary to examine the construct of emotion regulation and the interplay between emotion regulation skills and parenting in typical and atypical development.

Emotion Regulation and Temperament

Although children with emotion regulation deficits are at risk for externalizing and internalizing disorders (for a review, see [81]), differences in the operationalization of emotion regulation in the literature have led to difficulties researching this construct (for a review, see [82]). Research has focused on various aspects of emotion regulation, such as changes (a) in the activated emotion and (b) that result from the activated emotion [83]. Researchers generally agree that emotion regulation concerns internal processes related to manipulation of physiological, subjective, and/or behavioral components of emotional responding [84]. Much of the debate centers on the idea of emotion regulation as a voluntary or involuntary process and whether and to what extent it is internal or affected by external stimuli. Additionally, investigators have yet to reach agreement on whether to focus on the processes involved in emotion regulation rather than the amount of emotion expressed [81]. To frame the discussion of emotion regulation throughout this paper, we adopt the following definition of emotion regulation [81]:

The process of initiating, avoiding, inhibiting, maintaining, or modulating the occurrence, form, intensity, or duration of internal feeling states, emotion-related physiological, attentional processes, motivational states, and/or the behavioral concomitants of emotion in the service of accomplishing affect-related biological or social adaptation or achieving individual goals. (p. 338)

Temperament as an Index of Emotion Regulation

In addition to the challenges in defining and conceptualizing emotion regulation, researchers do not agree on the most useful method to measure this construct. Various tasks and procedures have been used to tap emotion regulation, including questionnaires, observations, and laboratory tasks [83]. Temperament, which may be an especially useful index of emotion regulation, similarly has been defined in various ways. The conceptualization we will use is “biologically rooted individual differences in behavior tendencies that are present early in life and are relatively stable across various kinds of situations and over the course of time” (p. 1101) [85]. Temperament provides the foundation upon which emotion development and emotion regulation are built and interacts with contextual factors to predict a range of youth outcomes [86].

One aspect of temperament that may be especially relevant in terms of emotion regulation is effortful control [84]. Effortful control is the “voluntary” branch of temperament and emotion regulation and refers to the ability to inhibit dominant responses and/or activate a subdominant response by voluntarily modifying one’s own attention and behavior [87, 88]. Inhibitory control (a facet of effortful control) is how well a child is able to control or suppress impulses and inappropriate approach responses when the context demands it, such as in a new situation or if an adult has given the child instructions to inhibit responses. Inhibitory control is correlated with not just behavioral regulation, but also emotional regulation [81]. Effortful control and inhibitory control are especially pertinent with regard to children with ADHD, who exhibit impulsivity as one of the core deficits of ADHD [89–92].

Eisenberg and Fabes’s [93] model describes how children’s temperaments may be characterized as under-controlled, highly inhibited, or optimally regulated. Optimally regulated children typically display greater flexibility, use effective emotion coping, and are more socially successful. Conversely, under-controlled children tend to be low in emotion regulation, reactively aggressive, and impulsive. Highly inhibited children may be withdrawn, sad, anxious, and lacking flexibility [93, 94]. Thus, child-specific temperamental features, such as effortful control, contribute to the development of emotion regulation. Further, research has demonstrated that certain difficult temperamental styles are associated with psychopathological symptoms [95], and that this link is not merely tautological [96]. The range of temperamental profiles suggests that examination of emotion regulation among children with ADHD is important to illustrate how their profiles may act as shared risk factors for other disorders.

Emotion Regulation Deficits Among Children with ADHD

Many children with ADHD exhibit emotion regulation deficits [13, 89, 90, 92, 97, 98], though there is much debate regarding the exact nature of these deficits [98]. Among youth with ADHD, 38 % of a community-based population sample and 24–50 % of a clinic-based sample exhibited emotion regulation deficits ([99], see [100] for a review, [101]). In terms of temperament, children with ADHD generally are highly reactive and exhibit low effortful control and high negative emotionality; however, they also exhibit extraversion and the capacity for overly reactive positive emotionality [102, 103]. Additional emotion regulation deficits may include low inhibitory control, low attentional control, and high activity level [104].

Barkley’s [105] model posits that emotion dysregulation in ADHD is due to an executive inhibition deficit, which includes difficulties with temperamental regulation and effortful control processes and may involve anterior fronto-striatal attentional networks, resulting in extreme positive approach [106, 107]. Subsequent reviews and research support and extend this model and indicate that youth with ADHD exhibit executive dysfunctions, such as abnormalities in reward circuitry, temporal processing, problem solving, and working memory, as well as delay aversion [108–111]. A review by Shaw et al. [100] describes abnormalities in the amygdala, ventral striatum, and orbitofrontal cortex contributing to emotion regulation deficits in ADHD. One issue that has not been resolved is whether ADHD with and without emotion dysregulation represent distinct conditions in terms of associated neurocognitive deficits [100]. These temperamental profiles and emotion regulation deficits among children with ADHD may play a contributing role in the development of comorbid psychopathology [104], a point to which we return later. However, consistent with a developmental psychopathology framework, contextual factors, such as parenting behaviors, likely interact with and impact a child’s emotion regulation and functioning across developmental periods and thus are considered next.

Parental Influences on Emotion Regulation and ADHD

Parenting factors may have important consequences for a child’s emotion regulation competencies, exacerbate children’s ADHD symptoms, and/or confer risk for comorbid conditions. Emotion socialization, which typically begins at an early age, is the manner in which parents teach, coach, and model appropriate emotional responses and is related to a child’s emotion regulation skill development [112]. Though many parenting dimensions play a role in child outcomes, parental coping with a child’s negative emotions is a particularly important facet of emotion socialization, and therefore emotion regulation. Consistent with Buck’s [113] model, parental use of non-supportive strategies intended to control children’s negative emotions may coerce children to suppress future negative emotions. By suppressing these emotions, children’s negative emotional arousal and anxiety may increase, essentially held internally until a similar situation elicits negative emotions, and initiating a cycle of increasingly intense and dysregulated negative emotions [81, 113]. Furthermore, lack of adaptive and appropriate emotional expressive responses may increase the likelihood of using other less adaptive behavioral responses [114]. Therefore, parents’ abilities to model effective emotion regulation skills and to teach children about negative emotions are compromised if parents display non-supportive reactions to children’s negative emotions.

Although parents sometimes may use non-supportive responses to their children’s emotions, children still may demonstrate appropriate and adaptive emotion regulation. Alternatively, even if parents do provide supportive responses to their children’s negative emotions, children may have problems with emotion regulation or develop psychological disorders. Thus, parenting may act as a risk or resilience factor as it interacts with child emotion regulation in predicting co-occurring conditions among children with ADHD.

Beyond their impact on emotion regulation, parenting behaviors are associated with comorbid conditions among children with ADHD. Johnston and Mash [10] provided support for a transactional model of ADHD and family functioning, such that children with ADHD may influence parenting behaviors, while family and parenting factors simultaneously influence the child’s behavior. However, there are inconsistencies in the literature regarding parenting and family factors among youth with ADHD. Some studies report more stressful, conflicted family environments; poorer parenting practices; and fewer authoritative parenting beliefs among families of children with ADHD compared to families of children without ADHD [115, 116]. Additionally, the Multimodal Treatment Study for ADHD showed that negative/ineffective discipline mediates the success of ADHD treatment [117]. However, other studies suggest family problems are associated with comorbid ODD or CD, rather than specific to ADHD alone [57, 118]. Indeed, negative parenting during the school-aged period contributes to the development of oppositional and conduct problems among children with ADHD symptoms [8, 10, 119, 120], and child disruptive behaviors likely influence parenting more than ADHD symptoms alone [121]. These findings suggest that temperamental features of ODD/CD may contribute to coercive family interactional patterns and that comorbid behavior problems may add difficulties for parenting above and beyond those associated with ADHD alone.

Nevertheless, social processes (e.g., parental consistency, family routine) may attenuate the presentation of ADHD symptoms [122]. In addition, positive parenting may act as a protective factor against the development of conduct problems among children with ADHD [123]. Thus, children with ADHD may be especially susceptible to harsh or non-supportive parenting styles, leading to the development of comorbid conditions, and the type of comorbidity that develops in addition to ADHD may be especially influenced by contextual factors [5, 10, 120, 124]. However, it is unclear based on the extant literature which children with ADHD develop which specific comorbid conditions, indicating that other factors, such as child emotion regulation abilities, are important to consider in explanatory models of comorbidity. Before turning to such models, we present a review of emotion regulation and interactions with parenting behaviors over the course of development to illustrate how these factors may be associated with ADHD and comorbid conditions.

Emotion Regulation During the Course of Typical and Atypical Development

Consistent with a developmental psychopathology perspective, child temperament moderates parental behaviors in the prediction of a variety of outcomes and core aspects of temperament may manifest in different behaviors over time, evidencing heterotypic continuity [125–127]. Given these potential differences related to temperamental features and contextual influences over time, it is important to consider emotion regulation across developmental periods. Such knowledge of typical and atypical development can facilitate understanding of a range of multifinality and equifinality in outcomes, highlight potential pathways to risk or resilience, illustrate how emotion regulation and parenting may serve as shared risk factors for or correlates of co-occurring psychological disorders among youth with ADHD, and inform prevention and intervention efforts that may be indicated for different developmental periods.

Infancy Through 3 Years Old

Infant emotion regulation may develop as a result of several factors. At this stage, inborn, temperamental factors are especially important and these biological or neuroregulatory mechanisms may predispose an infant to behave in a certain manner [128]. For example, genetics have been implicated in influencing certain aspects of temperament, such as reactivity or effortful control [129]. These aspects of temperament are conceptualized as more inborn, stable, and generalizable to a range of situations. At this point in development, social interactions with caregivers, such as the social smile, begin to train infants to regulate arousal [130, 131]. Though infants primarily rely on caregivers for soothing and to help them to regulate their emotions, they gradually learn to self-soothe and calm themselves during infancy and as toddlers [81]. Infants also demonstrate regulatory strategies, such as gaze aversion, which help them control anger and frustration, though they may be less successful at regulating and decreasing fear responses [83]. Thus, at this age, infants learn that they can change activated negative emotions with regulatory strategies.

Aspects of effortful control, such as inhibitory control, come online around 24–36 months, permitting toddlers to evidence certain emerging regulatory capacities [81]. Thus, impulsive or dysregulated behavior, crying, and temper tantrums are common among children of this age group and can be considered developmentally normative because the child is still learning to regulate basic arousal and control anger. Toddlers also demonstrate these behaviors as an effort to gain autonomy, test limits, and practice social skills [132]. As children gain an understanding of their social environment, parents’ strategies begin to shape emotion regulation capacities through emotion socialization (i.e., modeling of emotions by the parent). In addition, caregivers’ supportive and sensitive responses to toddlers’ positive and negative emotions may shape toddlers’ regulatory abilities, rather than the parents’ simply regulating their children’s emotions for them as in the infancy period [112, 133]. Thus, parental supportive responding and socialization of emotions may facilitate children’s emotion regulation capabilities.

During the early years of a child’s life, parent–child relations are especially critical for cultivating emotion regulation given that an infant with a difficult temperament may be especially challenging for parents. Consistent with temperamental features among children with ADHD, high levels of negative emotionality, such as continual crying, inability to self-soothe, or intolerance of frustration during infancy and the toddler years, may be markers of emotion dysregulation. These children may be at risk for conduct problems and aggression in later childhood and adolescence [35]. In addition, irritability, temperamental emotionality, and high activity level in the first few years of life are associated with comorbid ADHD and internalizing or externalizing disorders [101, 134–136]. These child behaviors may elicit harsh, inconsistent parenting, which can further exacerbate parent–child conflict and may in turn provoke further child noncompliance, aggression, and emotion dysregulation. Thus, deficits in emotion regulation during the first few years of life, given that emotion regulation is critical for forming basic regulatory abilities, might put the child on a trajectory for further dysregulation, exacerbation of ADHD symptoms, and the development of comorbid conditions, especially in the context of other risk factors such as harsh parenting.

Preschool and Early Childhood

As children move into the preschool and early childhood period, they demonstrate more sophisticated self-initiated emotion regulation because of advances in cognitive, motor, and language development [130]. At these ages, children improve on measures of effortful control as executive functioning skills also improve [137]. Thus, executive control of attention, action, and emotion develop simultaneously during the preschool and early childhood periods. Emotion regulation might be optimal when inhibition level is intermediate, suggesting that children who are over- or under-inhibited may be at risk for poorer emotion regulation [94] and the ability to regulate emotions enables the child to respond to life experiences in a socially acceptable and flexible manner [35]. Additionally, as children become more fluent in language and display more executive attention abilities during the preschool years, self-regulation continues to develop [138]. Among children with ADHD, different temperamental features might lead to distinct patterns of comorbidity however. For example, temperamental emotionality at age 3 predicts comorbid ADHD and internalizing disorders at age 7, whereas activity level at age 3 predicts comorbid ADHD and ODD at age 7 [136].

Children who continue to exhibit challenging temperamental styles and dysregulated emotions may elicit more negative, unresponsive, and/or punitive reactions from parents during this age period especially [139]. However, parental influences continue to interact with the child’s developing emotion regulation abilities. Thus, although children typically evidence a range of negative emotional responses to situations, such as feeling sad, angry, upset, or otherwise distressed, parents may provide non-supportive (e.g., harsh, insensitive) responses to these emotions, contributing to poor emotion regulation [140] and increases in aggression, impulsivity, and negative affect. These outcomes may be particularly likely among children with high levels of approach tendencies, high negative emotionality, and lower effortful control, consistent with the temperamental profiles of youth with ADHD [35, 103, 125, 141–144]. Thus, parenting strategies may operate as risk or resilience factors in the development of comorbid disorders among children with ADHD, depending on a child’s temperament and emotion regulation abilities.

Later Childhood and Adolescence

The majority of research on emotion regulation focuses on infancy, preschool, and early childhood as crucial periods of development [145]. However, emotion regulation skills may continue to develop throughout childhood and adolescence, developmental periods that are critical for the acquisition of cognitive, social, and emotional skills [145]. Typical and healthy development of emotion regulation during later childhood and adolescence includes incorporating emotion regulation skills into one’s emotion-related framework [81, 146]. Children begin to consider other people’s perspectives in relation to their own or others’ displays of emotions. Emotion regulation during these ages presents as a more multi-dimensional process than during earlier ages, as youth begin to learn more adaptive coping strategies for negative emotions such as cognitive reappraisal, positive refocusing, seeking help (e.g., from peers, parents, or other important relationship figures), and behavior diverting [147]. Normatively, as children enter late childhood and early adolescence, they tend to use suppression less frequently as they develop enhanced emotion regulation skills [147]. Children learn to internalize the explicit and implicit evaluations of their emotions by significant others. When parents validate and support children’s emotional reactions, children evince greater emotion regulation and social competence [145], which are crucial for this age period, particularly as peer relationships play an increasingly important role [148].

Children with inhibited temperaments, including higher fear response or attention to negative or threatening stimuli, may be predisposed to internalizing symptoms during these developmental periods; moreover, this effect may be pronounced among children with lower attentional control because these children have difficulty attending to more positive or neutral stimuli [149]. These youth also may exhibit dysregulation in the form of withdrawal from emotionally evocative stimuli, further exacerbating internalizing symptoms. Among children who are predisposed to depression, low effortful control and self-regulation might contribute to depressive symptoms as a result of difficulty with approach motivation [150, 151]. Thus, children’s temperamental features that are consistent with the ADHD profile (e.g., difficulties with self-control, emotion regulation) in early childhood may predict internalizing and externalizing behaviors in later childhood [125, 141].

We may see some of the effects of earlier temperament × environment (specifically, parenting strategy) interactions during the late childhood period as well. For example, negative parenting reactions during early childhood interact with restrictive parenting style to predict later externalizing behavior disorders [125]. Additionally, if the child has demonstrated emotion dysregulation and the parent in turn has displayed increasingly harsh and insensitive parenting, it is likely that the parent and youth will exhibit a cycle of coercive interactional patterns that may continue through late childhood and adolescence [152]. In particular, children and adolescents with low inhibitory control, consistent with temperamental features of children with ADHD, may demonstrate both internalizing and externalizing problems. For instance, if a child is prone to negative affect and exhibits low inhibitory control, he/she may have difficulty attenuating negative thoughts (e.g., rumination), and may exhibit negative withdrawal, possibly predisposing him/her to depression [153]. If this child has a parent who responds with anger or negative inferential feedback, the child’s risk for depression is further exacerbated [154]. Alternatively, to cope with negative emotions and thoughts, a youth with poor self-regulation might display CD and impulsive behaviors [35]. These externalizing behaviors could further elicit harsh, inconsistent, or disengaged parenting behaviors if the parent believes the adolescent is too challenging to discipline or monitor.

Further, parents exert relatively less influence on their adolescents than during earlier periods as teenagers become socialized by peers [148]. Children with ADHD are more prone to associations with deviant peers than children without ADHD, putting them at further risk for developing conduct problems; in addition, peer difficulties among children with ADHD may lead to depression or anxiety during adolescence [155]. Thus, a child’s basic temperament and emotion regulation abilities, as well as the foundation the parent has provided for emotion socialization, may predict the degree to which adolescents are influenced by peers and other environmental factors during adolescence, which consequently may contribute to or exacerbate different co-occurring conditions, consistent with multifinality.

In summary, previous research provides a foundation for a more integrated model of relations among child emotion regulation and contextual influences in the development of comorbid conditions among youth with ADHD. Consistent with the developmental psychopathology framework, the model also considers transactional relations between parents and children. In general, emotion regulation maps onto a diverse range of children’s behavioral and psychological functioning and may act as a shared risk factor for the development of co-occurring disorders, such as anxiety, depression, ODD, and CD, among children with ADHD. Temperament, especially effortful control, is an early marker of emotion regulation, and is consequently an important construct to consider among children with ADHD. As illustrated previously, emotion regulation skills develop from infancy through adolescence (and beyond), interacting with parenting behaviors and potentially protecting against or predisposing youth with ADHD to internalizing and externalizing problems. With these points in mind, we next present one potential model that focuses on the role of emotion regulation and its interrelations with parenting behaviors in the development of comorbid conditions among youth with ADHD.

A Conceptual Model for the Development of Comorbid Conditions Among Youth with ADHD

Although emotion regulation deficits and parenting behaviors (especially the way a parent fosters the development of emotion regulation) likely interact to predispose children with ADHD to other comorbid conditions [100, 136, 156, 157], there is a dearth of literature investigating emotion regulation deficits among children with ADHD and comorbid conditions [86]. To address this gap, evidence for specific deficits and interactional patterns among children with ADHD and comorbid conditions is reviewed below. As noted previously, certain emotion regulation deficits are associated with ADHD and comorbid disorders, suggesting that a shared risk factor explanation might account for disorder co-occurrence. An examination of the specific deficits associated with each comorbid condition considered in the present paper is important for understanding the potential emotion regulation profiles that may lead to one comorbidity rather than another. The specific profiles and associated deficits are presented in Table 1.

Table 1 Potential temperament and emotion regulation profiles for comorbid conditions among children with ADHD
Full size table

Children with behavioral disorders typically display negative temperamental styles and have difficulty regulating anger, aggression, and negative emotions; indeed, emotion regulation problems are particularly associated with aggressive symptoms among children with ADHD [35, 90, 98]. Further, children with ADHD who display negative affect and reactivity (as in Derryberry and Rothbart’s [149] model) often elicit harsher parenting reactions, further exacerbating child oppositional and behavioral problems [10, 62, 105, 123, 158]. Thus, given that ODD is associated with negative emotionality [102], one would expect that children with ADHD who also exhibit emotion regulation difficulties such as susceptibility to negative, angry, and aggressive behaviors might develop symptoms consistent with ODD, especially in the context of harsh, inconsistent, and non-supportive parenting strategies [10, 62, 123, 158].

Alternatively, because some research suggests that emotion regulation problems may be more linked to ODD than ADHD symptoms [98], emotional lability instead may be a risk factor for ODD, particularly among youth who exhibit ADHD. Thus, it is possible that emotional lability instead mediates the relation between ADHD and co-occurring ODD and aggression. In sum, among children with ADHD, co-occurring emotion regulation deficits, specifically negative emotionality [159], poor reactive control, low agreeableness, high irritability, and high approach tendencies [44], may contribute to the development of co-occurring ODD. Thus, these children are expected to be characterized by emotion regulation profiles of under-regulation (i.e., high lability; see Table 1).

CD is also characterized by lower levels of emotion regulation, though it has been examined less frequently in terms of emotion regulation dysfunction [160]. Children with CD are grouped into childhood-onset and adolescent-onset subtypes. The CD category recently included another specifier in DSM-5 intended to decrease heterogeneity among youth with CD; specifically, youth can be diagnosed based on the presence or absence of callous-unemotional (CU) traits (i.e., “with low prosocial emotions”) [2]. Little is known regarding the specific emotion dysregulation problems within these subtypes, though some research shows that it is likely that children with and without CU traits differ on emotion regulation profiles (for a review, see [161]). Overall, children with CD without CU traits likely evidence marked reactivity and executive functioning deficits, which may be manifested in reactive aggression [42, 162]. This subgroup may be more likely to be comorbid with ADHD, given their problems related to inhibition and impulsivity [42, 44, 162, 163].

Compared to youth with CD without CU traits, children with CD with CU traits may have fewer executive functioning deficits, lower physiological arousal to punishment cues, lower anxiety, and higher levels of proactive aggression [161, 164–168]. Additionally, children with CD and CU traits typically demonstrate fearlessness, sensation seeking, and disinhibition of aggression [160, 161]. Thus, we propose that there are two groups of children with ADHD and CD: (a) children with CD without CU traits, characterized by under-regulation, low effortful control, high reactive aggression, increased fear, and executive functioning deficits; and (b) children with CD with CU traits, characterized by low agreeableness, low conscientiousness, decreased fear/empathy, low effortful control, and proactive aggression (see Table 1). Regardless of subtype, children with both ADHD and CD likely demonstrate a poorer prognosis and deficient emotion regulation abilities relative to youth with ADHD only [169–171].

Children with internalizing problems generally exhibit decreased effortful control and high avoidance [149]. Specifically, children with anxiety tend to avoid events and situations that produce emotional arousal [172], experience emotions more intensely, exhibit dysregulated expressions and less adaptive coping, and evidence decreased ability to improve their mood relative to non-anxious children [86, 173]. Findings related to response inhibition among children with anxiety disorders have been mixed, with some research demonstrating increased response inhibition and other research failing to support this finding (for a review, see [28]). However, these mixed findings may be a function of the type of ADHD symptoms exhibited by youth. Children with anxiety disorders and both inattentive and hyperactive-impulsive symptoms of ADHD likely have certain executive functioning deficits (e.g., decreased attentional control, increased attentional biases to threat, low effortful control); high negative withdrawal; and low hostility compared to children with ADHD alone [5]. Those with predominantly inattentive symptoms of ADHD might exhibit anxious impulsivity, overarousal, and inattentiveness, but lower levels of impaired executive functioning relative to youth with both inattentive and hyperactive-impulsive symptoms [115, 174]. Thus, ADHD and anxiety might be characterized by poor regulation of both attention and affect, leading to both increased attention to negative stimuli and decreased regulation of accompanying affect [175] (see Table 1). However, the pattern of findings likely differs based on types of ADHD symptoms and the type of anxiety disorder experienced by youth, suggesting that further research into the specific emotion regulation deficits occurring among youth with comorbid ADHD and anxiety is needed.

Emotion regulation deficits among children with depression include negative emotionality and increased intensity of sadness, decreased positive emotions, emotional lability, and poor effortful control [88, 153, 176–179]. Thus, children with both ADHD and depression would likely exhibit increased emotional lability, low effortful control, increased negative emotions, and decreased positive emotions, relative to children with ADHD or depression only. These children are characterized by overregulation of emotional expression and low attentional control, especially with regard to disengaging from negative thoughts (see Table 1). Moreover, because emotion regulation deficits mediate the link between ADHD and depression symptoms among children, even when controlling for ODD/CD [75], emotion regulation deficits may be conceptualized as correlates of ADHD and depressive symptoms or shared risk processes that contribute to the symptoms of both conditions during different developmental periods. Additionally, emotion regulation deficits may act as mediators or risk factors for depression among youth with ADHD. Nevertheless, as reviewed earlier, it is important to consider the developmental periods during which these various comorbidities are likely to develop among youth with ADHD given typical age of onset of these comorbid conditions and the frequency and quality of risk and resilience processes (e.g., emotion regulation, parenting behaviors) during various developmental periods. Thus, the current model considers the development of comorbid conditions through adolescence to capture the range of heterogeneity, multifinality, and equifinality in individual outcomes.

Additional Gaps in the Current Literature

Although there is a large literature on inter-relations among components of the proposed model, there is little research jointly investigating relations among these constructs in an integrated model. Much of the literature to date has focused on elaborating the nature of ADHD and comorbid conditions (i.e., prevalence rates, symptom profiles) or has focused on risk factors for one particular disorder (e.g., ODD and/or CD) that co-occurs with ADHD, investigating comorbidity in isolation [e.g., 22, 62, 65, 75, 80, 120, 158]. However, there is a dearth of research examining correlates, sequelae, and/or mechanisms that may act as shared risk factors for ADHD and comorbid conditions. In particular, little research has investigated emotion regulation deficits in ADHD and comorbid conditions; additionally, research on bidirectional relations between children and parents in the context of ADHD and emotion regulation deficits is mixed [8, 44]. Further, there is a dearth of literature investigating mechanisms involved in the development of comorbid internalizing, as opposed to externalizing, disorders among children with ADHD [8]. Sex differences also warrant examination with regard to the current model, as the prevalence rate for ADHD among boys is higher than girls [1, 2]. Boys also may have fewer emotion regulation strategies and may display more negative emotions than girls, whereas girls may internalize symptoms, predisposing them to depression in adolescence [83]. Additionally, there may be sex differences that emerge in late childhood or adolescence, as boys might tend towards externalizing, substance use, or delinquent behavior compared to girls [26].

There is a paucity of longitudinal studies that evaluate the development of comorbid conditions among children with ADHD. According to Deault’s [8] meta-analysis of 22 studies examining ADHD, comorbidities, and parenting, only three studies utilized longitudinal designs. As described above, one of these studies found that parenting during early childhood predicted the developmental course of conduct problems among children with ADHD [123]; moreover, Burke et al. [159] found that child ODD and CD influenced parenting more than parenting influenced child symptoms, and that ADHD symptoms did not predict parenting. The direction of effects among parenting, ADHD, and comorbid conditions, despite the longitudinal design, remains unclear [8, 26].

The current model addresses these gaps by utilizing a developmental approach and considering processes that may be correlates of ADHD and/or risk factors for the development of comorbid conditions. Much of the existing research on comorbidity considers explanations that focus on developmental timing, such as ADHD precedes and/or confers risk for other disorders (see [23, 53] for overviews), though the mechanisms that confer risk for subsequent comorbid conditions have received little attention in the literature. Thus, investigations of shared risk processes and mechanisms, such as child emotion regulation, parenting, and other contextual factors, can allow a more nuanced test of the development of comorbid conditions among youth with ADHD from a developmental psychopathology perspective that takes into account multifinality and equifinality.

Implications and Future Directions

Given myriad negative outcomes associated with ADHD and comorbid conditions, clearer understanding of processes through which ADHD confers risk for other disorders has important clinical implications. Empirical investigations of the proposed model may afford specificity in identification of youth at risk for the development of comorbid conditions, as well as indicate targets and developmental points of prevention and intervention efforts aimed at child emotion regulation and associated parenting practices among youth with ADHD.

Research on emotion regulation and parenting among youth with ADHD could inform modifications to existing treatments and the development of tailored interventions. At present, although youth with comorbid conditions are included in prevention and intervention studies, children in the community are often more heterogeneous in presentation and comorbidity than youth who are typically included in randomized controlled trials [180]. Thus, assessments should include information regarding the quality and frequency of emotion regulation deficits among children with ADHD and comorbid conditions so that these processes could be targeted in treatment. Further, parent training may be especially useful among parents of youth with comorbid conditions for management of ADHD behaviors, teaching appropriate emotion regulation skills, and addressing other comorbidities [181]. Additionally, sex differences in temperamental styles and emotion regulation abilities among boys and girls should be incorporated into etiological and intervention models [44]. For example, girls who exhibit aggression or anger often do not express these feelings as overtly as boys [82], and parenting behaviors and child × parenting interactions may differ by child sex [e.g., 182]. Future research should examine how these mechanisms operate among boys and girls, which can inform whether treatment should address presenting issues among boys and girls differently.

A more modular treatment approach may benefit children with comorbidities and emotion regulation deficits, requiring an analysis of specific deficits before treatment [183]. Psychoeducation for emotion regulation skills typically has not been a target of treatments that have been deemed effective among youth with ADHD [86, 184, 185]. However, there is promising research on several preventive interventions for emotion regulation skills (e.g., PATHS—Promoting Alternative Thinking Strategies; [186] and EC—Emotions Course; [187]), as well as intervention models (ECBT—Emotion-focused Cognitive–Behavioral Therapy; [188] and CERT—Contextual Emotion-Regulation Therapy; [189]), each of which requires further evaluation among youth with ADHD.

Similarly, parent involvement in treatment has been deemed important among parents of children with ADHD, with behavior therapy as the only well-established evidence-based psychosocial treatment for child and adolescent ADHD (e.g., behavioral parent training, behavioral classroom management, and behavioral peer interventions) [181]. Thus, including child emotion regulation and signs or markers for the development of comorbid conditions among youth with ADHD would be useful adjuncts to parent training. Because emotion regulation and parenting are likely mediators, mechanisms, or shared processes for ADHD and comorbid conditions, they could serve as risk or protective factors, indicating areas to consider for prevention and intervention. Future research will be necessary to test the viability of targeting these processes in interventions and whether the addition of these variables to models of the development of comorbid conditions among youth with ADHD will facilitate improved understanding of multifinality among youth with ADHD.

Although the model presented focuses on emotion regulation as a shared process for the development of comorbid conditions among children with ADHD, there are additional factors that have received relatively less attention but also likely play a role. For example, academic or other learning difficulties [190, 191], peer factors [56, 192], perinatal problems [193], and broader family level processes [194] are associated with ADHD and may serve as more distal or proximal risk factors for comorbid psychological conditions. Future research should focus on these additional factors in addition to the parenting practices discussed earlier, and consider these processes within a developmental framework. For example, during childhood, peer rejection, parental discipline, and difficulty in school might set the stage for comorbid conditions among children with ADHD. During adolescence, deviant peer involvement and lack of parental monitoring might contribute to additional disorders that are more likely to onset during this developmental period. In the present paper, we focused on parenting behaviors related to emotion regulation because they are early-emerging processes that are crucial for the formation of important skills for future developmental periods and may buffer children against risk. Additionally, parenting practices and emotion regulation are potentially amenable to interventions as proximal influences and clear correlates of ADHD and co-occurring disorders. Thus, future research should explore these child-specific and contextual factors across development to determine their roles in the development or attenuation of co-occurring difficulties and whether these factors influence treatment outcomes among youth with ADHD.

Summary

The current paper considered developmental relations between ADHD and commonly co-occurring conditions using a developmental psychopathology perspective. Emotion regulation capabilities were discussed as potential shared risk processes of ADHD and co-occurring conditions, and in terms of bidirectional and transactional relations with parenting behaviors. A model linking temperament and emotion regulation processes to specific disorders was presented to illustrate potential different developmental pathways and multifinality among youth with ADHD. Future research in this area would inform prevention and specific interventions that target emotion regulation or parenting, emphasizing fit of intervention with the child’s emotion regulation, ADHD symptoms, and/or co-occurring conditions.