Psychologists, Angold and Costello[1], have coined the term ‘heterotypic comorbidity’ to describe individuals who have more than one mental disorder (comorbidity) in two separate domains (heterotypic). The two domains they are referring to are internalizing and externalizing. Where internalizing problems in childhood generally include anxiety, depression, being withdrawn, sad affect, somatic complaints, thought problems and emotional reactivity, externalizing problems in childhood take the form of aggression, defiance, substance use, impulsivity and attention problems [2], [3].
While heterotypic comorbidity is less common in adulthood, 7% of children between ages 5 and 15 met criteria for both anxiety and disruptive behavior disorders. This rate is three times greater than that of chance associations among disorders[1]. In a sample of 1st-6th graders, researchers found that about 12.5% of children were likely to have co-occurrence of heterotypic symptoms, which was more likely than having symptoms from only one domain [4]. The discrepancy in prevalence may be from examining disorders verses symptoms and that younger children tend to have more heterotypic problems than older children.
The most common heterotypic co-occurring disorders are conduct disorder (CD) and depression, CD or oppositional defiant disorder (ODD) and anxiety, followed by attention deficit hyperactive disorder (ADHD) and depression, and ADHD and anxiety[5]. Studies have shown that 23-63% of children with generalize anxiety disorder (GAD) have ODD or CD and 15% of children in an anxiety outpatient clinic were found to also have ADHD [6]. Psychologists, Tolan and Henry[4], investigated which heterotypic symptom clusters were most closely related, finding that the three highest rates of co-occurrence were anxiety/depression with social problems, social problems with somatization, and aggression with thought problems.
Research has found that there is actually a unique, stable trajectory of heterotypic comorbidity such that some children who have both internalizing and externalizing symptoms continue to have both symptom types over a 9-year period in childhood [7]. During adolescence, when co-occurrence decreases, girls tend to develop more internalizing and less externalizing symptoms [7][8]. Sterba and colleagues [9] examined this heterotypic trajectory compared to child trajectories of ‘only internalizing problems’ and ‘only externalizing problems’ at four different age points in childhood. At age 2, the co-occurring group was lower on social fearfulness than the internalizing group and higher than the externalizing group. At age 4, the co-occurring group did not differ from the internalizing group on any behaviors, however at age 5, this group switched to being no different than the externalizing group in their symptoms. At age 7, the co-occurring group displayed similar behaviors to the internalizing group, with the exception of being lower on fear behaviors.
Children with co-occurring internalizing and externalizing problems are typically more functionally impaired than those with singular domain symptoms[10] and are thought to be less likely to achieve socio-developmental milestones throughout childhood including making positive peer relationships[8]. In fact, these children are more likely to develop friendships with children engaging in delinquent and risky behaviors, in turn, making the children with co-occurring symptoms more likely to be engaged in these maladaptive behaviors[11] . Overall, children with co-occurring problems were more likely to be excluded by peers than other children[12]. Perhaps most salient, is the finding that children with internalizing symptoms and substance abuse problems are at highest risk for suicide,[13] and thus this unique subgroup of children should merit special attention from clinicians and prevention/intervention research in the future.
One theory is that children first develop internalizing problems, and that having these problems put some of these children at risk for also developing externalizing problems.[14] Several studies have found that children with anxiety disorders only at the initial interview met criteria for a disruptive disorder (ADHD, ODD and CD) up to a year later [15][16][17]. The frustration-aggression hypothesis[18] posits that when children with anxiety, who normally prefer to avoid threatening stimuli and negative emotions, are forced to confront those contexts, they may respond with aggressive impulses or reactive aggression, therefore exhibiting externalizing problems over time.
Externalizing problems may precede co-occurrence in children such that due to their aggressive behavior, children with externalizing problems are likely to be rejected by peers and, over time, the overwhelming feelings of rejection lead to anxiety and depressive symptoms[14]. Supporting this claim is one study that found that childhood ODD and ADHD predicted anxiety disorders in adolescence, but that the anxiety did not predict the externalizing disorders[19]. Other studies found that it is more common for children with pure externalizing problems to develop co-occurring symptoms than pure internalizing problems. Lastly, Gilliom and Shaw[2] found the baseline for externalizing symptoms was positively associated with the rate of change of internalizing symptoms in young boys.
Another theory is that heterotypic co-occurring problems develop together, having shared biological and environmental factors[20]. Studies show that mood lability and emotional dysregulation in childhood predict co-occurring symptoms and disorders later in life[21][22]. Internalizing and externalizing disorders also have some shared features such as an inability to appropriately regulate emotion and nuances in information processing. For example, both anxious and aggressive children exhibit hyperarousal and attend more to emotionally threatening stimuli [23], [24].
Longitudinal studies have found that children with co-occurring symptoms (depressive symptoms and conduct problems) experience the most hostile, and least warm, parenting compared to children with pure internalizing or externalizing symptoms[25]. Other studies have found similarities in parenting among symptom types such that both co-occurring and pure externalizing child groups experienced harsh discipline[7], and that both pure internalizing and externalizing groups had similarly high rates of negative maternal control[2].
Early child temperament has also been linked to the development of heterotypic symptoms such that difficult temperament was related to mother-reported co-occurring symptoms, but not pure internalizing or externalizing groups. In addition, mood lability (intense mood shifts) has been identified as a predictor unique to developing heterotypic disorders. Other measures of child temperament have been found to be similar among problem group. For instance, child resistance to control was associated with both co-occurring symptoms and pure externalizing symptoms. Negative emotionality and the role of fear are predictive of both pure internalizing and externalizing problems, where negative emotionality and fearfulness predict internalizing and negative emotionality and fearlessness predict pure externalizing problems [2]. However, the predictive nature of fear in children with both co-occurring symptoms has yet to be evaluated.
Genetic testing in twins has shown that pure internalizing and externalizing disorders are more genetically influenced than co-occurring disorders thus suggesting co-occurring symptoms are more related to environment and perhaps life stressors[26].
Cognitive difficulties have been found to be significantly different between children with heterotypic co-occurring symptoms compared to those with pure externalizing symptoms, but similar to those with pure internalizing problems[27], which merits further study.
One study, investigated premature children’s reaction to a stressful task by measuring a stress hormone called cortisol. Children with a heightened response to stress had more attention problems (included in externalizing symptom scales) and more emotional reactivity, anxious and depressive symptoms (included in internalizing symptom scales)[3],[28]. This stress hormone, cortisol, may be a biological marker for co-occurring symptoms and thus should be followed longitudinally to better understand its role.
Compared to pure internalizing and externalizing groups, children with heterotypic co-occurring symptoms have been found to experience more socioeconomic status distress and had higher familial life stress altogether. In addition, analyses by race suggested that mothers of European American children were more likely to report co-occurring symptoms, whereas teachers of African American children were more likely to report purely externalizing problems.[7]
Currently, there are many different types of therapy for internalizing and externalizing disorders with strong evidence of success, such as cognitive-behavioral therapy approaches for internalizing [6],[29]and Parent-Child Interaction Therapy (PCIT) or Parent Management Training (PMT) for externalizing. [30][31]. The child is referred to therapy based on his or her primary condition or the problem causing most concern at the time even if there are heterotypic disorders. Clinicians and researchers have raised the concern that having comorbid diagnoses may decrease the effectiveness of any treatment because of its association with greater overall impairment and more maladaptive outcomes. However, research has found reductions in externalizing symptoms (ODD) following anxiety treatments [6] as well as declines in anxiety (Separation Anxiety Disorder: SAD) after PCIT [32].
One study has modified an anxiety treatment program to include strategies targeting anger and aggression to better accommodate heterotypic comorbidity. They compared treatment outcomes of the anxiety treatment alone and the anxiety treatment plus the aggression supplement program and found that both treatments effectively reduced anxiety and aggression symptoms in children and were not significantly different from each other. They interpret their findings by stating that anxiety may play a role in the onset or maintenance of externalizing symptoms, however, other raised important limitations of their study which may account for the non-significant differences[33]. In a meta-analysis of the effects of treatment on comorbidity, researchers found that studies with homotypic comorbidity (two or more internalizing or two or more externalizing disorders) demonstrated better treatment effectiveness than those with heterotypic comorbidity[34], which suggests further study for heterotypic comorbidity treatment in children.
^ abAngold A, Costello EJ, Erkanli A. Comorbidity. J Child Psychol Psychiatry. 1999;40(1):57-87.
^ abcdGilliom M, Shaw DS. Codevelopment of externalizing and internalizing problems in early childhood. Development and Psychopathology. 2004;16(2):313-333.
^ ab Achenbach TM. Manual for the Child Behavior Checklist/4-18 and 1991 profile. Dept. of Psychiatry, University of Vermont; 1991 Cite error: The named reference "three" was defined multiple times with different content (see the help page).
^ abTolan PH, Henry D. Patterns of psychopathology among urban poor children: Comorbidity and aggression effects. Journal of Consulting and Clinical Psychology. 1996;64(5):1094-1099
^Ford T, Goodman R, Meltzer H. The British Child and Adolescent Mental Health Survey 1999: the prevalence of DSM-IV disorders. J Am Acad Child Adolesc Psychiatry. 2003;42(10):1203-1211.
^ abcKendall PC, Brady EU, Verduin TL. Comorbidity in childhood anxiety disorders and treatment outcome. J Am Acad Child Adolesc Psychiatry. 2001;40(7):787-794. Cite error: The named reference "six" was defined multiple times with different content (see the help page).
^ abcdKeiley MK, Lofthouse N, Bates JE, Dodge KA, Pettit GS. Differential Risks of Covarying and Pure Components in Mother and Teacher Reports of Externalizing and Internalizing Behavior Across Ages 5 to 14. Journal of Abnormal Child Psychology: An official publication of the International Society for Research in Child and Adolescent Psychopathology. 2003;31(3):267-283. Cite error: The named reference "seven" was defined multiple times with different content (see the help page).
^ abOland AA, Shaw DS. Pure versus co-occurring externalizing and internalizing symptoms in children: the potential role of socio-developmental milestones. Clin Child Fam Psychol Rev. 2005;8(4):247-270. Cite error: The named reference "eight" was defined multiple times with different content (see the help page).
^Sterba SK, Prinstein MJ, Cox MJ. Trajectories of internalizing problems across childhood: Heterogeneity, external validity, and gender differences. Development and Psychopathology. 2007;19(2):345-366.
^ Nottelmann ED, Jensen PS. Bipolar affective disorder in children and adolescents. J Am Acad Child Adolesc Psychiatry. 1995;34(6):705-708.
^Talbott E, Flemming J. The role of social contexts and special education in the mental health problems of urban adolescents. The Journal of Special Education. 2003;73(2):11-123.
^Fanti KA, Henrich CC. Trajectories of pure and co-occurring internalizing and externalizing problems from age 2 to age 12: Findings from the National Institute of Child Health and Human Development Study of Early Child Care. Developmental Psychology. 2010;46(5):1159-1175.
^ Verona E, Javdani S. Dimensions of adolescent psychopathology and relationships to suicide risk indicators. Journal of Youth and Adolescence. 2011;40(8):958-971.
^ abBubier JL, Drabick DAG. Co-occurring anxiety and disruptive behavior disorders: the roles of anxious symptoms, reactive aggression, and shared risk processes. Clin Psychol Rev. 2009;29(7):658-669.
^ Foley DL, Pickles A, Maes HM, Silberg JL, Eaves LJ. Course and short-term outcomes of separation anxiety disorder in a community sample of twins. J Am Acad Child Adolesc Psychiatry. 2004;43(9):1107-1114.
^ Bittner A, Egger HL, Erkanli A, et al. What do childhood anxiety disorders predict? J Child Psychol Psychiatry. 2007;48(12):1174-1183.
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^ Polman H, Orobio de Castro B, Koops W, van Boxtel HW, Merk WW. A meta-analysis of the distinction between reactive and proactive aggression in children and adolescents. J Abnorm Child Psychol. 2007;35(4):522-535.
^Somersalo H, Solantaus T, Almqvist F. Four-year course of teacher-reported internalising, externalising and comorbid syndromes in preadolescent children. European Child & Adolescent Psychiatry. 1999;8(Suppl 4):89-97.
^Gregory AM, Eley TC, Plomin R. Exploring the association between anxiety and conduct problems in a large sample of twins aged 2-4. J Abnorm Child Psychol. 2004;32(2):111-122.
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^Epkins CC. Cognitive specificity in internalizing and externalizing problems in community and clinic-referred children. Journal of Clinical Child Psychology. 2000;29(2):199-208.
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