Comorbidity (or coexisting disorders) is the rule rather than the exception (Angold, Costello, &Erkanli, 1999; Kring et al., 2007), indeed, an individual who doesn’t have difficulties beyond the core symptoms is something of a rarity. Barkley (2010) suggested that in clinical samples between 40-90% of children with ADHD have at least one other psychiatric disorder. In addition to these issues learning difficulties and disorder, a host of secondary psychological difficulties and negative emotional squealer are also commonly reported, as are developmental disorders, and a range of social and familial problems. This paper provides a brief overview of the theory behind why comorbidity exists, with particular emphasis placed on the complexities involved in ADHD, and the research with sample of individuals with ADHD.


Originating in the medical world, the term originally referred to the presence of at least two diseases. An issue inherent in mental health is that definitive biological structures are often not assured, and so in dealing with disorders and their classification makes the process more complex and ill-defined, however, it should be taken to mean the presence of two or more conditions or disabilities. For example, if a person is diagnosed with ADHD and an Anxiety Disorder, they are said to be comorbid for these two disorders. Conversely, those who report being easily distracted and forgetful is not comorbid for these symptoms, rather their co-occurrence suggest morbidity for a single disorder, ADHD (Kring et al., 2007).


According to Duvner (1994) development includes cognition, attention, perception, motor skills, behaviour, social relations, and also oral and written communication.   Moreover, these areas do not exist in isolation, but rather they are mutually interdependent.

Essentially, the scientific world has provided a multitude of evidence to support the notion that a child who is developmentally atypical in a single area is very unusual (Gilger& Kaplan, 2001). However, this has less to do with development and deficits, and more to do with how individual differences are classified.

Mario Maj (2005) suggested that co-morbidity is the norm because current classification systems artificially split complex conditions, sometimes even omitting symptoms in pursuit of discreet categories. Many symptoms overlap, for example, one of the diagnostic criteria for ADHD is hyperactivity, however, such behaviours are also said to be symptomatic of Bipolar Disorders (DSM-IV).

These issues mean that characterization and causative factors are difficult to determine (Gilger& Kaplan, 2001). Evidence from research into mental illness suggests that diagnostic categories may not be reflective of the ways the disorders affect real people (Armstrong et al., 1993). This has led to arguments in favour of studying symptoms, rather than categories, especially in relation to treatment research (Costello, 1992).

However, one cannot just disregard current classification and assessment procedures, since they are necessary to identify deficits, suggest and evaluate treatments. The issue with diagnostic systems such as the DSM-IV is that they are hierarchical in nature, and in the presence of two or more diagnosis, one may be considered primarily and used to account for other symptoms possibly better explicated by a secondary diagnosis (Kring et al., 2007). Mounting evidence suggests that conditions can exist concurrently (Farone, 2001). Thus, it is suggested that great care must be taken during assessment, and attention paid to ensuring differential diagnosis and the identification of all unique strengths and needs.


ADHD is not a single entity

ADHD (Attention Deficit Hyperactive Disorder) is a neurobiological condition, which affects 5% of children and young people and 4% of adults across racial, ethnic, and socioeconomic lines in Europe and around the world (Clark, Carr-Fanning, & Norris, 2011).   The symptoms (i.e., hyperactivity, impulsivity, and inattention), are believed to be due to deficits in executive functions (e.g., planning, organization, and time management), which are caused by a dysregulation in brain chemicals (or neuro-transmitters).

The condition is best understood as a bio-psychosocial condition, which means that it is medical in origin, but it is affected and influenced by the environment and the social and emotional aspects of the person and situation (Cooper, 2001).

The brain is involved in everything we think, feel, and do so ADHD affects many, if not all, aspects of a person’s life. However, ADHD is not a single entity. Rather, it varies across people and fluctuates with age, development, and environmental demands.

In addition, the situation is further exacerbated by the fact that individuals with ADHD demonstrate behaviours beyond the core symptoms (Harrison et al., 2010). According to DuPaul and Stoner (2004) the core symptoms act as a ‘magnet’ for other difficulties which can be more detrimental than the characteristics associated with the ADHD.


The consequences of comorbid condition, particularly if they go unidentified and untreated, may be severe.

Individuals with ADHD are at significant risk for developing secondary psychological problems, such as depression or aggression. These have been attributed to negative experiences, academic failures, frustration and rejection (Herbert, 2003; Silver, 2003).

Co-morbidity may increase this vulnerability further.

Furthermore, the combination of difficulties makes emotional and behavioural problems more likely. For example, young people with ADHD and a learning disability were found to have significant more behavioural problems (e.g., aggression or withdraw) that those who had either one of these diagnosis (Cruddace, 2006).

In addition, comorbidity creates more problems than just one disability added to another. In a study by Cruddance and Riddell (2006) young people with ADHD and a comorbid learning disability had more sever learning problems than children who have a learning disorder but no ADHD, and also more sever attantional problems than those with ADHD but no learning disorder.

ADHD affects everyone differently

But it usually affects many, if not all, areas of a person’s life …

  • Cognitive(e.g., thinking, learning, solving problems).
  • Communication(e.g., talking, listening, understanding).
  • Life skills(e.g., organisation, time management, etc.).
  • Behavioural(e.g., impulsivity, violence, disruptive, etc.).
  • Emotional dysregulation(e.g., highly sensitive, anger / aggression, etc.).
  • Emotional well-being(e.g., depression, anxiety, etc.).
  • Social / emotional(e.g., playing, feeling happy and accepted, etc.).


In a study based on data from the Australian National Survey of Mental Health and Well-Being; 21% of people fulfilling DSM–IV criteria for any disorder met the criteria for three or more co-morbid disorders (Andrews et al., 2002).

In Sweden a study of school aged general education children, Kadesjö and Gillberg (2001), found that 87% of those diagnosed with ADHD met the diagnostic criteria for one or more psychiatric disorder or developmental problem.


In a meta-analysis of the literature documenting referred clinical samples of children with ADHD; between 15% and 50% were found to have reading difficulties, between 24% and 60% mathematical difficulties, and anywhere from 24% to 60% show problems with spelling. Please see Barkley (1990) for an overview of these studies.

In a community based sample in the U.S.A., 50% of children diagnosed with ADHD also meet the diagnostic criteria for Dyspraxia; and of these 60% also met the diagnostic criteria.

Research indicates an overlap between ADHD andAutistic Spectrum Disorder (ASD) (Gillberg, 1999; cited in Gillberg et al., 2004), possibly attributed to deficits in Executive Functions. For example, a sample of young people with ADHD 80% were said to display significant ‘Autistic’ traits (Kirby & Salmon, 2007). In another study, Fitzgerald and Corvin (2001) found that 21% of CYP with ADHD also had a diagnosis of Asperger’s Syndrome. Most startling was Gilber et al.’s (2004) assertion that in very young children there are sometimes major problems determining whether one is dealing with severe combined subtype of ADHD or autistic disorder (or possibly both).


Harrison et al (2010) argued that different clinical profiles should exist for “pure ADHD” and ADHD with externalizing (e.g., aggression, oppositional, etc.) and/or internalizing (e.g., depression / anxiety) behaviours (Harrison et al., 2010). Given the MTA (1999) study identification of differential response to treatment based on the presence of anxiety and/or ODD indicates why distinct profiles might be advisable. The most common comorbid conditions are reported as Oppositional Defiant Disorder (ODD) followed by Conduct Disorder (CD) (MTA, 1999; Barkley, 2006); with an estimated 45-84% for the former (Farone&Biederman, 1997) and 44-55% for the latter (Barkley and Biederman, 1997).

Disorders of conduct are so prevalent in fact that under the ICD-10 there is a separate diagnostic category for ‘Hyperkinetic Conduct Disorder’. While some suggest that this is reactionary in nature to experiences of rejection and failure (Hartmann, 2002), Du Paul et al (2001) studied young children during free play and found that those with ADHD exhibited very significantly higher levels of negative social behaviour than their peers. Campbell (2002) found that problems with peer’s relationships are present from a very early age, and show severe deterioration with development.


There appear to be subtype specific patterns of co-morbidity, which some authors suggest is a sign of multiple disorders (Frick &Lahey, 1991). Individuals with the predominantly inattentive subtype (previously ADD) are reported to be less likely to develop conduct problems (Du Paul & Eckhart, 2004; Frick &Lahey, 1991), however, they may be significantly more likely to suffer from learning difficulties, academic failure (Barkley, 2006; DuPaul& Stoner, 2004; Weiss &Hetchmann, 1993), depression, anxiety, and self-harm than their hyperactive-impulsive counterparts who are also at risk for developing these difficulties (Clark, Carr-Fanning, & Norris, 2011).


Professionals need to ensure a differential diagnosis includes a process of excluding coexisting conditions which are symptomatically distinct (and require distinct management) (Zametkin& Ernst, 1999). Avoid the mistake of misattributing symptoms as secondary to an identified condition, and follow best practice guidelines in diagnosis and recommendations. Thus, if these are successfully done, and provided that all the individuals needs are identified and catered for; then every individual can meaningfully participate in education and society to the fullest of their potential.

Furthermore, it is always important to view each individual as unique in their strengths, deficits, and potentials. A diagnosis of one or more conditions / disorders does not define the individual. Classification is only a tool in identifying difficulties which may exist so they can be supported, and it also highlights the person’s strengths which can be a focus for positive development and feelings of success. This is why we say ‘a child with ADHD’ and not ‘an ADHD child’. Remember … labels only define the contents of canned goods!!!

But, if you feel like calling yourself an ADHD person we fully support your ownership and right to define yourself as whatever you want to be ????


  • Reduced intellectual functioning
  • Academic achievement and performance deficits
  • Specific Learning Disabilities Reading (8-39%), Spelling (12-26%), Math (12-33), and Handwriting (Common but unstudied)
  • Poor sense of time, inaccurate time estimation and reproduction
  • Decreased nonverbal and verbal working memory
  • Impaired planning ability
  • Reduced sensitivity to errors
  • Possible impairment in goal-directed behavioral creativity (??)


  • Delayed onset of language (up to 35% but not consistent)
  • Speech impairments (10-54%)
  • Excessive conversational speech (commonplace), reduced speech to confrontation
  • Poor organization and inefficient expression of ideas
  • Impaired verbal problem-solving
  • Co-existence of central auditory processing disorder (minority but still uncertain)
  • Poor rule-governed behavior
  • Delayed internalization of speech (30%+ delay)
  • Diminished development of moral reasoning


  • 10-30 standard score points behind normal


  • Delayed motor coordination (up to 52%)
  • More neurological “soft” signs related to motor coordination and overflow movements
  • Sluggish gross motor movement


  • Poor self-regulation of emotion
  • Greater problems with frustration tolerance
  • Under-reactive arousal system


  • Disruptive classroom behavior (commonplace)
  • Underperforming in school relative to ability (commonplace)
  • Academic tutoring (up to 56%)
  • Repeat a grade (30% +)
  • Placed in one or more special education programs (30-40%)
  • School suspensions (up to 46%)
  • School expulsions (10-20%)
  • Failure to graduate high school (10 to 35%)


  • Poor persistence of effort/motivation
  • Greater variability in responding
  • Decreased performance/productivity under delayed rewards
  • Greater problems when delays are imposed within the task and as they increase in duration
  • Decline in performance as reinforcement changes from being continuous to intermittent
  • Greater disruption when non-contingent consequences occur during the task


  • Greater proneness to accidental injuries (up to 57%)
  • Possible delay in growth during childhood
  • Difficulties getting ready for bed and sleeping (up to 30-60%)
  • Greater driving risks: vehicular crashes and speeding tickets
  • Greater medical expenses for family to bear
  • Start sexual intercourse earlier as teens
  • Greater risk of teen pregnancy (38%)
  • Greater risk of sexually transmitted disease (16%)


  • Hulme, C. J., and Snowling, M. J. (2009). Developmental Disorders of Language Learning and Cognition. Wiley-Blackwell: Massachusetts.
  • Neale, M. C., and Kendler, K. S. (1995) Models of comorbidity for multifactorial disorders. American Journal of Human Genetics, 57, 935-953. References:
  • Barkley, R. A. (1998). Attention deficit hyperactivity disorder: A handbook for diagnosis and treatment (2d ed.). New York: Guilford Press.
  • Cruddace, S. A., and Riddell, P. M. (2006) Attention processes in children with movement difficulties, reading difficulties or both. The Journal of Abnormal Child Psychology, 34, 675-83.
  • Duel, B. P., Steinberg-Epstein, R., Hill, M., Lerner, M. (2003) A survey of voiding dysfunction in children with attention deficit-hyperactivity disorder. The Journal of Urology, 170,1521–1523.
  • Faraone, S. V. (May, 2001) Patterns of comorbidity in ADHD: Artefact or reality? Program and abstracts of the 154th Annual Meeting of the American Psychiatric Association; New Orleans, Louisiana. Industry Symposium 46B.
  • Gaskell, M. G. & Altman, (2007). The Oxford Handbook of Psycholinguistics. Oxford University Press, New York.
  • Gilger, J. W., and Kaplan, B. L. (2001) Atypical brain development: A conceptual framework for understanding developmental learning disabilities. Developmental Neuropsychology, 20(2), 465-481.
  • Gillberg, C., Gillberg, I. C., Rasmussen, P., Kadesjo, B., Soderstrom, H., Rastam, M., Johnson, M., Rothenberge, A., and Niklasson, L. (2004) Co-existing disorders in ADHD – implications for diagnosis and intervention. European Child and Adolescent Psychiatry, 1(13), 80-92.
  • Herbert, Martin (2003). Typical and Atypical Development: From Conception to Adolescence. BPS Blackwell, MA.
  • Lyon, R. G. (1996) Learning Disabilities. The Future of Children: Special Education for Students with Disabilities, 6(1), 54-76.
  • Lyon, R. G., Shaywitz, S. E., and Shaywitz, B. A. (2003) Defining dyslexia, comorbidity, teachers’ knowledge of language and reading. Annals of Dyslexia, 53(1), 1-14.
  • Maj, Mario (2005) Psychiatric comorbidity: an artefact of current diagnostic systems? The British Journal of Psychiatry, 186, 182-184.
  • Mayes, S. D., Calhoun, S. L., and Crowell, E. W. (2000) Learning disabilities and ADHD: Overlapping spectrum disorders. Journal of Learning Disabilities, 33, 417-24.
  • Hinshaw, S. P. (2002) Preadolescent girls with attention-deficit/hyperactivity disorder: I Background characteristics, comorbidity, cognitive and social functioning, and parenting practices. Journal of Consult Clinical Psychology, 70, 1086–1098.
  • Kadesjö, B., and Gillberg, C. (2001) The comorbidity of ADHD in the general population of Swedish school-age children. Journal of Child Psychology and Psychiatry, 42, 487–492.
  • Kavale, K. A., &Forness, S. R. (1998) The politics of learning disabilities. Journal of Learning Disabilities, 33, 239-256.
  • Kring, A. M., Davison, G. C., Neale, J. M., and Johnson, S. (2007) Abnormal Psychology (10th ed.). J. Wiley, NJ.
  • Zametkin, A. J., and Ernst, M. (1999) Problems in the management of attention-deficit hyperactivity disorder. Northern England Journal Medicine, 340 , 40-46.
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