Comorbidity is the co-occurrence of more than one disorder in the same individual. One of the most frequent childhood examples is seen in children with Attention Deficit Hyperactivity Disorder (ADHD), who also frequently have comorbid symptoms of Oppositional Defiant Disorder or Conduct Disorder (CD). However, emotional disorders like anxiety and depression also frequently co-occur with behavioural disorders such as ADHD and CD, as well as with each other.  

Comorbidity has been reported in children as young as two years old, and the risk of comorbidity increases with each additional year from age 2 to 5. Furthermore, research has shown that preschoolers with comorbid disorders are significantly more impaired than those with just one disorder. On the whole, individuals with comorbid psychiatric disorders have poorer prognosis i.e. the long-term likely course of disorders and less optimal outcomes, due to having more severe symptoms, longer duration of illness and more functional disability.
Various explanations have been suggested for the occurrence of comorbidity. One is that there is symptom overlap across diagnostic categories, allowing a child to meet diagnostic criteria for multiple conditions. For example, symptoms of inattention can occur in ADHD, anxiety and mood disorders, meaning that a child with symptoms of inattention may meet diagnostic criteria for either one of these disorders. Conversely, because of this overlap of symptoms between disorders, it can be difficult to obtain a diagnosis and therefore treatment, particularly if an individual’s symptoms do not completely fit the diagnostic criteria for any one disorder. Another explanation for comorbidity may be that having one disorder causes or confers risk for the other. For example, young people with anxiety may be at increased risk for developing depression, if the primary disorder (anxiety), leads to social and academic difficulties that over time lead to the development of depression.
Finally, comorbidity may also be explained by shared or overlapping factors between disorders. These include common genetic factors, common environmental factors or a combination of both.
It is important to understand the mechanisms and causes of comorbidity because this may provide opportunities for disorder prevention or identification of children at risk who can then be targeted for early intervention which may improve their outcomes in later life.