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Comorbidity is the ´´co-occurrence of two or more mental health problems´´ that due to alterations in ´´psychiatric nomenclature´´, it has ´´emerged as a major clinical, public health and research issue over the past few decades´´. According to Angold and colleagues, there are two types of comorbidity: homotypic and heterotypic comorbidity. The homotypic comorbidity refers to the mental illnesses that exist within the same diagnostic category. For example, cocaine and alcohol are both substance use disorders. On the other hand, heterotypic comorbidity refers to mental illnesses that belong to different diagnostic categories.

One example of heterotypic comorbidity is anxiety disorders and eating disorders. Specifically, people suffering from eating disorders often exhibit ´´obsessions or compulsions´´ not related to food issues, such as ´´washing rituals, phobic obsessions´´ or fear of socialization. Generally, many studies have revealed a high comorbidity among eating disorders and anxiety disorders, with generalized anxiety disorders and social phobia to be most common among people suffering from Bulimia Nervosa, and social phobia along with obsessive compulsive disorder to be most common among Anorexia Nervosa. Specifically, obsessive compulsive disorder and eating disorders share strong psychopathological similarities, as they are both neurobiologically associated with the serotonin system. In addition, social phobia and eating disorders are strongly interrelated, due to two main reasons: firstly, due to personality disorders, and secondly, due to family influences and childhood sexual abuse. Another example of heterotypic comorbidity is schizophrenia and dementia. According to Arnold and Trojanowski, there are cases of people suffering from schizophrenia that exhibit ´´severe deterioration in cognitive and functional capacities´´. Dementia suggests loss and gradual decline from a previously attained level of functioning. Despite the co-occurrence of these two disorders, the evidence base is weak for this group. The reason for this is that both of these disorders are due partly to genetic predispositions and other biological factors that have not been fully comprehended yet. For example, molecular mechanisms have been found to be responsible for dementia in schizophrenia; however, the exact process is still under study.

Comparing these two groups would mean the comparison between case formulation approach and diagnosis approach. Eating disorders and anxiety disorders are triggered mainly due to sociocultural factors, such as peer and media influences, requiring the working psychological explanation for a patient´s feelings, behavior, and mentation. On the other hand, schizophrenia and dementia are triggered mainly due to biological factors, and thus requiring the synthesis of history, observation, and tests that will lead to the indication of the specific mental illness. Thus, the difference between their evidence base lies primarily on the factors that cause these disorders to manifest.

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