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The first studies relating to classification of creatures had been done in 18th century, by Carl Linnaeus (1) Linnaeus had been regarded as the father of morphologic classification (taxonomy). The creatures have been classified according to similarity of their appearances, in the morphologic classification. For instance; a shark that is a kind of fish and a dolphin that is a kind of mammalia had been examined within the scope of same family, although they are not similar as genetic structure. Flying squirrel (tribe of pteromyini) and Petaurus breviceps are very similar to each other with their big eyes, the white bellies and the stiff integument segment that takes place between their arm and legs and provides soaring and keeping the balance; therefore, they have been regarded as close relatives. However, the flying squirrel is a placental mammal (infraclass eutheria) and the Petaurus breviceps is a pouched mammal (infraclass marsupialia). Although they are different originally, there are creatures that appear differently even though they are similar genetically on the contrary the creatures that appear similar. Consequently, some bird species that have different appearances of cock and hen (for instance, Agelaius phoenicus) have been defined as different species by misunderstanding their physical appearances (2).
In the Linnaean classification; the structures have been classified to the same family according to their common functions (analogia) and not according to the common origin of structures (homology). Although this classification system includes very valuable information, it has been widely rejected at the present time. Nowadays, creatures are being classified by phylogeny. According to the phylogenetic classification, creatures are classified by the basic origin that forms their structures, i.e., their genetic structures (3).
The problem about classification of mental disorders has been a matter of debate for many years. Stengel has touched on the difficulties of this issue in his 63-page article (4). The systems that are currently used for the classification of psychiatric diseases have descriptive characteristics such as the morphologic classification of Linnaeus. The specifications, which are made on the basis of the similarity of symptoms, reflect the truth to what extent? Does schizo-affective disorder take place in the intersection set of mood disorder with schizophrenia? Can schizo-obsessive disorder be mentioned on the contact point of obsessive compulsive disorder and schizophrenia? (5) If not, why? Is olfactory reference syndrome (ORS) a delusional disorder that has somatic characteristics or an obsessive compulsive spectrum disorder? (6) Or should we accept ORS as a unique case which has a different genetic origin? (7)
There are many examples; however, there are critical comments based on the current classification systems (8,9). Many scientific and technological developments have occurred in the 20 years between the fourth and fifth editions of Diagnostic and Statistical Manual of Mental Disorders (DSM). However, these developments do not appear to be determinative on the classification of mental disorders.
Although the technical opportunities that we have at the present time are not sufficient, we can foresee that some of the psychiatric diseases’ situations in DSM can be changed in future. This development will inevitably discuss the current treatment strategies and alternatives. I am of the opinion that this perspective will provide a broader point of view when the patients are examined and treatment is planned.