The contribution of biological psychiatry to research on schizophrenia should not be seen only in terms of discovery of biological markers or identification of definite etiological factors. Although biological research has failed up to now to identify such markers or factors, its influence on the clinical conceptualization of the illness has been considerable. On the other hand, biological research on schizophrenia has not been insensitive to the continuous re-shaping of the clinical concept of the illness which occurred during the last decades. There are at least three issues for which this reciprocal influence can be identified: 1), the question of the symptoms to be included in the definition of schizophrenia; 2), the problem of the relationship between “core” schizophrenia and mild non-psychotic conditions such as schizoid and schizotypal disorders; 3), the issue whether schizophrenia represents a mixture of different entities or a basically unitary condition.
