Problems abound in defining schizophrenia. The two most common functional psychoses are schizophrenia and bipolar disorder (also known as manic-depressive illness). The distinction between the two is not easy to make and psychiatrists in different parts of the world at different times have drawn the boundaries in different ways. Bipolar disorder is an episodic disorder in which psychotic symptoms are associated with severe alterations in mood—at times elated, agitated episodes of mania, at other times depression, with physical and mental slowing, despair, guilt and low self-esteem.

On the other hand, the course of schizophrenia, though fluctuating, tends to be more continuous, and the person’s display of emotion is likely to be incongruous or lacking in spontaneity. Markedly illogical thinking is common in schizophrenia. Auditory hallucinations may occur in either manic-depressive illness or schizophrenia, but in schizophrenia they are more likely to be commenting on the person’s actions or to be conversing one with another. Delusions, also, can occur in both conditions; in schizophrenia they may give the individual the sense that he or she is being controlled by outside forces or that his or her thoughts are being broadcast or interfered with.

Despite common features, different forms of schizophrenia are quite dissimilar. One person, for example, may be paranoid but show good judgment and high functioning in many areas of life. Another may be bizarre in manner and appearance, preoccupied with delusions of bodily disorder, passive and withdrawn. So marked are the differences, in fact, that many experts believe that, when the causes of schizophrenia are worked out, the illness will prove to be a set of different conditions which lead, via a final common pathway of biochemical interactions, to similar consequences.

It is not at all clear what is schizophrenia and what is not. Scandinavian psychiatrists have tended to use a narrow definition of the illness with an emphasis on poor outcome. Russian psychiatrists have adhered to a broad definition with an emphasis on social adjustment. In the United States the diagnostic approach to schizophrenia used to be very broad. With the publication, in 1980, of the third edition of the American Psychiatric Association Diagnostic and Statistical Manual, however, American psychiatry switched from one of the broadest concepts of schizophrenia in the world to one of the narrowest.

Why is the diagnosis so susceptible to fashion? The underlying problem is that schizophrenia and manic-depressive illness share many common symptoms. During an acute episode it is often not possible to tell them apart without knowing the prior history of the illness. The records of people with manic-depressive illness, however, should reveal prior episodes of depression and mania with interludes of normal functioning.