Schizophrenia and manic-depressive illness (or bipolar disorder) are arguably the two most important conditions in psychiatry; the distinction between the two originated in 1919 with the work of E. Kraeplin. There are, however, serious doubts about whether these are really two distinct disorders rather than different points on a continuum of psychosis. Any psychiatrist will tell you that there are cases in which the unequivocal distinction between the two is all but impossible. Schizoaffective disorder, which has characteristics of both, is born of that reality.
The disorder may represent a component that is intrinsic to the individual rather than a disease caused by an external cause, such as a virus. Rather the symptoms may represent an extreme variation of normal.
The hallmark, first rank, or nuclear symptoms of schizophrenia include: experiencing feelings and/or actions as externally controlled; hearing thoughts spoken aloud; hearing voices discussing one in the third person; hearing a commentary on one’s actions; and experiencing thoughts being removed from or inserted into one’s head or broadcast to others. These symptoms seem to represent a loss of the boundary between self and others.
When defined by these nuclear symptoms, schizophrenia is constant across populations that differ widely in geographic, climatic, industrial and social environments. It seems that schizophrenia is a characteristic of human populations rather than typical diseases such as diabetes or heart disease that are clearly affected by environmental factors in a way that schizophrenia is not.
People with schizophrenia are at a reproductive disadvantage. They do not have as many children as those without schizophrenia for a whole variety of reasons. According to evolutionary theory, the disorder should become less common and eventually disappear with such a significant reproductive disadvantage. Nature selects for characteristics that promote the transmission of an individual’s genes. It is therefore suggested that perhaps the symptoms of schizophrenia are associated with other human characteristics that are reproductively advantageous and therefore serve to maintain these symptoms in the population. Language has been suggested as one such human characteristic. It is thought that language is what defines our species and separates us from all others. Language is believed to have originated with a critical genetic change—the speciation event—between 100-250,000 years ago.
The speciation event led to differentiation of cerebral function. In modern man language is largely a function of the left, dominant hemisphere of the brain. Schizophrenia is associated with a failure of lateralization of brain function. It represents one extreme of natural variation within the population in which critical components of language are divided between the two hemispheres more equally than usual. This compromises a person’s ability to distinguish self from other and therefore makes it possible to hear one’s own thoughts as if they were someone else’s. All of the central symptoms in schizophrenia can be understood in this way.
If this theory is correct, psychosis is the price we pay as a species for the advantage of language – and it will likely always be thus. We may find ways to alleviate symptoms and help people function in society, but traditional approaches to finding disease causation will likely not be effective for this disorder.