In this chapter we shall cover genetic work bearing on the major classes of mental illness, namely, schizophrenia, manic-depression, melancholia, senile dementia, and psychoneurosis. We shall also touch on investigations of the role of heredity in such disorders as criminality, suicide, homosexuality, mental defect, and epilepsy.
Definitions and descriptions of mental illness present complex problems. One of the most obvious criteria separating the normal from the abnormal person is that the latter, whether voluntarily or involuntarily, is undergoing some form of hospital treatment or in some other way is on a psychiatrist's records. We cannot assume, however, that persons having no contact with a hospital or with psychiatrists are all normal. Many mentally ill individuals, for one reason or another, live their lives in the outside world. Thus hospitalization or case-record criteria are useful only as starting points. They tell us that most individuals in hospitals are abnormal; they do not tell us that all those outside hospitals are normal. Because of this difficulty, genetic surveys of mental illness should always include precautions against the biased selection that can arise from relying on these criteria too rigidly.
More exact ways of diagnosing the abnormal have been described in many text books. One schema, adopted by The American Psychiatric Association ( Noyes and Kolb, 1958), divides mental illness into three major categories as follows: (I) Disorders caused by or associated with impairment of brain tissue function. These may be due to various agents such as trauma, toxins, pre- or peri-natal accidents, infections, and a number of others. (II) Mental deficiency, both endogenous and exogenous. (III) Psychogenic disorders without any clearly defined physical causes or structural brain changes. This last group includes involutional psychoses, the various types of schizophrenia, manic-depression, paranoid reactions, undefined psychotic reactions,