In the development of psychiatric genetics progress has been very largely dependent on the progress of clinical psychiatry. In the early days the theory of polymorphism held the field—that is the idea that there is a single hereditary basis for what may be called degeneration, which might show itself in many forms, any form of insanity, psychopathic character, criminality or mental defect. This theory was built up on the unclear psychiatry of the day, when the boundaries between different clinical states were vaguely drawn and largely in dispute. We are now passing out of this stage in clinical psychiatry, but the genetic theory that was built on it, the idea of polymorphism, still lingers on. It has been the stimulus to an immense amount of muddled research of no value. The time has come when this theory should be finally discarded. We are not entitled to think that different clinical conditions have the same genetic basis until such has been proved. From a factual point of view the accumulation of knowledge has shown it in a number of cases to be untenable. From a heuristic point of view it offers no possibilities of advance.