People are surprised that mental illnesses are hereditary. In fact, mental illnesses are always based on physical ailments; they arise from a malfunctioning of the body. Neither the spirit nor the soul can fall ill. Though mental illnesses are always rooted in physical problems, people wonder how they can occur through heredity, which indeed they can do. If a parent, particularly the mother, suffers from tuberculosis or another disease like arteriosclerosis, which admittedly occurs rarely in younger persons, the children do not necessarily become afflicted with these illnesses but instead can suffer from mental deficiencies.
People are surprised about this, but need it puzzle us, gentlemen? Whatever the child can inherit must be inherited first of all from its head. Therefore, if the mother is consumptive, one need not be surprised that her condition is not passed on to the lungs of the unborn child, which, after all, are not even functioning yet. The condition is rather carried over into the head and comes to expression in the brain.
Thus, nobody should be surprised that the disease inherited is quite different from that of the parent. Venereal disease, for example, can appear in children as an eye disease. It is no wonder, for when the child’s head is developing, its eyes are exposed to what afflicts the parents; its eyes are in an environment that’s venereally diseased! So it is not at all surprising.
Source: Rudolf Steiner – GA 348 – Health and Illness I: Lecture II: Illnesses Occurring in the Different Periods of Life – Dornach, October 24, 1922