There have been several inquiries into the incidence of various “neuropathic” conditions among the relatives of the mental defective, and it has been found that mental defectives often have relatives who have suffered from such conditions as cerebral hæmorrhage, epilepsy, alcoholism, etc. Thus it has been stated that 80% of mental defectives have a “positive” family history, and it is suggested that this figure (80%) supports the theory that most mental deficiency is due to hereditary factors. Before the significance of such a figure can be discussed we must first know the corresponding figure for the normal population.