Feeble-mindedness is largely handed on by heredity. It was formerly supposed that idiocy and feeble-mindedness are mainly due to environmental conditions, to the drink, depravity, general disease, or lack of nutrition of the parents, and there is no doubt an element of truth in that view. But serious and frequent as are the results of bad environment and acquired disease in the parentage of the feeble-minded, they do not form the fundamental factor in the production of the feeble-minded.
Feeble-mindedness is essentially a germinal variation, belonging to the same large class as all other biological variations, occurring, for the most part, in the first place spontaneously, but strongly tending to be inherited. It thus resembles congenital cataract, deaf-mutism, the susceptibility to tuberculous infection, etc.
Exact investigation is now showing that feeble-mindedness is passed on from parent to child to an enormous extent. Some years ago Ashby, speaking from a large experience in the North of England, estimated that at least seventy-five per cent of feeble-minded children are born with an inherited tendency to mental defect. More precise investigation has since shown that this estimate was under the mark. Tredgold, who in England has most carefully studied the heredity of the feeble-minded,29 found that in over eighty-two per cent cases there is a bad nervous inheritance. In a large number of cases the bad heredity was associated with alcoholism or consumption in the parentage, but only in a small proportion of cases (about seven per cent) was it probable that alcoholism and consumption alone, and usually combined, had sufficed to produce the defective condition of the children, while environmental conditions only produced mental defect in ten per cent cases.30