The authors show that the progeny of two mentally defective individuals indicates an accumulation and multiplication of bad traits and demonstrate that certain traits follow specified lines of descent such that, after one generation, related families may each have different characteristics. Further, the authors hold that feeble-mindedness is not a biological, but rather a legal or sociological term. They argue that subvarieties of feeble-mindedness are inherited as unit characters and conclude that defectives should not be allowed to marry, that there is an enormous cost of allowing defectives to propagate, and that the outcome of children depends on the comparative weight of heredity and environmental influences. The nature-nurture issue is addressed by investigating whether the children of defectives improved when removed from their homes. Some do and even children who are raised in bad environments will occasionally do well. 9 tables, 7 figures.