There were 41 matings where both parents were feeble-minded. They had 222 feeble-minded children, with two others that were considered normal. These two are apparent exceptions to the law that two feeble-minded parents do not have anything but feeble-minded children. We may account for these two exceptions in one of several ways. Either there is a mistake in calling them normal, or a mistake in calling the parents feeble-minded; or else there was illegitimacy somewhere and these two children did not have the same father as the others of the family. Or we may turn to the Mendelian law and we discover that according to that law there might be in rare instances such a combination of circumstances that a normal child might be born from two parents that function as feeble-minded. For practical purposes it is, of course, pretty clear that it is safe to assume that two feeble-minded parents will never have anything but feeble-minded children.
Again, we find that there were eight cases where the father was feeble-minded and the mother normal, and there were ten normal children and ten defective.
There were twelve cases where the father was normal and the mother feeble-minded, with seven feeble-minded children and ten normal. Both of these are in accordance with Mendelian expectations.
We further find that in the cases where one parent was feeble-minded and the other undetermined, the children were nearly all feeble-minded, from which we might infer that the probabilities are great that the unknown parent was also feeble-minded.
We shall not go further into this matter in the present paper, but leave the detailed study of this family from the Mendelian standpoint for further consideration, when we take up the large amount of data which we have on three hundred other families. Enough is here given to show the possibility that the Mendelian law applies to human heredity. If it does, then the necessity follows of our understanding the exact mental condition of the ancestors of any person upon whom we may propose to practice sterilization.
From all of this the one caution follows. At best, sterilization is not likely to be a final solution of this problem. We may, and indeed I believe must, use it as a help, as something that will contribute toward the solution, until we can get segregation thoroughly established. But in using it, we must realize that the first necessity is the careful study of the whole subject, to the end that we may know more both about the laws of inheritance and the ultimate effect of the operation.
CONCLUSION AND RÉSUMÉ
The Kallikak family presents a natural experiment in heredity. A young man of good family becomes through two different women the ancestor of two lines of descendants,—the one characterized by thoroughly good, respectable, normal citizenship, with almost no exceptions; the other being equally characterized by mental defect in every generation. This defect was transmitted through the father in the first generation. In later generations, more defect was brought in from other families through marriage. In the last generation it was transmitted through the mother, so that we have here all combinations of transmission, which again proves the truly hereditary character of the defect.
We find on the good side of the family prominent people in all walks of life and nearly all of the 496 descendants owners of land or proprietors. On the bad side we find paupers, criminals, prostitutes, drunkards, and examples of all forms of social pest with which modern society is burdened.
From this we conclude that feeble-mindedness is largely responsible for these social sores.