The family study views feeble-mindedness as hereditary and contends that such social inefficiency cannot be dealt with from the standpoint of the criminologist alone. Feeble-minded persons are defined as those who are incapable of competing on equal terms with normal persons because of mental defects existing from birth or from an early age. The families are characterized by pauperism, alcoholism, criminal behavior, and immorality due to their mental deficiences. The stories of several families interwoven by marriage demonstrate family solidarity among the feeble-minded and the perpetuation of defective traits in successive generations. The societal costs of feeble-mindedness are discussed, and it is contended that institutionalization alone provides the eugenic segregation necessary to stop the flow of degeneracy from one family generation to another. 10 figures.