An important advance in the legal definition of criminal responsibility of deficients should be made by avoiding all subtle questions of psychological analysis such as would be involved in deciding, for example, under the New York statute whether the accused “was laboring under such a defect of reason as not to know the nature and quality of the act he was doing or know the nature of the act as wrong.” Obsolete legal descriptions could easily be cleared away by adopting the statement of the law suggested by the Committee of the Institute of Criminal Law and Criminology for criminal responsibility and insanity. In substance such a law would then state that the accused was mentally deficient “so as not to be responsible ... for his acts or omissions at the time when the act or omission charged was made.” The New York law places an emphasis on knowledge which should be placed on will, only one feature of which is an understanding of the situation. 4. What should be the aim in the care and control of deficients and delinquents after diagnosis also depends upon a proper understanding of the causes of these conditions. We have summarized some of the best and most recent investigations in which a notable advance toward solving this problem has been made by means of the correlation method. This has proved to be a new and vigorous force for directing social progress. By no other method have we approached so near the solution of the cause of delinquency. It enables us to restate the problem of criminality as mainly a problem in the treatment of a hereditary criminal diathesis in which mental deficiency is the largest factor. These recent scientific measurements have deprived neither the eugenist nor the euthenist of the opportunity for service. There is plenty of congenial work to be done by those whose sympathies may exaggerate the influence of heredity, contagion, or training. As in the control of tuberculosis, so with the diathesis of delinquency, some effect is produced by predisposition, by training, and by external influences. Unless the present evidence, however, is outweighed by improved data obtained in the future, the most strategic point for attacking persistent delinquency is through the relation to deficiency, with heredity holding the heights.