The phrase 'mental deficiency' is applicable not only to defect in intelligence, but also to defect in temperament and character. The importance of intellectual deficiency in crime has been exaggerated. Burt finds 7 per cent. of his cases of juvenile delinquency mentally deficient. After puberty more girls than boys are defective and delinquent, but before puberty it is commoner in boys. Special classes for the backward child would result in immense diminution in crime. In considering deficiency in character, Burt agrees with East, that in moral imbecility there may be no intelligence defect. He disagrees with the popular medical and legal opinion which assumes the existence of an innate moral sense and