In this paper Dr. Audiffrent deals with the mental conditions that commonly prevail in cases of infanticide, and with the influence which such conditions should exert on the social and legal attitude towards infanticide. With special reference to a case in which a young woman escaped from an asylum and killed herself and her child, the author considers the peculiar psychological conditions of pregnancy and the puerperal period, and suggests that there are biological reasons why a woman in whom, from whatever cause, abnormal mental conditions are set up, should be impelled to destroy her child. This impulse is not confined to the human female, but is found throughout nature, and leads, for instance, a bird whose young have been confined in a cage to enter the cage and kill them, while many animals, if interfered with after parturition, will kill or eat their young. The author considers that this tendency is recognised outside civilisation, and that it lies at the basis of the wide-spread belief that the mother is the mistress of the child she has carried in her womb, that it belongs to her like any other object that she produces, and that no one has any right to contest her rights over her infant's life.