Abstract
Mental defect is a state of both intellectual retardation and social incompetence. In considering the differential diagnosis of mental defect, it becomes necessary to review conditions where only one of these factors may be present. But since intellectual retardation can be simulated by specific defects of visual, auditory and speech functions, it is also necessary to bear these further conditions in mind. Hence in differentiating mental defect, attention must be paid to three separate groups: conditions characterized by intellectual inadequacy alone, specific defects which may simulate intellectual inadequacy, and conditions characterized by social incompetence alone. Conditions characterized by intellectual inadequacy alone include uncomplicated intellectual retardation, dullness, and backwardness. Specific defects impairing visual, auditory and speech functions include congenital blindness, congenital word-blindness, congenital deafness, congenital word-deafness, elective mutism, delayed development of speech and congenital motor aphasia. Where social incompetence is the outstanding feature there is a risk of mistaking for mental defect, behaviour which is primarily delinquent, convulsive or psychotic in character.