In recent years the approach to psychiatry has become more and more biological. The point of interest is shifting from the kind of mental illness which a person has, to the kind of person who has the illness. The greatest stimulus in this direction to psychiatry in the last few years was undoubtedly the publication, in the year 1921, of Kretschmer's Körperbau und Charakter. Indeed, the stimulus was so violent that nowadays hardly any psychiatric field (not to mention other branches of the biological sciences) has escaped exploration along Kretschmer's lines; and the vast bibliography which has piled up is enough to appal the student of constitutional biology and kindred subjects. For some reason, surprisingly little work has been done on the constitution and temperament of the weak-minded. The object of this present research is to fill the lacuna in some small measure. The neglect of this field of study is surprising, for the reason that weak-mindedness is classified by Kraepelin, the father of modern psychiatry (Psychiatrische Klinik, Barth, Leipzig, 1921, Bd. i, S. 23), in the group of the congenital mental disorders; and a congenital anomaly should surely be expected to provide interesting material for biological research. The present writer, then, approached his subject without prejudice and without a thesis. It was his business to study a group of congenitally weak-minded people, and observe and record their physical and temperamental anomalies.