Genetic investigations, particularly research on twin development, have been thought to offer a clue for the detection of hereditary and environmental factors causing mental defects in twins. Recent studies on this subject have been made by a number of psychiatrists. Luxenberger began these investigations in 1928, and they have been continued by Lange, Kranz, Stumpfl and Borgström on criminal twins, by Conrad on epileptic twins and by Smith and Juda on idiot twins. A recent study on epileptic twins and on the various psychotic types has been made by Rosanoff and his co-workers. Erik Essen-Möller in his monograph on psychiatric studies on a series of twins sets out to determine the possible association between certain external diseases and symptoms and innate, hereditary factors by studying almost exclusively a series of identical twins. However, a group of fraternal twins was investigated to provide a control for comparison with the identical twins.