This paper, which is a contribution to a discussion held by the Medical Section of the British Psychological Society in March, 1940, lays emphasis upon the importance of selecting lines of investigation along which psychologists and sociologists can co-operate. Small social units like the family (which is what mostly interests psycho-analysts) are not the proper groups upon which to base a study of the changes effected in the individual by war from a 'peace ego' to a 'war ego', to quote Freud. The conflicts engendered by war concern groups of a larger kind, such as pacifists or communists or religious bodies; and it is the relation of these to each other, and, more especially, of the individual to them, that should be the principal object of our enquiry.