There is a type of social psychology which finds the explanation of man's social behaviour in a hypothetical “disposition” of his mind. This motive-complex is conceived as specialized for the function of adapting conduct to social life, and as being in itself relatively closely integrated, developing and functioning as a whole. Of this hypothetical “gregarious instinct” McDougall goes so far as to say: “For it is highly probable that instinctive dispositions are Mendelian units” (Journ of Abn. Psych, and Soc. Psych., vol. xvi, p. 316). This plainly suggests that the unity of the social disposition (its existence as a discrete factor in development) is to be regarded as antedating experience—that it is an ultimate datum for psychology not susceptible to analysis, and is not a derivative of any other known motive such as “love,” “fear,” or “hope of reward.” This “instinct” interpretation of social behaviour has been criticized on many grounds (as unfruitful for psychology and incompatible with biological fact); but of course the demonstration of a Mendelian transmission of the social disposition would compel us to regard it as an element of character. Our conception of mental development and of the “socialization” of the individual, of the relative significance of upbringing as compared with organic endowment and our whole psycho-pathology depend upon our acceptance or rejection of McDougall's view. If he is right in regard to the germinal “unit” determination of the social disposition, criminological studies should offer verification. I propose, therefore, to consider how far we are justified in regarding moral insanity and moral imbecility as true “morbid entities.”