For purposes of discussion, sex differences are divided into three groups: a. primary anatomical differences, b. secondary sex characters, and c. psychological differences. It is noted that the last two groups are determined by the action of endocrine secretions. It is suggested that some psychological differences must be due to tertiary rather than secondary sex characters, resulting from self-consciousness of sex. Psycho-analysis and other methods have been unable to decide how far observable differences are innate and how far acquired; we know, however, that the