It will be necessary at the outset to draw a clear distinction between the _literature of opinion_ and the _literature of fact_. The literature of opinion includes all written statements, made by scientific men and others, not based on experimental evidence. The literature of opinion on the subject of sex differences in mental traits is voluminous. It appears in the writings of Nietzsche, Schopenhauer, Mill, Moebius, and others. By the literature of fact is meant those written statements based on experimental data, which have been obtained under carefully controlled conditions, and which may be verified by anyone competent to understand and criticize them. In this chapter we shall seek the answers to the propounded questions in the literature of fact alone, neglecting as irrelevant to the discussion the entire literature of opinion. Since the discussion is limited to the literature of fact, it will of necessity refer only to literature of a comparatively recent date. Until about fifteen years ago there had been practically no attempt to collect precise data on the subject of sex differences in mental abilities. Before experimental data were sought the hypothesis was accepted that human females are, by original nature, different from and inferior to human males, intellectually. The factor of sex determined everything; the way to discover whether a given individual was capable of any given intellectual task was not to let the individual undertake the task and to judge by the result, but to indicate the sex of the person in question. Coincident with the intense controversy which rose in the nineteenth century over the higher education of women, a number of statistical studies were carried on by the questionnaire method. These were followed by experimental studies, and at the opening of the twentieth century several experiments were being made to investigate the matter of sex differences in intellect. About this time also the idea began to gain headway that whatever differences exist between the sexes as we find them in the world may be due to training and not to original nature; and it began to be pointed out that this aspect of the matter complicates even experimental investigation in ways difficult to control. We may speak here of the experiments on brain weight which were published and much discussed about thirty years ago. Romanes, among others, insisted that the male brain was, on the average, several grams heavier than the female brain, and for a time it was supposed that the fact of innate female inferiority had been thus satisfactorily established. However, it was later demonstrated that relative to total body weight the female brain is as heavy as the male brain. It was also found that no positive correlation can be established between brain weight and intellect.