The principal causes for individual differences are sex, remote ancestry, near ancestry, maturity, and training.  The question to be answered in the discussion of each of these causes is how important a factor is it in the production of differences and just what differences is it responsible for.  That men differ from women has always been an accepted fact, but exact knowledge of how much and how they differ has, until recent years, been lacking.  Recently quantitative measurement has been made by a number of investigators.  In making these investigations two serious difficulties have to be met.  First, that the tests measure only the differences brought about by differences in sex, and not by any other cause, such as family or training.  This difficulty has been met by taking people of all ages, from all sorts of families, with all kinds of training, the constant factor being the difference in sex.  The second difficulty is that of finding groups in which the selection agencies have been the same and equally operative.  It would be obviously unfair to compare college men and women, and expect to get a fair result as to sex differences, because college women are a more highly selected group intellectually than the college men.  It is the conventional and social demands that are primarily responsible for sending boys to college, while the intellectual impulse is responsible to a greater extent for sending girls.  Examination of children in the elementary schools, then, gives a fairer result than of the older men and women.  The general results of all the studies made point to the fact that the differences between the sexes are small.  Sex is the cause of only a small fraction of the differences between individuals.  The total difference of men from men and women from women is almost as great as the difference between men and women, for the distribution curve of woman’s ability in any trait overlaps the men’s curve to at least half its range.  In detail the exact measurements of intellectual abilities show a slight superiority of the women in receptivity and memory, and a slight superiority of the men in control of movement and in thought about concrete mechanical situations.  In interests which cannot be so definitely measured, women seem to be more interested in people and men in things.  In instinctive equipment women excel in the nursing impulse and men in the fighting impulse.  In physical equipment men are stronger and bigger than women.  They excel in muscular tests in ability to “spurt,” whereas women do better in endurance tests.  The male sex seems on the whole to be slightly more variable than the female, i.e., its curve of distribution is somewhat flatter and extends both lower and higher than does that of the female; or, stated another way, men furnish more than their proportion of idiots and of geniuses.