It is clearly, however, not due to any intensity of this strain that we find the incidence of insanity in men of genius, as illustrated, for example, by senile dementia, so much more marked than its incidence on their parents. There is another factor to be invoked here: convergent morbid heredity. If a man and a woman, each with a slight tendency to nervous abnormality, marry each other, there is a much greater chance of the offspring manifesting a severe degree of nervous abnormality than if they had married entirely sound partners. Now both among normal and abnormal people there is a tendency for like to mate with like. The attraction of the unlike for each other, which was once supposed to prevail, is not predominant, except within the sphere of the secondary sexual characters, where it clearly prevails, so that the ultra-masculine man is attracted to the ultra-feminine woman, and the feminine man to the boyish or mannish woman. Apart from this, people tend to marry those who are both psychically and physically of the same type as themselves. It thus happens that nervously abnormal people become mated to the nervously abnormal. This is well illustrated by the British men of genius themselves. Although insanity is more prevalent among them than among their parents, the same can scarcely be said of them in regard to their wives. It is notable that the insane wives of these men of genius are almost as numerous as the insane men of genius, though it rarely happens (as in the case of Southey) that both husband and wife go out of their minds. But in all these cases there has probably been a mutual attraction of mentally abnormal people.