2. If men who were above the average height always selected as brides women who were equally above the average height and short men selected similarly, the coefficient of correlation between height in husbands and wives would be 1, and there would thus be perfect assortative mating. If only one half of the men who differed from the average height always married women who similarly differed and the other half married at random, there would be assortative mating for height, but it would not be perfect: the coefficient would only be half as great as in the first case, or .5. If on the other hand (as is indeed the popular idea) a tall man tended to marry a woman who was shorter than the average, the coefficient of correlation would be less than 0; it would have some negative value. Actual measurement shows that a man who exceeds the average height by a given amount will most frequently marry a woman who exceeds the average by a little more than one-fourth as much as her husband does. There is thus assortative mating for height, but it is far from perfect. The actual coefficient given by Karl Pearson is .28. In this case, then, the idea that "unlikes attract" is found to be the reverse of the truth. If other traits are measured, assortative mating will again be found. Whether it be eye color, hair color, general health, intelligence, longevity, insanity, or congenital deafness, exact measurements show that a man and his wife, though not related by blood, actually resemble each other as much as do uncle and niece, or first cousins. In some cases assortative mating is conscious, as when two congenitally deaf persons are drawn together by their common affliction and mutual possession of the sign language. But in the greater number of cases it is wholly unconscious. Certainly no one would suppose that a man selects his wife deliberately because her eye color matches his own; much less would he select her on the basis of resemblance in longevity, which can not be known until after both are dead. Sigmund Freud and Ernest Jones explain such selection by the supposition that a man's ideal of everything that is lovely in womankind is based on his mother. During his childhood, her attributes stamp themselves on his mind as being the perfect attributes of the female sex; and when he later falls in love it is natural that the woman who most attracts him should be one who resembles his mother. But as he, because of heredity, resembles his mother, there is thus a resemblance between husband and wife. Cases where there is no resemblance would, on this hypothesis, either be not love matches, or else be cases where the choice was made by the woman, not the man. Proof of this hypothesis has not yet been furnished, but it may very well account for some part of the assortative mating which is so nearly universal.