CHAPTER XI
THE IMPROVEMENT OF SEXUAL SELECTION
"Love is blind" and "Marriage is a lottery," in the opinion of proverbial lore. But as usual the proverbs do not tell the whole truth. Mating is not wholly a matter of chance; there is and always has been a considerable amount of selection involved. This selection must of course be with respect to individual traits, a man or woman being for this purpose merely the sum of his or her traits. Reflection will show that with respect to any given trait there are three ways of mating: random, assortative and preferential.
1. Random mating is described by J. Arthur Harris[95] as follows:
"Suppose a most highly refined socialistic community should set about to equalize as nearly as possible not only men's labor and their recompense, but the quality of their wives. It would never do to allow individuals to select their own partners—superior cunning might result in some having mates above the average desirability, which would be socially unfair!
"The method adopted would be to write the names of an equal number of men and women officially condemned to matrimony on cards, and to place those for men in one lottery wheel and those for women in another. The drawing of a pair of cards, one from each wheel, would then replace the 'present wasteful system' of 'competitive' courtship. If the cards were thoroughly shuffled and the drawings perfectly at random, we should expect only chance resemblances between husband and wife for age, stature, eye and hair color, temper and so on; in the long run, a wife would resemble her husband no more than the husband of some other woman. In this case, the mathematician can give us a coefficient of resemblance, or of assortative mating, which we write as zero. The other extreme would be the state of affairs in which men of a certain type (that is to say men differing from the general average by a definite amount) always chose wives of the same type; the resemblance would then be perfect and the correlation, as we call it, would be expressed by a coefficient of 1."
If all mating were at random, evolution would be a very slow process. But actual measurement of various traits in conjugal pairs shows that mating is very rarely random. There is a conscious or unconscious selection for certain traits, and this selection involves other traits because of the general correlation of traits in an individual. Random mating, therefore, need not be taken into account by eugenists, who must rather give their attention to one of the two forms of non-random mating, namely, assortative and preferential.
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.