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.
- 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