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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.
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