2. The Measurement of Assortative Mating'
The precise meaning of the term "assortative mating" may perhaps be made a little clearer in the process of explaining how the similarity or dissimilarity of husband and wife is measured.
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 a definite type; the resemblance would then be perfect and the correlation, as we call it, would be expressed by a coefficient of 1.
Fortunately, the meaning of correlation can be illustrated by a character for which the reader knows that there is a high degree of assortative mating. The table shows the age of bride and groom in 2,500 Chicago couples. The swarm of figures showing the frequencies of different combinations spreads diagonally across the table, demonstrating that while men or women of any class marry consorts of varying ages, there is a pronounced tendency for individuals of the same relative age to mate. From such a table the statistician calculates the equation to a straight line (or to a curve of a higher order) which describes approximately the change in the average age of brides associated with increase in the age of the grooms. The diagram (Fig. 1) shows that the agreement of the theoretical line with the empirical means is very close indeed. Or he may express the closeness of correlation quite independently of the absolute values of the two characters on the scale of
- From a paper by F. E. Lutz, "Assortative Mating in Man," Science, N. S., Vol. 22, pp. 249-250, 1905.
- Since women marry younger than men, relative, not absolute, age must be specified.
- Such differences as occur are probably due to errors of random sampling.