Fig . 35.—Given a population divided in two equal parts, one of which produces a new generation every 25 years and the other every 33⅓ years, the diagram shows that the former group will outnumber the latter two to one, at the end of a century. The result illustrated is actually taking place, in various groups of the population of the United States. Largely for economic reasons, many superior people are postponing the time of marriage. The diagram shows graphically how they are losing ground, in comparison with other sections of the population which marry only a few years earlier, on the average. It is assumed in the diagram that the two groups contain equal numbers of the two sexes; that all persons in each group marry; and that each couple produces four children.
It will be noted that these studies merely show that the brighter and prettier girls were preferred by men as a class. If the individual men whom the girls married had been studied, it would probably have been found that the mating was also partly assortative.
If the choice of a life partner is to be eugenic, random mating must be as nearly as possible eliminated, and assortative and preferential mating for desirable traits must take place.
The concern of the eugenist is, then, (1) to see that young people have the best ideals, and (2) to see that their matings are actually guided by these ideals, instead of by caprice and passion alone.