The application of the principles of heredity to our subject of Eugenics is of such great importance that it is reserved for separate consideration in the next chapter. We may, therefore, devote the remainder of this chapter to the consideration of data of another kind, which are commonly treated by this same method of determining correlation coefficients between two sets of varying phenomena in order to determine whether there is any actual relation between them or not. This will serve to illustrate the use of this method.
We shall turn then to the subject of differential or selective fertility in human beings and consider its relation to Eugenics. As a starting point we may take the self-evident statement that a group of organisms will tend to maintain constant characteristics through successive generations only when all parts of the group are equally fertile. If exceptional fertility is associated with the presence or absence of any characteristic the number of individuals with or without that trait will either increase or diminish in successive generations, and the character of the distribution of the group as a whole will gradually become altered, the average moving in the direction of the more fertile group. Or if infertility is so associated, then the average of the whole group moves away from that condition. Eugenically, then, we should ask whether in human society there is at present any such association of superfertility or infertility with desirable or undesirable traits. It is obviously the aim of Eugenics to bring about an association of a high degree of fertility with desirable traits and a low degree of fertility with undesirable characteristics.
First, let us look at certain data gathered relative to the size of the family in both normal and pathological stocks (Table II). In order that a stock or family should just maintain its numbers undiminished through successive generations and under average conditions, at least four children should be born to each marriage that has any children at all. The table given shows clearly what stocks are maintaining, what increasing, and what diminishing their numbers.
| Authority. | Nature of Marriage. (Reproductive period.) | No. in Family. | |
|---|---|---|---|
| Deaf-mutes, England | Schuster | Probably complete | 6.2 |
| Deaf-mutes, America | Schuster | Probably complete | 6.1 |
| Tuberculous stock | Pearson | Probably complete | 5.7 |
| Albinotic stock | Pearson | Probably complete | 5.9 |
| Insane stock | Heron | Probably complete | 6.0 |
| Edinburgh degenerates | Eugenics Lab | Incomplete | 6.1 |
| London mentally defective | Eugenics Lab | Incomplete | 7.0 |
| Manchester mentally defective | Eugenics Lab | Incomplete | 6.3 |
| Criminals | Goring | Completed | 6.6 |
| English middle class | Pearson | 15 years at least, begun before 35 | 6.4 |
| Family records—normals | Pearson | Completed | 5.3 |
| English intellectual class | Pearson | Completed | 4.7 |
| Working class N. S. W. | Powys | Completed | 5.3 |
| Danish professional class | Westergaard | 15 years at least | 5.2 |
| Danish working class | Westergaard | 25 years at least | 5.3 |
| Edinburgh normal artisan | Eugenics Lab | Incomplete | 5.9 |
| London normal artisan | Eugenics Lab | Incomplete | 5.1 |
| American graduates | Harvard | Completed | 2.0 |
| English intellectuals | Webb | Said to be complete | 1.5 |
All childless marriages are excluded except in the last two cases. Inclusion of such marriages usually reduces the average by 0.5 to 1.0 child.
This subject has been investigated recently in a rather extensive way by David Heron, for the London population. Heron concentrated his attention upon the relation of fertility in man to social status. He used as indices to social status such marks as the relative number of professional men in a community, or the relative number of servants employed, or of lowest type of male laborers, or of pawn-brokers; also the amount of child employment pauperism, overcrowding in the home, tuberculosis, and pauper lunacy. Twenty-seven metropolitan boroughs of London were canvassed on these bases, which are certainly significant, though not infallible, indices to the character of a community. His results are shown in the briefest possible form in Table III.
| Correlation Coefficient. | |
|---|---|
| With number of males engaged in professions | -.78 |
| With female domestics per 100 females | -.80 |
| With female domestics per 100 families | -.76 |
| With general laborers per 1,000 males | +.52 |
| With pawnbrokers and general dealers per 1,000 males | +.62 |
| With children employed, ages 10 to 14 | +.66 |
| With persons living more than two in a room | +.70 |
| With infants under one year dying per 1,000 births | +.50 |
| With deaths from pulmonary tuberculosis per 100,000 inhabitants | +.59 |
| With total number of paupers per 1,000 inhabitants | +.20 |
| With number of lunatic paupers per 1,000 inhabitants | +.34 |
This table gives the results of the calculation of coefficients of correlation between the birth rates and the conditions enumerated. We may just recall that this coefficient is a measure of the regularity with which the changes in two varying conditions or phenomena are associated: and further that a coefficient of 1.0 indicates perfectly regular association, 0.75 a very high degree of regularity. The first line of the table then, for example, means that when these twenty-seven districts were sorted out, first, with reference to the number of professional men dwelling in them, and then with reference to their respective birth rates, there was found a very high degree of regularity (coefficient of correlation=-.78) in the association of these two conditions—birth rate and number of professional men. Here is a very close relation, but, the sign of the coefficient is negative. The significance of this negative sign is that among the communities studied those where the number of professional men is the larger show always, at the same time, the lower birth rates. Coming to the second line of the table, it seems fair to assume that the number of servants employed in a district in proportion to the total number of residents or families there, gives a fairly though not wholly satisfactory indication of the social character of the community. Measurement of the actual relation between the proportional number of servants employed in a community and the birth rate in that community, gave practically the same result as in the case of the number of professional men. The more servants employed in a district the lower its birth rate. Two methods of measuring this relation gave essentially the same result; comparison of the birth rate with the ratio of domestics, first to the number of families, second to the number of females, gave -.76 and -.80 respectively—very high coefficients and both negative.
But the sign changes and becomes positive when we come to other comparisons. When we count the relative number of pawnbrokers and general dealers, of "general laborers" (that is, men without a trade and without regularity of occupation and employment), of employed children between the ages of ten and fourteen, of persons living more than two in a room, when we consider the infant death rate, the death rate from pulmonary tuberculosis, and the relative number of paupers,—then we find the signs of the coefficients are all positive, and on the average the coefficients are more than 0.50—a moderate to high degree of regularity of the relation. The districts characterized by the larger numbers of such individuals or by higher death rates of these kinds, are at the same time the districts where the birth rates are the higher.