Some authorities declare that the results of inbreeding are similar in the human species, that the marriages of near kin are less fruitful than the average. Darwin writes in this connection: “With regard to human beings, the question whether breeding in-and-in is also deleterious, will probably never receive a direct answer, for man reproduces his kind so very slowly, and cannot be made the object of experiment. The very general disinclination of nearly all races to the marriage of near kin, which has existed from the very earliest times, is of weight in relation to this question. Indeed we appear almost justified in applying to the human race the experience gained by experiment on the higher mammals.”
Darwin’s assumption regarding the effect upon fertility of the marriage of near kin in the human species, cannot, however, be accepted without qualification. In ancient times there was no uniformity of opinion on this topic. It is well known that among the Phœnicians, a son might marry his mother, and a father his daughter; and among the ancient Arabs it was the legal duty of the son to marry his widowed mother. Moses, on the contrary, forbade marriages between parents and children, between brothers and sisters, also marriage with a father’s sister, with a wife’s mother, and with an uncle’s widow.
Darwin considered that the marriage of first cousins was not unfavourable to fertility. Of 97 such marriages, 14 were sterile, whilst of 217 marriages of those not akin, 35 were sterile; the percentage in both cases being almost identical. Mantegazza, who regards kinship in marriage as unfavourable to fertility, found nevertheless that among 512 marriages of near kin, only 8 to 9% were sterile. It is widely believed that the dying out of many aristocratic families is dependent on the inbreeding so common in this class—but it must be admitted that scientific evidence in support of this belief is lacking. Incest in the human species may certainly result in fertilization. Among the Jews, marriages of near kin are very common, and often prove extremely fruitful.
Göhlert made a statistical investigation of the fertility of the reigning families of Europe, in order to throw light on this question. In the Capet dynasty, 118 marriages of near kin took place, and of these 41 were sterile; in the Wettin dynasty (Saxony), there were 28 such marriages, of which 7 were sterile, and 1 produced one child only; in the Wittelsbach dynasty (Bavaria), 29 such marriages, of which 9 were sterile, and 3 produced only one child each. Thus of 175 marriages of near kin, 57, or 32.6% remained sterile. Further, in the Habsburg-Lothringen dynasty, of 110 marriages, 25 were marriages of near kin, and of these 33% remained sterile.
It has been assumed since the days of antiquity that temperament and constitution exercise some influence on fertility. Hippocrates, Soranus, and Diokles, are among the ancient authors who refer to this matter. Soranus says very justly: “Since most marriages are contracted, not from love, but for the procreation of children, it is irrational, when choosing a wife, to have regard, not to her probable fruitfulness, but instead of this to the social position and the wealth of her parents.”
It would appear that a certain dissimilarity in physical constitution and temperament between husband and wife is favourable to the fertility of the marriage. For instance, a vivacious, dark husband, and a lethargic, fair wife, are better suited to one another than a husband and wife both extremely active, or both of extremely phlegmatic temperament.
Toussaint Loua published the following figures regarding the fertility of the women of the various countries of Europe:
| Country. | Number of births per hundred inhabitants. | Fertility of Women Between the Ages of 15 and 45 Years. | ||
|---|---|---|---|---|
| Married. | Unmarried. | Average. | ||
| Hungary | 4.94 | 17.8 | ||
| Russia | 4.12 | 20.5 | ||
| Austria | 3.93 | 16.4 | ||
| Germany | 3.77 | 34.8 | 2.9 | 17.7 |
| Italy | 3.67 | 28.8 | 2.4 | 16.1 |
| Holland | 3.67 | 35.3 | 1.0 | 16.0 |
| Finland | 3.63 | 15.8 | ||
| England | 3.58 | 29.7 | 1.6 | 15.5 |
| Scotland | 3.53 | 32.8 | 2.5 | 15.8 |
| Belgium | 3.25 | 33.7 | 1.8 | 14.8 |
| Denmark | 3.12 | 28.5 | 2.8 | 14.4 |
| Roumania | 3.12 | 13.5 | ||
| Norway | 3.10 | 29.3 | 2.2 | 14.0 |
| Sweden | 3.05 | 29.1 | 2.5 | 13.7 |
| Switzerland | 3.04 | 29.7 | 1.1 | 13.1 |
| Greece | 2.96 | 13.2 | ||
| Ireland | 2.69 | 29.8 | 0.5 | 12.3 |
| France | 2.63 | 20.3 | 1.8 | 11.6 |
In towns, conjugal fertility is less, extra-conjugal fertility greater, than in the country. An increase in factory labour gives rise to an increase in the population, but to a decline in the vitality of the offspring; that is to say, it causes a quantitative increase, and a qualitative decrease, in fertility. An increase in agricultural labour has precisely the opposite effect. The influence of war upon fertility is unfavourable both quantitatively, and qualitatively. According to Tschouriloff, the introduction of universal military service, by withdrawing for a time all the most vigorous men from domestic life, tends to diminish fertility. Extensive emigration from a country in which the soil is fertile, and where the vital conditions are generally favourable, is stated by Bertillon to cause an increased fertility in the mother country; he further states that an increase in the number of the proprietors of the soil is followed by diminished fertility, and vice versa.
Prostitutes show as a rule a very low fertility. According to the data of Tarnowskaja, the fertility of prostitutes in Russia is 34%, whilst married women of similar ages in Russia exhibit a fertility of 51.8%. Gurrieri found 60% of prostitutes childless.