It thus appears that stress should be laid upon the advantages of outbreeding rather than upon the supposed ill effects of inbreeding. Nevertheless we must admit that there exist biological factors of such a kind as to be capable of influencing the psychological attitude towards incest in the way that Westermarck's theory requires. But although the theory that incest prohibition is due to natural selection working on the relative disadvantages of inbreeding may be correct so far as it goes, this does not absolve us from the duty of looking elsewhere for other factors which may have worked in the same direction. For it appears very doubtful whether the factors we have just been considering can be regarded as an adequate explanation of incest prohibition as we know it. If it is the advantages of outbreeding rather than the disadvantages and can scarcely account for the intensity of the incest prohibition of inbreeding that are potent; if the evil effects of inbreeding are so relatively slight as to leave room for doubt as to their nature and even the fact of their existence; if they are of such kind as to leave healthy stocks but little if at all affected and to become serious only after long continuance without admixture of fresh blood from outside (a state of affairs that can but rarely have occurred); and if they are liable to be counteracted by a change of locality or of life's conditions (which must sometimes have occurred, especially among nomadic peoples): then it is not easy to understand how such a widespread and powerful human characteristic as the aversion to incest can have arisen solely as the result of natural selection, working through the bad effects of incest or the superior advantages of outbreeding. The largeness of the result would be manifestly out of proportion to the cause, and it would seem that although we may allow some considerable influence to this factor, we have to admit that it must be supplemented by some other cause or causes of appreciable magnitude[253].
(4) According to one type of explanation (e. g. that Influence of non-sexual factors held by Wundt[254]) the horror of incest is not the cause of exogamy, but the consequence of it—the origin of exogamy itself being due to some other influences only indirectly connected with the sex life. Of such theories of the origin of exogamy a number have been put forward by eminent authorities.
Thus McLennan[255], the discoverer of both Totemism and Exogamy, held the view that exogamy was a consequence of the wide prevalence of marriage by capture; this latter being itself a result of the preponderance of the male sex over the female in primitive communities and of the general condition of hostility existing between neighbouring tribes. Herbert Spencer[256] similarly thought that exogamy arose from marriage by capture, men belonging to successful tribes nearly always acquiring their brides in this way, so that it eventually became a disgrace to marry within the tribe. Lord Avebury[257], believing that group marriage or promiscuity was the rule in primitive society, suggests that women taken in war belonged to their individual captor, marriage in a narrow sense being thus exogamous from the start. Kohler[258] believes on the contrary, that exogamy arose as a means of bringing about inter-tribal friendships or alliances. Andrew Lang[259] has suggested that exogamy is due to the fact that the younger brothers of a family were frequently driven out by the stronger and older ones in order to ward off any want that might arise from the living together of a large number of brothers and sisters—the younger brothers being then obliged to marry outside the family group.
Now all these factors—and others too perhaps—may very Such factors cannot fully explain exogamy well have had their influence in bringing about the practice of exogamy: in particular, it would seem that the difficulty of supporting a large family living together under really primitive conditions would be very likely to have had the effect of driving the younger members of the family away from the immediate circle of their parents. Nevertheless there appears to be a pretty general agreement that none of these theories affords a complete and sufficient account of the origin of exogamy. The conditions postulated by McLennan's theory (shortage of women and frequent wars between neighbouring groups) are by no means universally found among primitive peoples, and even if there exists the required preponderance of men over women, it is by no means obvious why the men should refuse to avail themselves of such fellow-tribeswomen as they could find. On Spencer's view, it is difficult or impossible to account for the existence of exogamy among all the tribes of a given area, since in constant warfare there must be some which are vanquished, and among these endogamy rather than exogamy should be the rule. Lord Avebury's theory rests on the assumption that communal marriage was the original condition of mankind—an assumption that is now abandoned by many of the best authorities. Kohler's view can scarcely explain how the objection to sexual unions within the tribe should have come to apply not only to marriage but (equally strongly) to less regular or purely temporary connections. Andrew Lang's theory similarly fails to explain why the rule of exogamy is made to apply to the elder members of the family with the same force as to the younger[260].
Moreover, even supposing that the existence of such an institution as exogamy could be in itself satisfactorily accounted for on some such grounds as those advanced by theories of this kind, it is at once evident that we have here no adequate explanation of the strictness with which the system is enforced, The psychological nature of the incest prohibition shows the insufficiency of these influences the severe penalties that are exacted for infringement of its rules (which is very often punishable by death), the intense nature of the incest horror generally, and the fact that this horror persists even where, as in civilised countries, there is no organised system of exogamy in the technical sense. The psychological researches of Freud and his followers would seem to have shown conclusively that this intense aversion to incest (like all repugnances and taboos of a similar kind) is the negative expression of a correspondingly intense desire for the forbidden thing, and therefore no explanation which neglects to take into consideration this desire can be regarded as even approximately satisfactory. If the aversion to incest had arisen merely as a consequence of the age-long practice of exogamy—which itself is due to other causes—there would be no reason why this aversion should be intimately connected with the positive tendency to incest or of sufficient strength to overcome this tendency, with the powerfulness of which we are now well acquainted. In view of the existence of this strong tendency to incest (which was not appreciated before the work of Freud), it seems no longer possible to maintain, either that exogamy can have arisen independently of the counter-impulse to repress this tendency, or that this counter-impulse, which finds its psychic expression in that incest horror so generally observable both in primitive and cultured man, can be satisfactorily explained as the result of any institution or custom itself unconnected with the tendency to incest.
(5) Turning to factors that have in general received less The biological absurdity of parent-child incest explicit recognition at the hands of the authorities who have written on the subject, we may note first that as regards the most fundamental type of incest as revealed by psycho-analytic study—that of parent and child—there is involved a sort of biological absurdity, which may well have been to some extent instrumental, through the agency of natural selection, of bringing about that inhibition of the incestuous tendencies for which we have to account. Parents and children necessarily differ considerably in age, though less so in most primitive communities than in the civilised societies of to-day, where marriage is so often postponed till relatively late in life. Even among primitive peoples however the difference is very appreciable, especially in view of the fact (which must be borne in mind in this connection) that with such peoples, owing to the harder conditions of existence, life is often shorter than in civilised communities, while the enfeeblement that accompanies advancing age comes on proportionately sooner. If men were to follow blindly the impulses manifested in the primitive and fundamental forms of incest tendency, sons would cohabit with their mothers, daughters with their fathers. In such unions one of the partners would be relatively aged, and the offspring would in consequence very probably be lacking in that degree of vitality or health normally possessed by the children of parents of more equal age; they would moreover fail, in the majority of cases, to enjoy that degree of provision and protection which could be afforded where both parents are still youthful. Even if such unions were to occur, they could not (in the case of one sex at any rate) be continued in the following generation; for if a son were to cohabit with his mother and if a male child resulted from their union, this child could not in turn fruitfully unite with his mother (who would also be his grandmother), as she would be now definitely past the reproductive age. Further, such unions would come into opposition with the almost universal tendency to find sexual attractiveness in youthful rather than in aged persons—a tendency which, like the appreciation of beauty in the opposite sex in general, we may suppose has been shaped largely, if not wholly, by the operation of natural selection, which has ensured that men and women should in the main be attracted to those who are most likely to produce strong and healthy children.
Thus it can easily be understood that any races which tended to indulge to any large extent the impulses which prompt to incestuous unions between parents and children would be at a disadvantage as compared with those races in which these unions did not occur or occurred less frequently; the latter races tending therefore to supplant the former. Given then the existence of a strong impulse towards parent-children unions, we can see how biological factors may very well have favoured the growth of strong counteracting factors, such as manifest themselves in the repression of the tendencies towards this form of incest.
These considerations would of course apply not only to relations between parents and children but to all other unions in which the age difference between the partners is considerable. They would not however apply to unions between brother and sister or between cousins who are of approximately equal age. The influences which lead to the aversions to these latter unions must be sought elsewhere[261].