(1) The reasons which are given by primitive peoples for their obedience to the rule of exogamy are various; sometimes it is considered that harm would come to the pair who are guilty of the forbidden union, this perhaps being usually of the nature of some disease, or else very frequently, impotence or barrenness; sometimes it is the offspring of the guilty pair who will incur the penalty; quite often, however, the evil results of such a union are supposed to affect the whole community to which they belong and consist not uncommonly in general infertility of women, animals and plants. These reasons, though they no doubt exercise a powerful influence among those who hold them, are for the most part too obviously of the nature of superstitions, inventions or rationalisations to be taken at their face value; though the study of them on psycho-analytic principles would no doubt bring interesting and suggestive results. Hardly much more satisfactory, if regarded as attempts at affording complete and ultimate explanations, are some of hypotheses that have been put forward by modern students of exogamy. [Sidenote: Durkheim's Theory of the sanctity of blood] (2) Thus, Durkheim[246] suggests that exogamy arose as a result of the religious respect for blood, particularly menstruous blood; the divine totemic being is supposed to be resident in blood, hence blood is sacred, especially to those of the totem clan, and no man of this clan may trespass on the very spot where the sacred blood periodically manifests itself. Even if this theory should afford a satisfactory proximate explanation of exogamy, it is obviously very far from revealing the true ultimate biological and psychological factors that have led to the practice. Even apart from this, however, it gives rise to certain difficulties and objections (more especially connected with the lack of the close correspondence between exogamous classes and totemic clans which we should expect upon this theory) and has almost certainly at best but a very limited field of application[247]. [Sidenote: Westermarck's theory of an innate aversion to incest] (3) Westermarck[248] would explain exogamy and the avoidance of incest generally as due to the fact that there is an innate aversion to sexual intercourse between persons living together from early youth, and that, as such persons are in most cases related by blood, this feeling would naturally display itself in custom and law as a horror of intercourse between near kin[249]. [Sidenote: resulting from the injurious effects of inbreeding]