The theory put forward by McLennan as an explanation of the origin of exogamy is very simple and on superficial examination very convincing. He assumed that exogamy arose from a scarcity of women, which forced men to obtain wives by capture from other groups and thus gradually led to a general preference for strange women. The cause of this assumed scarcity of women was considered to be the infanticide of new-born females, which was carried on systematically, for savage people foresaw that in the struggle for existence it would be a hindrance to have a great number of women, who could take no share in the battle with enemies, and who presumably would contribute less to the food supply than the men.
H. Cunow also traces back the origin of exogamy to the scarcity of women and wife capture. He starts from the assumption that among the Australian and other uncivilised races the number of persons in a horde is very limited. "If one assumes that the number of members of a horde is sixty, the youngest class would contain, according to present-day reckoning, about twenty-five persons, the middle class twenty, and the oldest class about fifteen persons. In the middle class there would, therefore, be only about ten women. Among these a young man entering the middle class would often not find a single woman that he could take for his wife, for, after pairing marriage had become general, the few existing women had already found a spouse; they had already been disposed of. There was nothing left for the young man but to capture a woman from a strange horde as soon as possible, or to try to persuade a comrade of the same age class to let him share in his marriage relationship on the understanding that his hunting bag would contribute towards the 'household of the three.' This multiple conjugal partnership is customary among most of the Australian tribes even to-day." To this it must be added that the man needs to show much less consideration for a captured strange woman than for one of his own tribe, who would run away if badly treated. Nor can the young man remain single, for he himself would then have to drag his property about, which would hinder him in the hunt and expose him to the ridicule of his companions. (In reality there are many unmarried men even in Australia.) The search for wives led ultimately, according to Cunow, to wife capture and exogamy.
Infanticide, which McLennan assumes, is at present a rare exception among primitive people. Almost all explorers praise their great love for children, and even malformed children are not always killed. Even where infanticide does occur, the sex of the child is certainly not the factor that decides whether it is to be killed or not. The assumption that scarcity of women is brought about by girl infanticide is not correct. The female sex is, indeed, in the minority among uncivilised natives where they have been counted; but the excess of men is only small. Mutual capture of women could not alter this disparity, for it is unlikely that some tribes permitted the capture of their women without retaliation. Besides, even among primitive people men are careful in risking their lives. Capture of women is, therefore, nowhere the rule, but is everywhere the exception. Had it been the rule anywhere, the continuous fighting would have led to the extermination of the tribes in question. Frazer is right when he says: "If women are scarce in a group, many men will prefer to remain single rather than expose themselves to the danger of death by trying to capture women from their neighbours." This is what really happened among many tribes of the Australian natives who lived on a friendly footing with each other. It even happens that the old men who claim the women expressly forbid the young men to steal women from other tribes, because that will lead to bloodshed. Further, scarcity of women is most likely overcome, as previously mentioned, by several men's sharing one wife, which arrangement, unlike the capture of women, avoids arousing the hostility of neighbours. Among peaceable tribes, therefore, a numerical preponderance of men results not in exogamy, but in polyandry. But admitting that a warlike tribe has not sufficient women and therefore captures them from their neighbours, it is still unexplainable why the men should altogether avoid sexual relationship with their own women, few as they are, and have no desire for them whatsoever. This will certainly not be the result; on the contrary, the few women obtainable without force will be all the more in demand.
Frazer thinks that the origin of exogamy has been rightly explained by the American ethnologist L. H. Morgan, who for many years lived among the exogamic Indians as one of them, and thus came into direct contact with exogamy. Morgan assumed that sexual promiscuity was general at a very early period in the history of mankind, and that exogamy was instituted for the deliberate purpose of preventing cohabitation between blood relations, particularly between brothers and sisters, as was previously customary. This struck promiscuity at the root; it removed its worst peculiarity, and resulted at the same time in a powerful movement towards the establishment of sexual monogamy.
Frazer, in supporting Morgan's theory, relies exclusively on the Australian natives, who, according to him, though extremely primitive savages, "carry out the principle of exogamy with a practical astuteness, logical thoroughness, and precision such as no other race shows in its marriage system."
Frazer finds that the effects of the Australian marriage class system are in complete harmony with the deeply rooted convictions and feelings of the natives as regards sexual intercourse, and concludes that the successive tribal subdivisions have been brought about deliberately in order to avoid marriage of blood relations. According to him, it is not going too far to assert that "no other human institution bears the stamp of deliberate purpose more clearly than the exogamous classes of the Australians. To assume that they serve only accidentally the purpose that they actually fulfil, and which is approved by them unreservedly, would be to test our credulity nearly as much as if we were told that the complicated mechanism of a watch has originated without human design."
Nearly all Australian tribes have the system of division into marriage classes. Every tribe consists of two main groups (called in ethnographical literature phratries or moieties), and each of these groups is again divided into two, four, or eight classes. Sometimes the phratries and classes have special names, but not always. In the latter case it may be assumed that the names have been lost, while the division of the tribes into marriage groups remains. These groups are strictly exogamous. In no case are the members of the main group of the tribe (phratry) or of the same class allowed to marry each other. Only members of two given classes may marry, and their children are again assigned to given classes. Among some of the tribes there exists paternal descent, among others maternal descent. Which of the two modes of descent prevails in Australia can hardly be determined. Among some tribes property is inherited in the female line. Other rights of the female sex connected with mother descent are unknown. An example of the Australian marriage classes is given here, namely, that of the tribe Warrai, who live on the railway line running from Port Darwin to the south. Among this tribe indirect paternal descent is the custom; i.e., the children belong to the main group (phratry) of the father, but to other marriage classes.
| Phratry I. | Phratry II. |
| Adshumbitch *Aldshambitch | Apungerti *Alpungerti |
| Apularan *Alpularan | Auinmitch *Alinmitch |
The female marriage classes are marked with an asterisk.