Familiarity with women within the group and unfamiliarity with women without the group is the explanation of exogamy on the side of interest; and the system of exogamy is a result of exchanging familiar women for others.  We have seen that capture was not an important means of securing wives outside the group, and that exogamy was fully developed before property and media of exchange were developed to any extent, and consequently before the purchase of women had become a system.  We have seen also that the Australian who wants a woman at the present time gets her by exchanging another woman for her.  Social groups were necessarily small in the beginning.  Before invention and co-operation have advanced far, the group must remain small in order to pick up enough food to sustain life on a given area.

Starting out with a single pair, when the family increases in size a separation is necessary; and clans are an outcome of the process of division and redivision, the bond between the clans and their union in a tribe resulting from their consciousness of kinship.  Now, it is a well-known condition of exogamy that, while a man must marry without his clan, he must not marry without his tribe, and for the most part, in fact, the clan into which he shall marry is designated.  In other words, allied clans gave their women in exchange mutually.  This was a natural arrangement, both because the two groups were neighbors and because they were friendly, and at the same time the psychological demand for newness was satisfied.  When a family was divided into two branches, Branch A had a property interest in its own women, but preferred the women of Branch B because of their unfamiliarity.  The exchange took place at first occasionally and not systematically, and the women parted with in each case were not, perhaps, in all cases the youngest, and we may assume that they had in all cases been married before they were given up.  But gradually, and when the habit of exchange had been established, men came to look forward to the exchange and to desire to secure the girl at the earliest possible moment, until finally young women were exchanged at puberty, and virgins.  When for any reason there is established in a group a tendency toward a practice, then the tendency is likely to become established as a habit, and regarded as right, binding, and inevitable:  it is moral and its contrary is immoral.  When we consider the binding nature of the food taboos, of the couvade, and of the regulation that a man shall not speak to or look at his mother-in-law or sister, we can understand how the habit of marrying out, introduced through the charm of unfamiliarity, becomes a binding habit.