"As mentioned, no society has a prescriptive practice. The rules are always adjusted to fit the bumpy and contradictory world of experience. It must nevertheless be admitted that absolute rules exist everywhere. The incest prohibition exists in all societies, even if it has often been pointed out that it varies in its significance and compass; in some societies, it is limited to the kin we might call close family, that is, people with the same biological mother and father and their relatives in direct lines of descent; but usually half-siblings are included in the incest prohibition, and often the prohibition is extended to include what we might call more remote relatives."