A social response to the costs of incest
One theory is that the observance of the taboo would lower the incidence of congenital birth defects caused by inbreeding. A society that had noticed this might tend to form an incest taboo.
Anthropologists reject this explanation for two reasons. First, inbreeding does not directly lead to congenital birth defects per se; it leads to an increase in the frequency of homozygotes. A homozygote encoding a congenital birth defect will produce children with birth defects, but homozygotes that do not encode for congenital birth defects will decrease the number of carriers in a population.
One might complain that a society would have to have a fairly advanced understanding of genetics to recognise this potential "benefit" of incest, whereas the increased prevalence of birth defects is relatively easy to spot.
Second, anthropologists have pointed out that in the Trobriand case a man and the daughter of his father's sister, and a man and the daughter of his mother's sister, are equally distant genetically. In that particular case, the prohibition against relations is not based on or motivated by concerns over biological closeness.
Sociologist Ian Robertson gives three main social reasons why incest taboo exist as a cultural universals. The first is that early human beings-living primarily in small kinship groups of hunters and gatherers- needed to protect themselves by forming alliances with other groups. By forcing their children to marry into families outside their own, each group widened its social links and provided itself with allies in time of famine or other hazards. These groups faced the alternatives of marrying out or dying. Marriage in most traditional societies is a practical alliance between groups, not a love match between individuals. That is why marriages are arranged by the parents, often when their offspring are still children and sometimes even before they are born. The second reason for the incest taboo is that the family itself could not function without it, for the statuses of family members would be utterly and hopelessly confused. As Kingsley Davis points out: " The incestuous child of a father-daughter union would be a brother of his own mother, i.e. the son of his own sister; a stepson of his own grandmother; possibly a brother of his won uncle; and certainly a grandson of his own father." The third reason is that without an incest taboo, sexual rivalry among family members would disrupt the normal roles and attitudes of the various relatives. the father, for example, might experience role conflict as both the disciplinarian and the lover of his daughter; the mother might be jealous of both; and the child, of course, would be caught in the middle. Faced with constant conflict and tension, the family institution might simply disintegrate. The incest taboo has developed over time because it is vital to the survival of the family and thus of society itself. Of course, neither traditional nor modern societies consciously appreciate the reasons for the taboo. They and we simply accept it as natural and moral.