Modern
anthropology developed at a time when a great many human societies were illiterate, and much of the research on incest taboos has taken place in societies without legal codes, and, therefore, without written laws concerning marriage and incest. Nevertheless, anthropologists have found that the institution of marriage, and rules concerning appropriate and inappropriate sexual behavior, exist in every society. The following excerpt from
Notes and Queries on Anthropology (1951), a well-established field manual for
ethnographic research, illustrates the scope of ethnographic investigation into the matter: These theories anthropologists are generally concerned solely with brother–sister incest, and are not claiming that all sexual relations among family members are taboo or even necessarily considered incestuous by that society. In these theories, anthropologists are primarily concerned with marriage rules and not actual sexual behavior. In short, anthropologists were not studying "incest" per se; they were asking informants what they meant by "incest", and what the consequences of "incest" were, in order to map out social relationships within the community. This excerpt also suggests that the relationship between sexual and marriage practices is complex, and that societies distinguish between different sorts of prohibitions. In other words, although an individual may be prohibited from marrying or having sexual relations with many people, different sexual relations may be prohibited for different reasons, and with different penalties. For example,
Trobriand Islanders prohibit both sexual relations between a woman and her brother, and between a woman and her father, but they describe these prohibitions in very different ways: relations between a woman and her brother fall within the category of forbidden relations among members of the same clan; relations between a woman and her father do not. A man and his father's sister will often have a flirtatious relationship, and, far from being taboo, Trobriand society encourages a man and his father's sister or the daughter of his father's sister to have sexual relations or marry.
Instinctual and genetic explanations An explanation for the taboo is that it is due to an instinctual, inborn aversion that would lower the adverse
genetic effects of
inbreeding such as a higher incidence of congenital
birth defects. Since the rise of modern genetics, belief in this theory has grown.
Birth defects and inbreeding The increase in frequency of birth defects often attributed to inbreeding results directly from an increase in the frequency of
homozygous alleles inherited by the offspring of inbred couples. This leads to an increase in homozygous allele frequency within a population, and results in diverging effects. Should a child inherit the version of homozygous alleles responsible for a birth defect from its parents, the birth defect will be expressed; on the other hand, should the child inherit the version of homozygous alleles not responsible for a birth defect, it would actually decrease the ratio of the allele version responsible for the birth defect in that population. The overall consequences of these diverging effects depends in part on the size of the population. In small populations, as long as children born with inheritable birth defects die (or are killed) before they reproduce, the ultimate effect of inbreeding will be to
decrease the frequency of defective genes in the population; over time, the gene pool will be healthier. However, in larger populations, it is more likely that large numbers of carriers will survive and mate, leading to more constant rates of birth defects. Besides recessive genes, there are also other reasons why inbreeding may be harmful, such as a narrow range of certain
immune systems genes in a population increasing vulnerability to infectious diseases (see
Major histocompatibility complex and sexual selection). The biological costs of incest also depend largely on the degree of genetic proximity between the two relatives engaging in incest. This fact may explain why the cultural taboo generally includes prohibitions against sex between close relatives but less often includes prohibitions against sex between more distal relatives. Children born of close relatives have decreased survival.
Melford Spiro argued that his observations that unrelated children reared together on Israeli Kibbutzim nevertheless avoided one another as sexual partners confirmed the Westermarck effect. Joseph Shepher in a study examined the second generation in a
kibbutz and found no marriages and no sexual activity between the adolescents in the same peer group. This was not enforced but voluntary. Looking at the second generation adults in all kibbutzim, out of a total of 2769 marriages, none were between those of the same peer group. However, according to a book review by John Hartung of a book by Shepher, out of 2516 marriages documented in Israel, 200 were between couples reared in the same kibbutz. These marriages occurred after young adults reared on kibbutzim had served in the military and encountered tens of thousands of other potential mates, and 200 marriages is higher than what would be expected by chance. Of these 200 marriages, five were between men and women who had been reared together for the first six years of their lives. A study in Taiwan of marriages where the future bride is adopted in the groom's family as an infant or small child found that these marriages have higher infidelity and divorce and lower fertility than ordinary marriages; it has been argued that this observation is consistent with the Westermarck effect.
Third-parties' objections Another approach is looking at moral objections to third-party incest. This increases the longer a child has grown up together with another child of the opposite sex. This occurs even if the other child is genetically unrelated.
Counter arguments One objection against an instinctive and genetic basis for the incest taboo is that incest does occur. Anthropologists have also argued that the social construct "incest" (and the incest taboo) is not the same thing as the biological phenomenon of "inbreeding". For example, there is equal genetic relation between a man and the daughter of his father's sister and between a man and the daughter of his mother's sister, such that biologists would consider mating incestuous in both instances, but
Trobrianders consider mating incestuous in one case and not in the other. Anthropologists have documented a great number of societies where marriages between some first cousins are prohibited as incestuous, while marriages between other first cousins are encouraged. Therefore, it is argued that the prohibition against incestuous relations in most societies is not based on or motivated by concerns over biological closeness. Other studies on cousin marriages have found support for a biological basis for the taboo. Also, current supporters of genetic influences on behavior do not argue that genes determine behavior absolutely, but that genes may create predispositions that are affected in various ways by the environment (including culture). Steve Stewart-Williams argues against the view that incest taboo is a Western phenomenon, arguing that while brother–sister marriage was reported in a diverse range of cultures such Egyptian, Incan, and Hawaiian cultures, it was not a culture-wide phenomenon, being largely restricted to the upper classes. Stewart-Williams argues that these marriages were largely political (their function being to keep power and wealth concentrated in the family) and there is no evidence the siblings were attracted to each other and there is in fact some evidence against it (for example,
Cleopatra married two of her brothers but did not have children with them, only having children with unrelated lovers). Stewart-Williams suggests that this was therefore simply a case of social pressure overriding anti-incest instincts. Stewart-Williams also observes that anti-incest behaviour has been observed in other animals and even many plant species (many plants could self-pollinate but have mechanisms that prevent them from doing so).
Sociological explanations Psychoanalytic theory—in particular, the claimed existence of an
Oedipus complex, which is not an instinctual aversion against incest but an instinctual desire—has influenced many theorists seeking to explain the incest taboo using sociological theories. Some anthropologists argue that nuclear family incest avoidance can be explained in terms of the ecological, demographic, and economic benefits of exogamy. While Lévi-Strauss generally discounted the relevance of alliance theory in Africa, a particularly strong concern for incest is a fundamental issue among the age systems of East Africa. Here, the avoidance between men of an age-set and their daughters is altogether more intense than in any other sexual avoidance. Paraphrasing Lévi-Strauss's argument, without this avoidance, the rivalries for power between age-sets, coupled with the close bonds of sharing between age-mates, could lead to a sharing of daughters as spouses. Young men entering the age system would then find a dire shortage of marriageable girls, and extended families would be in danger of dying out. Thus, by parading this avoidance of their daughters, senior men make these girls available for younger age-sets and their marriages form alliances that mitigate the rivalries for power.
Endogamy Exogamy between households or descent groups is typically prescribed in
classless societies. Societies that are stratified—that is, divided into unequal classes—often prescribe different degrees of endogamy. Endogamy is the opposite of exogamy; it refers to the practice of marriage between members of the same social group. An example is
India's caste system, in which unequal castes are endogamous. Inequality between
ethnic groups and
races also correlates with endogamy. An extreme example of this principle, and an exception to the incest taboo, is found among members of the ruling class in certain ancient states, such as the Inca, Egypt, China, and Hawaii; brother–sister marriage (usually between half-siblings) was a means of maintaining wealth and political power within one family. Some scholars have argued that in Roman-governed Egypt this practice was also found among commoners, but others have argued that this was in fact not the norm. ==See also==