Incest/Inbreeding Taboos

There are several reliable examples of human communities where incest and/or close inbreeding have occurred on a regular and systematic basis. These examples include not only the well known cases of royal family incest but also incestuous practices among commoners. This social class distinction is important to note because human sociobiologists have dismissed the many instances of royal incest as exceptional and of no consequence to the debate. Cases involving commoners, where sibling or other incestuous marriages are usual and systematic, strongly challenge sociobiological suggestions that a selection mechanism exists to prevent inbreeding.

One of the more conspicuous examples of incestuous marriage involves the Roman Egyptians of the first three centuries C.E. A great deal of documentary evidence with genealogical information (mostly census records, but also personal letters, marriage contracts and other types of contracts, petitions, and documents addressed to the administrative authorities) has been unearthed and reveals that Egyptian commoners frequently practiced full brother-sister marriage (Scheidel 1996; Middleton 1962). Russel Middleton argues that there is little uncertainty in these documents. "Unlike some of the earlier types of evidence which may be subject to differing interpretations, these documents of a technical character have an 'indisputable precision'" (1962, p. 606).

It is evident that full sibling marriages accounted for 15 to 21 percent of all unions. When considering how many sibling marriages were demographically possible and socially acceptable (i.e., some families would not have children with siblings of the opposite sex that survived to marriageable age; or have children with opposite sexed siblings; or have children with siblings with the customary age differences—Egyptian marriages conventionally occurred between an older man and younger woman), we find that almost all possible brother-sister marriages were, in fact, contracted. This strongly suggests that sibling marriages were not only common but the preferred norm.

The documents also demonstrate that sibling marriages sometimes continued through two and three generations, and that the overwhelming majority of brother-sister marriages produced children. This practice lasted for at least three centuries and ended only when the Romans discouraged the custom by withholding Roman citizenship from persons continuing the practice.

Another example of a brother-sister incest custom is presented by Edward E. Evans-Pritchard when writing about the African Azande. "[W]hen a boy reaches puberty he may take his sister and with her build their little hut near his mother's home and go into it with his sister and lay her down and get on top of her—and they copulate" (1974, p. 107). Middleton (1962, p. 603) also notes that Azande kings married their daughters and that father-daughter incest was common among the Thonga.

Among the Greeks, Keith Hopkins notes that "[t]he Athenians allowed marriage between half-siblings of the same father but different mothers; the Spartans allowed marriage between half-siblings of the same mother and different fathers" (1980, p. 311). The ancient Hebrews permitted a similar practice as noted in the Old Testament by Abraham's marriage to his half-sister Sara.

John M. Goggin and William C. Sturtevant (1964) list eight other societies that allowed sibling marriage among commoners as well as thirty-five societies that allow sibling marriage between persons of high status.

In addition to cases of sibling marriage, there is abundant evidence of close inbreeding provided by human isolates—small isolated communities where the degree of inbreeding is determined by the size, extent, and length of isolation of the population (Leavitt 1990). These small isolated communities were numerous in the past and represent the norm for preagricultural Paleolithic societies.

A well-documented illustration of a human isolate is the Samaritans of the Middle East. From about 200 B.C.E., when the Samaritans broke completely from Jewish society, until the twentieth century, the Samaritan population declined dramatically (largely due to persecution by more powerful neighbors). At the end of World War II, the Samaritan population numbered 146 individuals, and this population had remained relatively stable for 100 years. By the 1980s, however, the population had increased and the Samaritans consisted of two communities of about 250 individuals (Bonne-Tamir 1980; Jamieson 1982; Talmon 1977).

Inbreeding in the Samaritan communities has been intense, not only because of their small population, but because of three other well established customs. First, Samaritan religion prohibits marriage with individuals outside of their faith. Second, the Samaritans limit their marriages to extended family lineages. Third, they prefer cousin marriage. Batsheva Bonne-Tamir (1980) has observed that nearly 85 percent of all Samaritan marriages are between first and second cousins. However, over a long period of time, the Samaritans have revealed neither a higher rate of genetic disease nor lower fertility than other populations.