The incest taboo is one of the oldest and most perplexing mysteries encountered by students of human society. Historically, western scholars believed that the incest taboo—long proposed as a cultural universal—is vital to understanding the human condition. Thus, interest in the incest taboo has an extensive history.
Although the incest taboo varies in meaning by society, it is frequently an important rule of prohibition, commonly encompassing religious sanctions, and usually forbidding sexual contact between particular categories of relatives and family members. Closely related to the incest taboo are the rules of exogamy that usually prohibit marriage between the same categories of kin forbidden by incest rules (Murdock 1949). Typically included in the taboo are nuclear (parents and children) and immediate (e.g., grandparents, aunts and uncles, nieces and nephews, and first cousins) family members. In societies composed of unilineal descent groups (e.g., lineages, clans, and moieties), the incest rule often includes all or most of a person's descent (kinship) group. This includes distantly related individuals to which an actual genealogical connection cannot be made (Murdock 1949). A thorough understanding of the incest taboo necessarily recognizes this rule as an important part of a larger system of sexual regulations. In turn, these sexual regulations are an important component of the extensive normative structure regulating family, marriage, and kinship systems, and ultimately the larger society.
There are many cross-cultural variations in the incest taboo. Whereas it appears that most societies have some sort of incest prohibition, the rule is not strictly universal. Likewise, many societies deem the incest taboo extremely serious, whereas other groups view the taboo more casually. Sanctions for taboo violations reflect a similar cross-cultural diversity. In some societies, members simply express disapproval or distaste when incest occurs, as might be expected in the presence of bad manners. In other communities, the act of incest is considered horrifying or unthinkable, and transgressors may be put to death or expelled from the society. In many instances, the incest taboo is intricately entwined with religious tenets and proscribes supernatural sanctions against violators or against the society as a whole. In technologically advanced societies scientific explanations have commonly replaced religious beliefs, and religious sanctions have been replaced by legal penalties and concerns about genetic harm to progeny.
Plutarch (c.e. 46–120?) was one of the earliest Western scholars interested in the incest taboo. His writings anticipated two modern theories: alliance theory and familial conflict theory. Alliance theory concludes that the incest taboo exists to create an outward reaching network of cooperative kin, which is a primary social structure essential for human survival. This network works because rules of incest force individuals to find sexual and marriage partners outside their own families. Familial conflict theory argues that incest restrictions exist to prevent destructive conflicts within the family. If family members were to engage in sexual relationships with each other, role conflicts and jealousies would destroy the effectiveness of the family institution.
The Roman historian Tacitus (c.e. 56–120) offered a theoretical framework similar to Plutarch's, suggesting alliance networks as the reason for the incest prohibitions in Roman society (Honigmann 1976). In addition to alliances, Augustine (c.e. 354–430) proposed a natural aversion to incest and an "inherent sense of decency" that prevents incestuous relationships. Thomas Aquinas (c.e. 1225–1274) advocated alliance theory and asserted that incest hindered child development. Aquinas believed that close kin marriages encourage lust and result in disruptive role conflicts that could destroy the family (Honigmann 1976).
The development of the social sciences in the nineteenth and twentieth centuries continued these themes. George Murdock (1949) and Yehudi Cohen (1978) accepted alliance theory, whereas Sigmund Freud (1950) and Talcott Parson (1954) continued the argument that incestuous relations are destructive to the family.
In The Descent of Man (1871), Charles Darwin acknowledged the family conflict model but proposed an evolutionary foundation by hypothesizing that inheritable traits allowing incest would be selected against in the evolutionary process.
Edward Westermarck, in The History of Human Marriage (1891), employed Darwinian evolutionary theory and posited that incest avoidance emerged as an instinct to prevent the genetic harm produced by inbreeding. Westermarck hypothesized that this instinct was activated when people were raised in close proximity, such as in families. He believed that this aversion would be evident most commonly among siblings, but Westermarck also proposed that sexual repugnance would develop when unrelated children were reared together. This same thesis (Westermarck's hypothesis) is currently asserted by sociobiologists of human behavior (human sociobiologists), who assume that many complex social behaviors are grounded in genetic inheritance shaped by natural selection.
Beyond these historical accounts, notable explanations of the incest taboo include demographic theory, proposed by Mariam Slater (1959) and elaborated by Charles Case (1969). It is these theorists' contention that the demographic characteristics of human breeding populations (e.g., life expectancy, birth order, and the distribution of sex among siblings) make incestuous activity in the immediate family unlikely and, at best, short-lived.
Talcott Parsons's (1954) socialization theory asserts that the incest taboo is part of a normative structure employing eroticism—and its withdrawal—as a system of sanctions in the socialization of children. The affection offered by parents and other adults (often relatives) acts as a powerful reward for "proper" behavior in children, just as its withdrawal acts as a forceful punishment. Parsons claimed that this is an effective socialization process because of the deeply social nature of the human species.
For Parsons, the incest taboo is part of the system of sexual regulations that draws a boundary beyond which the family may not wander when imparting erotic rewards. Withholding erotic rewards forces the adolescent child to participate in the larger society in order to find greater sexual fulfillment. This ties the society together through marriage and kin relationships.
Nature versus Nurture
In current incest taboo literature, the most pronounced dispute reflects the mind/body debate. Scholars prescribing inheritable social behavior, as opposed to those postulating that social behavior is produced through environmental experiences, presently dominate much of the discussion. With the publication of Edward O. Wilson's Sociobiology (1975), the extension of Darwin's natural selection theory to complex human behaviors experienced a zealous revival. Central to this revival is the assertion by human sociobiologists that because of the universal character of the incest taboo, and the prevalence of inbreeding avoidance in other species, incest/inbreeding avoidance in humans represents the best example of a naturally selected behavior.
Sociobiologists of human behavior have supported their theory of incest/inbreeding avoidance by employing research from four major areas. These include research exploring the universal nature and compliance with incest rules; studies of inbreeding harm; ethological and animal research on inbreeding avoidance; and investigations of marriage practices among children raised together (Ruse 1981–1982; Leavitt 1990). A critical look at these research areas, however, raises significant questions regarding their support for human sociobiological hypotheses. Specifically, the deleterious hypothesis of inbreeding, which underlies the sociobiology thesis on incest avoidance, has been called into question.
Sociobiologists believe that life—and its evolution— results from the competition between individual species members to spread their genes by producing the most progeny. (Progeny, by definition, carry parental genes to future generations.) The central sociobiological thesis concerning incest/inbreeding avoidance simply states that natural selection favors outbreeding behavior because inbreeding more often results in genetically debilitated offspring—in other words, inbreeding is not the best adaptive strategy for producing the most descendants. Although the deleterious thesis is widely accepted, and has taken on a law-like stature, a careful examination of the simple Mendelian mathematics involved quickly refutes this notion (Shields 1982; Livingstone 1969).
Almost all harmful genes are recessive, requiring that both parents carry the gene to produce offspring that manifest the deleterious effect. Since relatives share some common ancestry, they are more likely than nonrelatives to share the same harmful recessive genes. In this respect, the more closely related the mates, the more common their ancestry, and hence the more likely they will share the same deleterious genes. Thus, mating between relatives is thought to more readily produce genetically harmed descendants.
However, if a society customarily practices inbreeding, such as first and second cousin marriage, harmful recessive genes will quickly pair up and wash out of the gene pool. This occurs because deleteriously effected individuals are far less likely to reproduce and pass along the harmful genes to descendants. The result of systematic and recurring inbreeding in a population is to reduce the "genetic load" (the number of harmful recessive in the gene pool). Thus, inbreeding is no more harmful than outbreeding. Indeed, the advantage of an inbreeding system, especially for slow breeding mammals like humans, is that it preserves genotypes that have already proven successful in the environment.
Inbreeding is no more harmful than outbreeding unless inbreeding is practiced erratically in an otherwise outbreeding population. In outbreeding populations, mates are less likely to share close ancestry and thus the same harmful recessives traits. In this kind of reproductive system, recessive genes do not wash out of the population and thus accumulate as a large genetic load. The result can be the manifestation of harmful characteristics in the offspring of mating relatives.
For most of human history, breeding populations were small and isolated, and the community often practiced cousin marriage. The results were a relatively homogenous population of inbred individuals. Such homogenous populations are also common in other species (Shields 1982). It is unlikely, therefore, that a naturally selected mechanism would evolve to prevent incest/inbreeding.
Sibling Marriage and Human Isolates
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.
Westermarck's Hypothesis: The Israeli Kibbutzim and Chinese sim pua Marriage
To support the deleterious theory of incest/inbreeding avoidance, human sociobiologists have repeatedly emphasized Edward Westermarck's hypothesis (1891) that children raised in close proximity will develop an aversion to sexual relationships with each other. Sociobiologists assume that this aversion originated as a naturally selected mechanism. Human sociobiologists site evidence from two case studies of human communities in support of Westermarck's hypothesis.
One group, the Israeli kibbutzim, separate children from their parents' household at birth and raise them in age-graded cohorts. In these cohorts boys and girls are raised without segregation, even sharing sleeping, bathing and toilet facilities; the proximity and intimacy of their upbringing is greater than what would usually be expected among siblings. Joseph Shepher (1983) studied these kibbutzim as a test of Westermark's hypothesis and reported that of the nearly 3,000 kibbutzim marriages he examined there was not one case of intra-cohort marriage.
However, several other researchers reported compelling research results which demonstrate that there are numerous social structural and ideological reasons why individuals of the same kibbutzim cohort might not marry (Talmon 1964; Spiro 1965). Mordecai Kaffman (1977), on the other hand, reported that by the late 1970s sex and marriage between cohort members had become common. John Hartung (1985), in re-analyzing Shepher's research, reported that not only did cohort members from Shepher's samples marry but did so at a disproportionately higher rate than would be expected for marriages involving non-cohort members.
A second case study, often sited in support of Westermark's hypothesis, was published by Arthur Wolf (1995). Wolf studied a form of Chinese marriage known as minor marriage (or sim pua marriage). In this marriage custom a family adopts a young girl and raises her as a sister to their son. This adopted sister will eventually be the son's wife. Because these marriages had a higher divorce rate and produced fewer children than Chinese "major" marriage, Wolf and the human sociobiology community presented this research as primary evidence in support of Westermarck's hypothesis.
Wolf makes it clear, however, that the Chinese consider sim pua unions to be low-status marriages for the poor, and these marriages are often the object of public ridicule and scorn. Adopted daughter-in-laws are frequently mistreated and unhappy, and given the suggestion of incest in a sibling relationship, it is a wonder that such marriages worked at all. In other words, there is no need to invent a complicated Darwinian mechanism to understand why the Chinese minor marriages more often failed.
Another area of research that refutes Westermarck's hypothesis is from studies of nudist communities. Dennis Craig Smith and William Sparks (1986) found that nudist children experience more sexual play with siblings and have more incestuous relations with immediate family members than non-nudists even though their early associations are quite intimate.
Sociocultural Factors in the Development of Incest Regulations
The incest taboo is better understood when examined in the context of the sociocultural development of human systems (Leavitt 1989). This development is best understood in light of the general organizational framework of human kinship groups (or descent systems) that comprise a fundamental component of many human communities.
In general, descent systems are either bilateral or unilineal. Bilateral systems, like American society, are diffused and include equally the relatives of both an individual's parents. In this system, a person is not usually acquainted with relatives beyond first or second cousins, and ritual or formal activities beyond the most immediate family are typically absent.
In a unilineal kinship system, all members of the kinship group trace their ancestry to a common ancestor (either mythical or actual). If this founder was male, descent is traced through the male line (patrilineal); if the founder was female, ancestry is traced through the female line (matrilineal). In a few rare instances, there are societies of double or dual descent in which each individual inherits two descent group memberships. Because membership in unilineal descent groups is determined by an individual's descendancy from a single ancestor, only some of a person's relatives will belong to their kinship group.
Societies with unilineal descent systems are commonly organized around lineages or clans, in other words, kinship organizations, which include hundreds of people recognized as blood relatives. These groups are the organizational backbone of the society and orchestrate most societal activities, including political, economic, military, religious, and educational functions.
The earliest, simplest societies (hunting and gathering or foraging societies) typically consisted of a tribe incorporating a number of small nomadic bands organized through bilateral descent. Band exogamy was practiced to ensure that the bands remained unified, even though they ranged far apart and saw each other only occasionally. This meant that a person had to marry and have sex with a partner outside of the immediate family and band. Thus, the survival of the tribe was ensured through bonds of blood and marriage ( Johnson and Earle 1987). When bands, which consisted normally of twenty-five to fifty people, found themselves in trouble their marriage and blood ties ensured that they could seek aid from other groups.
At the end of the Ice Age (10,000 b.c.e.), the earth's climate, flora, and fauna changed sufficiently to begin moving human communities toward agriculture (Harris 1977). With this change came a need for more sophisticated social structures, especially for politico-military and economic activities. As life became increasingly sedentary and communities grew in size, access to resources became increasingly crucial, as did the need to defend relatively scarce fertile farmland. These structural and institutional changes encouraged the appearance of unilineal descent groups.
By extending the incest taboo to encompass lineage or clan members, the kinship group compelled its children into marriages of alliance with other descent groups. Such marriages carried reciprocal obligations for economic and military assistance essential for survival. These marriage bonds were so important that parents and other kin commonly determined whom their young would marry (arranged marriage). Frequently, kin group associations were made strong and stable through an exchange of gifts between family groups (bride price, dowry, groom wealth) and the encouragement of the couple to have many children ( Johnson and Earle 1987).
As human societies continued to grow and evolve technologically and structurally (including larger settlements), the incest taboo began to contract, encompassing fewer relatives. New organizational structures not based on kinship ties or descent became increasingly common, and these structures were more efficient for operating larger, more complex societies. These new institutions were part of the development of the state, which ensured political and economic alliances through its own bureaucratic agencies.
With the appearance of modern industrial societies, the incest taboo contracted to encompass only the nuclear family and a few other immediate relatives (Cohen 1978). Punishments for violating the incest taboo have followed a similar evolutionary path. Where the incest taboo has been extended to ensure survival, its violation has generally been punished more severely. As the incest prohibition became less essential for enhancing alliances, it contracted to include fewer categories of relatives, and sanctions for violations of the incest proscriptions have become less severe (Leavitt 1989).
By understanding human sociocultural and environmental conditions, it is possible to understand the incest taboo, as well as its many variations, without tortuous allusions to genetic evolution.
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gregory c. leavitt