There are many incestuous relationships in the Bible. It is best to concentrate on the stories where there is some moral judgement, positive or negative, of those relationships. Some instances are merely described and not commented on. But some of those occasions include some of the most highly revered figures of the Bible, who are said to be righteous and just, like the person of Lot, who fathered children with his own daughters. Others, such as Abraham and his half-sister, are actively rewarded by God for incest (in their case, rewarded with a child). God made incest necessary by creating just two humans to start off with (Adam and Eve), and later, by killing off all humans except Noah and his family (at which time, animal-kingdom incest was just as necessary). Although Noah's family was already a spiderweb of close family marriages: the five generations of parents that preceded both Noah and his wife Emzara all descend from just three individuals1. Inbreeding causes countless genetic problems in families, which get worse per occurrence and leave detectable dents in the genetic makeup of species. Yet, in our genetic record there are no signs of periods of intense incest in Humans resulting from Adam and Eve, nor are there such signs in animals nor humans resulting from inbreeding after Noah's Ark. After creation, and after the flood, incest was rife and necessary, as part of God's plan. So it can't be bad or immoral, and to condemn incest is the same as saying that God's plan is evil. Even if you take the story of Adam and Eve and Noah as myths, their moral teachings imply that incest is ok. Predictably from such a disparate collection of writings, incest is explicitly condemned in quite a few other places in the Bible.
Most societies2 and primitive religions have had stern rules against incest. This has always made sense: the results of inbreeding including many genetic problems including retardation and infertility3, two problems that no kingdom or empire wanted stifling its growth. Although medieval Christianity was notably stricter, most other societies have prohibited incest at least to marital unions within but not including first cousins4. But there are two main forces working against this healthy moral constraint:
Power Games. When power is concentrated in a single ruling family, there has arisen in history a strong impetus for that family to avoid sharing power with outsiders by dictating who must marry who. Sometimes, nations engage in kinds of extended-family marriage-swap schemes, but often, these continue for too long resulting in increasing inbreeding amongst a class of inter-national royals. The ruling families of Egypt from the time of Ahmes, who ruled from 1580 to 1558BCE, to the Cleopatras, saw incest as a royal tradition for power-game reasons, as did the Peruvian royalty5.
Holy Dogmas About Bloodlines and Cultural Purity. Strict rules against marrying non-believers has led many religious communities into necessary incest; the long-term effect of such rules is always a gradual decay in genetic diversity and increase in genetic disease as the result of slow effects of inbreeding. In small or new communities in history and in the present, such negative effects can manifest quickly. The early Mormon church saw much incest due to membership shortage, up to 1892, and, the gypsies are often inbred due to a reluctance to marry outsiders4.
Anthropologists suspect that in some situations, the argument that "the bloodline must be kept pure" is actually an excuse to justify practices that are really just power-games (i.e., the prevention of land becoming inherited by non-family-members).
When the above two factors combine - a family in power, and a religion that enforces rules against marrying impure outsiders; entire cultures can stand on the verge of mass degeneration and isolation. Such tribes leave distinct biological markers upon our genes and easily traceable through history by examining modern genes: hence we often discover periods of inbreeding amongst groups through the study of family genetics.
Luckily, such rules are very hard to enforce and there is always a sneaky undercurrent of occasional sexual union with outsiders. Although in itself immoral, occasions when a child is fathered in secret by an attractive foreigner but raised as a local with two local parents, are perhaps one of the saving graces of strict religious communities that are obsessed with obtained a just-out-of-reach state of impurity. Such doctrines cause much harm and suffering and make a simmering parental deceit necessary if any new blood is to be brought in.
There are other exceptions to common societal instincts against incest.
“The Kaniagmut tribe allowed marriage between brothers and sisters or parents and children. The Karens of Tenasserim, some ancient Peruvians, [...], the Hindu Sakta sect all at times allowed marriage between brother and sister. In Iran brother-sister marriage was practised to keep land in the family, while the Azande in Africa even allowed father-daughter marriage for high chiefs.”
In the era from BCE Greek permissiveness bloomed and spread to the entire Middle_East. Gender roles were blurred, the arts could produce effeminate men and even hermaphrodite public statues appeared. In Egypt also incest continued occasionally amongst the rulers, with Greek influence "in the royal house of the Ptolemies brother regularly married sister, and the poet Theocritus produced verse in favour of incest"7.
Wayward Families sometimes become prone to incestuous relations. You can view this is either a small-scale form occasional tribe or culture incest, or, as the extended influence of wayward individuals. Either way, "there is evidence that the structure of families in which incest occurs is unusually patriarchal and traditional, especially with respect to the subservient position of women relative to men"8.
Wayward Individuals who eschew social norms through mental instability or for other reasons that fall under the remit of abnormal psychology. Investigations by psychologists have found that statistically "perpetrators of incest are often rigidly religious and moralistic"9. A fine example is Warren Steed Jeffs, the President of the Fundamentalist Church of Jesus Christ of Latter Day Saints (a spin-off from the Mormon church). Warren Steed Jeffs got himself on to the FBI's "Ten Most Wanted List" on account of the scale of his sex crimes within his congregation, including sex with minors and incest10. These isolated criminal cases are probably best discussed by the relevant experts, so I devote no time here.
Some periods that saw incest rice in occurrence are hard to categorize. The medieval period from 1400-1600CE saw the institution of the Pope succumb to a long period of indulgence; orgies, prostitutes, permissiveness and every kind of sexual service had its centre in the Papacy. "John Burchard, the slightly biased servant and biographer of the Borgias, described the orgies in which, for instance, at a papal feast fifty naked prostitutes crawled round the floor for chestnuts. Both Alexander Borgia (1431-1503) and Sigismondo Malatesta (1417-68) were openly accused of incest. In 1514, under Pope Leo X, Rome boasted 7,000 official prostitutes. [... and elsewhere] the murdered Bishop Henry III of Liege had sixty-five bastard children; the priests of France had been forbidden to stay with their mothers and sisters because of the rise in incest"11. These highly confusing incidents are documented across multiple pages by Thomson. This isn't power-games incest; it is probably a case of culturally-accepted incest during a period of particular immorality for the Catholic world.
“Many world religions have religious doctrine that forbids or frowns upon the marriage of outsiders (exogamy). Sometimes this is defined as people with wrong beliefs and is designed to protect believers from being exposed to outside ideas. Scriptures warn that intermarriage brings god's wrath and makes people impure. Anthropologists suspect that in some situations, the argument that "the bloodline must be kept pure" is actually an excuse to justify practices that are really just power-games (i.e., the prevention of land becoming inherited by non-family-members). Often, such rules stem from racist and xenophobic instincts. Nearly always, dogmas against exogamy result in prejudice and de-humanization of outsiders, leading in some cases to faith-based sectarianism, religious intolerance and extremism.
The Hebrew Scriptures / Christian Old Testament has many stories warning against marrying foreigners. Deuteronomy 7:3-4 and Ezekiel 20:32-34 says believers are not to marry nor live among non-believers because foreign women will "turn away" men from worshipping God - the punishment is God's anger and losing God's favour. Malachi 2:11-12 says the same thing, but also adds that the descendants of such unions will also be punished (so much for free will). Ezra has long been a source of racism and pointless sectarianism. It says when his people marry outsiders, it offends God and "corrupts" the community (Ezra 9:2). Learning of intermarriage causes the prophet Ezra to tear his clothes, pluck out his beard hair and sit down astonished (9:3). This prejudice and intolerance is found again in Ezra 10:2-3, 10-12 where 113 men are forced to abandon foreign wives else face God's wrath. Continuing this, Neh 13:23-27 has a holy man chastising and punishing mixed-culture families and forces believers to promise they will no longer marry or let their sons marry foreign women. In Numbers 25: 6-15 another holy man is rewarded by God for murdering a newly wed husband and foreign wife (Zimri and Cozbi) for the offence of marrying an outsider, because God had sent plagues as a result of such impurities.
Rules against marrying outsiders can lead to widespread incest, especially in small communities, and this leaves distinct biological markers upon our genetic ancestry, hence we have often discovered periods of inbreeding amongst religious groups through the study of family genetics. Luckily, in the modern world, most people ignore their religions' prohibitions against exogamy, and scriptural verses on the matter are rarely repeated by religious leaders. The world has moved on morally, and negative judgements based on others' faith or skin colour are no longer popular barriers to marriage.”
Part of God's Plan & Abraham was Rewarded For It:
(1) Adam and Eve's children were Cain, Abel (Genesis 4), Seth and others (Genesis 5:4). They must have had sex with their parents or with each other, and thus had children of their own. Incest with the very closest relatives is a necessary part of God's plan, according to the story of Adam and Eve. Even if you accept that this story is mythical, then, the moral truth behind it still insists that incest must be ok, being such a fundamental part of God's good plan. (See: "Christian Mythology: Adam and Eve, and the Serpent, in the Garden of Eden" by Vexen Crabtree (2013).)
(2) After The Flood (Genesis 6:6-8,7:1,20-23,9:1,18-19), which was only survived by one single family (Noah's), incest was once again rife and necessary, a situation God itself had caused. (See: "Noah, the Ark and the Flood, from the Bible Book of Genesis" by Vexen Crabtree (2013).)
(3) Abraham married his sister. He was one of the most holy men of the Old Testament. God rewarded them for it. "And God said unto Abraham, as for Sara thy wife...I bless her, and give thee a son also of her..." (Genesis 17:15-16, Genesis 20:11-12). There was a lot of this going on in this holy family:
(4) Lot fathered children with his own daughters after they took turns to seduce him while he was drunk. Lot is considered favourable by god, was saved by God's angels (Genesis 19:11-13, 15-17,19) and is described as just and righteous in 2 Peter 2:6-8.
(5) Joshua gives Caleb's daughter to her cousin as a wife in Joshua 15:16-17 as a battle reward for conquering Kirjath Sepher.
There are many other cases of incest - too many to list here!
Incest is Condemned by God:
"Cursed be he that lieth with his sister, the daughter of his father, or the daughter of this mother..." (Deuteronomy 27:22).
"None of you shall approach to any that is near of kin to him, to uncover their nakedness" (Leviticus 18:6).
Union your father's wife, or, a father with his daughter in law, both merit punishment by death (Leviticus 20:11-12).
"If a man shall take his sister, his father's daughter, or his mother's daughter...it is a wicked thing" (Leviticus 20:17).
Deuteronomy and Leviticus across various chapters describe many other (but not all) forms of incestuous unions as prohibited.
A Secular Incident:
Absalom was a non-religious figure, featuring in political schemes and power struggles. Yet he had the moral strength to bring justice against King David's son Amnon who raped Absalom's sister (Amnon's half-sister) (2 Samuel 13). His means of going about it were dishonest because he did not think that the holy court of King David would have brought justice, the event being family-on-family.
The main problem is not morals, it can be argued that just because it is immoral does not mean that it didn't happen. The main problem is biological. Interbreeding two families causes severe retardation, mutation and infertility. This happens to isolated Human populations even when there are more than two families. The problem increases with severity the longer the inbreeding occurs.
“Full-sibling or parent-child incest results in about 17% child mortality and 25% child disability, for a combined result of about 42% nonviable offspring.”
Donald Brown, 'Human Universals' pp123
The phenomenon of nonviable offspring from breeding between closely related family members is not limited to Humans, but to most life, especially amongst mammals and multicellular organisms:
“A study of 38 captive mammalian species found a cross-species average of around 33% offspring mortality resulting from closely incestuous matings.”
Donald Brown, 'Human Universals' pp124
Due to the non-viable offspring that result from incest, which gets worse with each generation, the Adam and Eve story cannot be the literal whole truth. When a Christian next time relies on the urban myth of "Christian Family Values" then wander how they would explain to someone the big question of "What happened after the Flood?" The only moral escape route is to admit that the Adam and Eve story is a metaphor. The only biologically correct explanation known is that we evolved slowly from lower animals so that incest was never a problem as there was an entire pool of families to draw from, which were all slowly evolving into humans en masse just as we are right now slowly evolving, like all other species, into future species.
A Christian once argued with me and explained that the inbreeding of Adam and Eve's children did not result in retardation and genetic disease, because our genes were much more pure and perfect, and that the genetic problems caused by inbreeding only appeared once our gene pool, alongside mankind, had fallen out of grace. Whether or not you accept this line of reasoning (and the genetic evidence is not that we are degenerating over time), it only applies to Adam and Eve and their offspring, and not Noah and his wife, who had exactly the same problem in a time period somewhat after Adam and Eve. By the time of Noah, humanity had fallen so far that God tried to wipe us all out. There is hardly scope to argue that at this time, genetic diseases did not occur as a result of inbreeding.
More likely than any convoluted apologist explanations of why the side-effects of incest did not appear is that (1) the authors of Genesis did not know anything about genetics, (2) they were knowingly writing myths and complicated real-life issues were not important to the myth and (3) the story was simply not thought out in that much depth.
Zoroastrianism heralded the monotheistic era of religion, and its doctrines and style carried forwards into Judaism and Christianity. Its laws and principles on marriage were also inherited by those later religions.
“Zoroastrianism, like Judaism, has strict rules against marrying outsiders (called exogamy)13. Followers of Zoroaster in India are called Parsis, and some of that community moved to Hong Kong, but at even their height there were never more than 100 of them13. Such a small community, only marrying each other and the occasional Parsi from Mumbai in India, struggled to expand14. Incest is part of official Zoroastrian doctrine, wherein it is called Xvaetvadatha - marriage between cousins in particular was called 'marriages made in heaven'. Zoroastrian culture is also very wary of accepting conversions13, and this combined with incest and a shrinking base of adherents has placed Zoroastrianism in a poor position globally, and the religion is "significantly declining"13.”