Things that Yoram knows" Do only humans avoid incest?

Corbus asks: In human culture there is a taboo on sex between family members, but not in animals. For example, it is not uncommon for a cat to fertilize its biological mother. Does incest really lead to the birth of "defective" offspring?

Incest does increase the chance of genetic defects. A mutation is a random change in the genetic material and it happens from time to time in each of us. Billions of "letters" make up the genome and every copying error that occurs during cell division is a mutation. Each of us has several dozen defective genes, some of them fatal defects. What protects us are the reserve genes. Each gene appears in us at least twice: once from the genetic load of the egg and once from what the sperm brought with it and almost always one normal copy is enough for us. We are in trouble when the same defective gene appears both in the cell in the sperm and in the egg, and of course the likelihood of such an event occurring increases the greater the genetic similarity between mother and father. This is the biological basis for aversion to incest.

But two false assumptions are hidden in the question: first, the taboo is not unique to us, we are not so different from our fellow creatures: they also have prevention mechanisms for incest. Second, our taboo is far from perfect or even stronger than the animals'.

The plant kingdom is built around preventing separation from self-pollination

As mentioned, mechanisms that prevent incest are present in animals and plants. In the plant kingdom, several mechanisms have been developed to prevent self-pollination, meaning the fertilization of the ovules with pollen originating from the same plant. Some species have developed dioeciousness, meaning that the entire plant will bear only male or only female flowers, this is the case for example with date trees. Monoecious plants in which flowers of both sexes are on the same plant or stamens and ovules in one flower (a bisexual plant) avoid self-pollination by separating in time the maturation of the sperm cells and the ovules (as in avocado trees) or the anatomical structure of the leaf column and stamens does not allow "discovery Such "fornication" (as in the marigold flowers). There are also plants that do not shy away from self-fertilization, including wheat.   

In the animal world, we see around us domestic cats being impregnated by their brothers, but with the cat's ancestors, the kittens that reached maturity separated from each other and wandered far away: the chance that during the heat season the wild cat will actually meet her brother is very small, and therefore there is no motivation to develop an instinctive reluctance against incest. In the wild, the males migrate a greater distance when they reach adulthood and thus further reduce the chance of an awkward family meeting. In the case of domestic animals, the migration is stopped at the fence of the yard or the bars of the cage, and this is how hybrids are obtained that are unlikely in the wild. "Family values" that prevent incest have evolved in animals in dense herds. For this, mechanisms are needed that allow the animal to recognize relatives. Chimpanzees, our closest living relatives, are able to match pictures of foreign chimpanzee mothers to their children based on facial features, and this ability helps them avoid incest. Mice avoid mating if the smell of their partner's urine is too similar to their own.

Even in birds, genetic studies have shown that cases of incest are very rare. In long-term monitoring of a dense population of sparrows on an isolated island: a situation in which it is impossible to stay away from relatives, the researchers identified only 16 cases of incest over 9 years (it is likely that monitoring a human population of a similar size would have provided many more cases). The cases that did occur were always either a mother to her son or a sister to her brother whose appearance and voice have changed since he grew wings and flew but never between father and daughter because the external appearance and singing style of the adult do not change and the female will avoid mating with a male whose appearance and voice are too similar to those she saw and heard from her father when she was in the nest. The female is more reluctant to the possibility of incest than the male because the burden of pregnancy of damaged offspring falls on her, therefore the female sparrow will be careful to avoid contact with her father more than the male avoids mating with his mother or sister.

 Every rule has an exception: in arid areas in Africa, a strange animal digs: the naked rat (Naked mole-rat). These blind rodents live in underground burrows and hardly ever come out. In those isolated burrows, a unique form of reproduction developed that only two closely related species of African rats need of all mammals. In this organization reminiscent of ant colonies and bee hives, only one female in a fertile colony and only one male inseminates it. All other members of the band are first degree relatives and are not sexually active at all. This strange species is perhaps the only mammal in which the taboo on incest has disappeared: when the queen dies, one of the workers will become queen and one of her brothers will fertilize her. The genetic similarity between members of the swarm is even greater than that between bees working in a hive. In such a situation, any genetic defect will be fatal, but a population that has survived several generations will be free of defects and therefore will not be harmed by the discovery of incest. The other species of these strange rats (Damaraland Mole Rat) preserved, even in extreme isolation conditions, the universal taboo. When the queen dies, there is no breeding until a foreign female happens to arrive - an event that may only happen years later or not at all. Only then, when reproduction becomes possible without incest, one of the males will assume the role of the father and the joy of the species will return to the lair.

And what about us?

In humans, a psychological mechanism named after Edvard Westermarck prevents us from feeling sexual attraction to those who grew up with us in our childhood: usually brothers and sisters. In a study of children raised in children's homes in a kibbutz, it was found that there are almost no marriages between the members of the same kibbutz, and there are no couples raised in the same children's home during the critical period: up to the age of 6. In accordance with Westmark's theory, the list of prohibitions of celibacy in the Koran is quite short, but it includes "your nursing sisters" i.e. those who breastfeed From the same nurse and grew up side by side in the first years of life. The psychological barrier is not absolute and its weakness is evidenced by many cases of incest between father and daughter or between an adult brother and his younger sister. As with the animals, the human female is more sophisticated and cautious than the male and cases of incest between a mother and a child or between an older sister and a younger brother are very rare. A study in which subjects were asked to simulate sexual relations with people of different degrees of closeness and to report their reaction (from disgust to attraction) found that women extended the reluctance from family members to close friends as well, meaning they created a kind of emotional security range.

Man is allowed - culture

But we must remember what distinguishes us from cats, sparrows or avocado trees: culture. The human taboo is flexible and changes from period to period. Since the barrier to sexual attraction is created when children grow up together throughout childhood, in polygamous societies (in which one man has several wives) an attraction can certainly be created between half-siblings from the father's side, each of whom was raised by a different mother with her children. The first recorded case of such a pull appears in the book of XNUMX Samuel. Amnon, the eldest son of King David, falls in love with his half-sister Tamar, David's daughter from another wife. The lack of a close connection in childhood makes her not a close relative from a psychological point of view, and indeed Amnon describes her not as his sister but as Absalom's sister (brother to the same father and the same mother) "Eth-Tamar, Absalom's sister, my brother, I love". Tamar herself, when she is attacked by Absalom, does not see the sexual relationship as a violation of the law "and she said to him, O my brother, do not ask me - because he will not do that, in Israel: do not do this vile thing... and now Speak, please, to the king, for he will not keep me from you. ".

The Book of Genesis provides many events that today we would define as incest. Terah's three sons: Avraham, Haran and Nahor create a genealogy that is the nightmare of every geneticist; Avraham carries Sarah, his half-sister from Terah's side, "My sister is my father's daughter, not my mother's daughter; And be me, for a wife" while Nahor carries Milcha, the daughter of his brother Haran. In the next generation, Yitzchak marries Rivkah, the granddaughter of Haran and Milcha, and their son Yaakov marries Rachel and Leah, his cousins ​​to the Armenian son. Jewish law not only permits the marriage of cousins ​​and nieces, but the Sages even encourage such marriages: "The one who loves his neighbors and draws closer to his relatives and carries his sister's daughter and accompanies a rock to the poor when he is pressed, the scripture says, 'Then call and the Lord will answer.'" In Hellenistic Egypt, brother-sister marriage was the norm in the royal family, Cleopatra VII (Julius Caesar and Antony's lover) not only married her brother but was the home of Cleopatra V and her brother Ptolemy (Ptolemy) XII, the grandparents were also brothers so Cleopatra only had one great-grandmother and one great-great-grandfather .

In some human societies consanguineous marriages - mostly cousins ​​- are a prevalent practice despite the increased risk of birth defects. Some speculate that consanguineous marriages had an advantage under conditions of high exposure to infectious diseases. In such situations, there is sometimes an advantage not to a single gene, but to a set of genes that confers resistance to the cause of the disease: if such a useful set of genes is common in the family, there will be an advantage to the one who will keep it, that is, will give birth to offspring with those close to him. A comparative study between different cultures revealed that indeed consanguineous marriage is a more accepted practice in areas where contagious diseases are a significant cause of death.

A clear correlation between the failure of state institutions and the level of close marriages (which create clans)

In the modern world where infectious diseases are often easily prevented or treated, this kind of adjustment no longer has any value and is harmful. But the damage is not only to health: countries with a high rate of consanguineous marriages are also those that struggle to build functioning democratic institutions. When cousin marriages are not accepted, the genetic closeness and with it the family commitment is diluted in each generation. Most of us barely know second cousins ​​and it is likely that we will not recognize a fourth cousin when we see him on the street. In such a society a person is forced to form social, professional, commercial and political ties that are not based on family ties and accordingly learns to trust people, organizations and institutions. In societies where the family tree is less divided, the relatives are also the neighbors, partners and friends, in such societies it is difficult for institutions common to the entire population such as the judicial system, the public service, media, professional unions and political parties to gain trust and cooperation from the public. Cross-cultural comparative research shows a significant correlation between consanguineous marriages and failure to build stable democratic institutions.

The Catholic Church forbids the marriage of cousins

In the fifth century AD, the Western Roman Empire collapsed and the continent was taken over by the Barbarians: warrior tribes such as the Huns, the Vandals, the Franks and the Goths. The social structure of those tribes is reminiscent of the institution of the extended family (عشیرة) in Bedouin society. The Roman historian Tacitus describes the barbarian tribe in which the heads of the family are also the legislators, judges and military commanders and in which the family members are obliged to "take upon themselves the conflicts of their father or other flesh and their ties of friendship". It is hard to imagine a more opposite value system than that of a state of law, yet the descendants of those barbarians were, about 1,000 years later, the founders of the first states and today they are a model for functioning democracies.

 There are those who see the initial development towards a modern state in a conference of Catholic bishops that met in 506 in the city of Agda in France. The decisions of the conference mainly dealt with the behavior of priests and the prohibition of the sale of church lands, but one clause in the margins of the decisions forbade the marriage of the sons of David. Later, the Catholic Church in the West extended the ban up to seventh degree descendants of David. In the Byzantine Empire, a ban on the marriage of sons of David was accepted in 692 and the eastern churches did not extend the ban beyond first sons of David (common grandfather). The effect of this halachic ruling was not immediate, but over the generations the rate of close marriages steadily decreased, the power of the extended family decreased and at the same time the power of formal institutions and public legal systems increased. Comparative historical research has shown a correlation between the length of time a society weaned itself from consanguineous marriages and the growth of state institutions that would form the basis of a democratic regime and even the degree of democracy of modern societies.

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