The original question was: Does the bible support brother and sister sexual relationships? If not, how did the population ever increase after Adam and Eve?
Answer by Diane Eager
Genesis is very clear that God created one man, Adam, and one woman, Eve. Eve was given this name by Adam because she was “the mother of all the living”. Therefore, the human race started out as one couple, and all people on earth are descended from them. This means for the first few generations all marriages were close family marriages, well within the prohibited relationships of current marriage law in most modern western countries. In the first generation there were certainly brother and sister marriages, and these would have continued into the next few generations, along with cousin marriages, and maybe uncle/niece or aunt/nephew marriages.
God did not prohibit any of these until well after the days of Abraham.
After Noah’s Flood the world’s human population was reduced to one family of eight people: Noah and wife, three sons and their wives. Therefore, the next generation consisted entirely of siblings and cousins. It was very likely there were brother/sister marriages in this generation, and in the next few generations. When God judged the people at the Tower of Babel He split up the population into small groups. As God created the family it is most unlikely He divided families, but split the population into clan groups. These clan groups moved away from one another, and would have only bred amongst themselves for many generations.
One of the main reasons today for laws against close family marriages today is the increase in genetic diseases in the offspring of closely related parents, due to shared recessive genes coming together. For single gene disorders such as phenylketonuria or cystic fibrosis brother/sister unions are a real problem. If both parents have one of the defunct genes there is 25% probability for the children to inherit two defunct genes and have the disease. There is less but still significant increase in the probability of children having multigene problems such as congenital deformities, intellectual deficits and disorders of the immune system.
None of these would have been a problem in the beginning. When God created the world, including Adam and Eve, it was very good. Therefore, there were no genetic diseases. Furthermore, the environment was also very good, so there was nothing to damage genes and cause heritable diseases. Even after sin came into the world the environment was still much better than it is today, and the mutation rate in the first few generations would be very low. The long lives lived by people in the pre-Flood world are a good indicator of this.
After the flood the environment was devastated and the mutation rate increased. This is reflected in the decrease in life-spans between Noah’s time and time of the Hebrew patriarchs, Abraham, Isaac and Jacob. During this period of history it was still acceptable to marry within the family, and it must have happened often in the immediately post-Flood period when Noah’s family began the process of repopulating the earth, and also in the immediate post-Babel period, when the population was split into small groups who would only breed amongst themselves for many generations.
Even in Abraham’s time there would have been fewer harmful mutations than there are now, and this is reflected in the relatively long lives of the patriarchs: Abraham 175, Isaac 180, Jacob 147. Therefore, it was quite in order for Abraham to marry his half-sister. Abraham’s wife Sarah was the daughter of Abraham’s father, but not the daughter of his mother. (Genesis 20:12)
However, the world has continued to go downhill and the human race has accumulated many mutations. By the time of Moses God laid down the formal prohibition of brother/sister sexual relations in the Law of Moses where we read: “Do not have sexual relations with your sister, either your father’s daughter or your mother’s daughter, whether she was born in the same home or elsewhere. … Do not have sexual relations with the daughter of your father’s wife, born to your father; she is your sister.” Leviticus 18:9, 11.
This has continued to be Jewish law, but has also been incorporated into the laws of other nations, and continues to this day.
For more information, see the question: If Adam & Eve only had Cain and Abel, who was the woman Cain took as a wife? Answer here.
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