Question of the week: My atheist colleagues frequently bring up the “incest” during the time of Adam and Eve (I believe they mean in the generation of Cain, Abel, and Seth). What would your thoughts on this be?
My answer: If all humanity is descended from one man and one woman, as the Bible claims, then one of Adam and Eve’s sons had to have married one of Adam and Eve’s daughters. Evidently, God tolerated brother-sister marriages during the Genesis era. This tolerance is affirmed in the account of Abraham. Abraham was married to his half-sister (Genesis 20:12).
This tolerance did not last, however. In the Levitical law, intercourse between a brother and a sister was strictly prohibited. Indeed, it is prohibited in virtually every nation and culture today.
There is a genetic reason for the initial tolerance and the later intolerance. Animal breeding experiments show that one can have a brother mate with a sister for about twenty consecutive generations before there is a significant risk of propagating a serious genetic defect. On the human level, this became a problem for the Egyptian pharaohs. After many generations of the pharaohs marrying their sisters, the pharaohs became afflicted with hemophilia.
In Deuteronomy 7:15 and Deuteronomy 28:58–60, Moses tells the Israelites that if they obey the laws God has given to them they will avoid many of the diseases and afflictions of the Egyptians. Evidently, what God had in mind included genetic disorders. To avoid such genetic disorders, in certain parts of the world and even in the United States it would be wise to ban not only brother-sister marriages but also marriages between first cousins.