Why was incest allowed in the Bible?

Cain was married to either his sister or his niece. Sarah was Abraham's half-sister. Abraham's brother Nahor married his niece Milcah. Why was incest allowed in the Bible?

The Bible doesn't say, but we can speculate. First, there was an obvious need for marriage between close relatives, as the number of human beings on the earth at that time was limited. Adam and Eve’s children married their close relatives, by necessity, without it being considered incest. But, even in those days, marriage between parent and child seems to have been disallowed: “That is why a man leaves his father and mother and is united to his wife, and they become one flesh” (Genesis 2:24).

Also, Adam and Eve were created genetically pure. They had no congenital defects to pass on to their offspring. DNA damage accumulates over time, but even 2000 years later, when Abraham was born, genes were healthy enough to allow for him to marry his half-sister. Genetic abnormalities did not influence marriage partners until the mutations were common enough to be a health risk.

Another possible reason is the patriarchy and tribalism common at the time. When Isaac needed a wife, Abraham did not want him to marry one of the local Canaanite women, whose religion was anathema to the worship of God. So he sent his servant back to his homeland to find a woman from his relatives (Genesis 24). Later, Rebekah convinced Isaac to send their son Jacob away to her family to find a wife (Genesis 27:46). In doing so, the family unit was kept strong, and ungodly religious influences were kept to a minimum. This was particularly true among Egyptian royalty, where incest was used to maintain the royal line—and make for some very unhealthy pharaohs.

God tightened incest standards when He gave the law to Moses. Leviticus 18:6 says, "None of you shall approach any one of his close relatives to uncover nakedness. I am the LORD." It goes on to elaborate: A man could not marry his mother, step-mother, sister (half or full, brought up within his family or in another home), granddaughter, step-sister (brought up as his sister), paternal or maternal aunt, father's brothers' wife, daughter-in-law, sister-in-law, or both a woman and her daughter or granddaughter, nor sisters while both are alive. It is thought that there was no mention of a man marrying his daughter because the disgrace of the relationship was obvious. Notice how these laws maintain honor and peace within the larger family unit (Leviticus 18:6–18), and they also establish a morality still applicable today.

In ancient times, it was often taboo to have relations with your child or sibling, but the first comprehensive mandate against marriage among family members in the Bible didn't arrive until the Mosaic Law.

Related Truth:

What does the Bible say about having a marital/sexual relationship with a close relative?

What does the Bible say about eunuchs?

Does the Bible talk about asexuality?

Why did God tell Adam and Eve to fill the earth—if no one died, wouldn't the earth have become overpopulated?

Creation vs. evolution - What does the Bible say?

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