The Rationale for Prohibiting Inbreeding

Deuteronomy 27:20, 22-23 state (in the King James Version): "Cursed be he that lieth with his father's wife; because he uncovereth his father's skirt. And all the people shall say, Amen...Cursed be he that lieth with his sister, the daughter of his father, or the daughter of his mother. And all the people shall say, Amen.  Cursed be he that lieth with his mother in law. And all the people shall say, Amen."

What is the rationale for these laws?  Many of us would read those laws and say "Because that activity is gross!", and it is. But not everyone in ancient times regarded it as gross.  The Intervarsity Press Bible Background Commentary's comment on Deuteronomy 27:20-23 states:

"Incest was abhorrent in most other societies as well (see, for example, the prohibitions in *Hittite laws).  The exception is Egypt, where it was a common practice in the royal family (but little attested elsewhere) as a means to strengthen or consolidate royal authority.  This concept is also seen among *Elamite kings."
Keil-Delitzsch state in their comment on Leviticus 18:6-7:

"The marriage laws and customs were much more lax among the Gentiles. With the Egyptians it was lawful to marry sisters and half-sisters (Diod. Sic. i. 27), and the licentiousness of the women was very great among them (see at Gen_39:6.). With the Persians marriage was allowed with mother, daughter, and sister (Clem. Al. strom. iii. p. 431; Eusebii praep. ev. vi. 10); and this is also said to have been the case with the Medians, Indians, and Ethiopians, as well as with the Assyrians (Jerome adv. Jovin. ii. 7; Lucian, Sacriff. 5); whereas the Greeks and Romans abhorred such marriages, and the Athenians and Spartans only permitted marriages with half-sisters (cf. Selden de jure nat. et gent. v. 11, pp. 619ff.). The ancient Arabs, before the time of Mohammed, were very strict in this respect, and would not allow of marriage with a mother, daughter, or aunt on either the father's or mother's side, or with two sisters at the same time. The only cases on record of marriage between brothers and sisters are among the Arabs of Marbat (Seetzen, Zach's Mon. Corresp. Oct. 1809). This custom Mohammed raised into a law, and extended it to nieces, nurses, foster-sisters, etc. (Koran, Sure iv. 20ff."

I'm hesitant to accept the authority of Jerome on what the Assyrians did, since Jerome lived long after the time of the Assyrians.  But there may be something to what Keil-Delitzsch say about the other nations.  Within the cultures of the ancient Near East and other Gentiles, there was a prominent abhorrence of marrying immediate family, but there was also some acceptance of the practice.

Even within the Bible, Abraham married his half-sister (Genesis 20:12), and, after Amnon raped his half-sister Tamar, Tamar informed Amnon that he now had to marry her (II Samuel 13:4), which implies that she regarding the practice of half-siblings marrying as permissible, even though v 12 indicates that she believed that Amnon was doing something that was stigmatized in Israel----which could have been the rape itself, or perhaps Amnon sleeping with his half-sister.

Why, then, do Deuteronomy 27, Leviticus 18, and Leviticus 20 prohibit sex with immediate family?  One explanation is that God wanted to prevent the diseases that result from inbreeding.  Maybe the ancient Israelites had observed over the years the bad effects of inbreeding, and that was the basis for their prohibition of it.  And yet, Numbers 36 requires women whose dead fathers had no sons to marry someone from their father's tribe, so as to keep their father's land within the family.  Moreover, as far as I can see, there is nothing within Leviticus 18 that prohibits Israelites from marrying their cousins.

Is the prohibition based on the rationale that people should not sleep with someone who raised them?  But Ezekiel 16 presents God marrying Jerusalem after he had raised her, and I once heard a professor remark that this reflects a custom within the ancient world.

Perhaps the problem was that Israelites should not sleep with someone else's wife.  We know that adultery was considered to be wrong, and yet there seems to be something more going on in the prohibitions on inbreeding.  After all, Leviticus 18:12-13 prohibits Israelites from sleeping with their aunt, and it's not said that the aunt was necessarily somebody else's wife.  Rather, the passage just notes that the aunt is the kinswoman of the Israelite's parent, and so it obviously deems that to be what's wrong.  But what is its rationale?

I wouldn't be surprised if biblical scholars have speculated about this question.  I've just not found much speculation about it in the commentaries that I've read.