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.