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Is brother-sister intermarriage intrinsically immoral?

123rf.com/Maryna Pleshkun

Rodrigo F. from Brazil writes in response to Dr Sarfati’s previous feedback Cain’s wife and brother-sister intermarriage. This takes issue with the claim that there was nothing intrinsically wrong with this practice before God expressly forbade it. In essence, it is attempting to reach the same conclusion as MJ, to whom Dr Sarfati responded in Cain’s wife explanation ‘gross and disgusting’? but following the advice to argue from Scripture rather than emotions. The arguments are important enough for Dr Sarfati to respond to below.

First of all, God bless you for defending the word of God! Regarding this particular issue I have some questions to point out:


1-Lv 18.17b for they are her near kinswomen: it is wickedness.

2-Lv 18.24 Defile not ye yourselves in any of these things (including brother intermarriage): for in all these the nations are defiled which I cast out from before you;

If God said that “the nations are defiled which I cast out from before you” it would mean He considered incest to be immoral (wickedness) before the law be given as well.

Lv 18.27–30 (for all these abominations (including brother intermarriage) have the men of the land done, that were before you, and the land is defiled);that the land vomit not you out also, when ye defile it, as it vomited out the nation that was before you. For whosoever shall do any of these abominations, even the souls that do them shall be cut off from among their people. Therefore shall ye keep my charge, that ye practise not any of these abominable customs, which were practised before you, and that ye defile not yourselves therein: I am YHWH your God.

abominable customs—Which means Lv 18 6–18. That includes “The nakedness of thy father’s wife’s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness.”

Here YHWH is saying explicitly that “abominable customs” were “practised before” the Mosaic Law, and He considered this defiled. How could we understand that God considered some of these abominable customs as a eternal immoral behaviors but only brother’s intermarriage was not intrinsically considered immoral.

Another question about brother intermarriage is that nowadays geneticists are developing many resorts to check the probabilities of consanguinous children developing any sort of problem through the DNA analysis of both. So, just for the sake of the argument, if two Christian brothers say that they made an exam with a biologist attesting they will have a good probability to have perfect children, thus there is no more any moral problem because the necessity of the Law in that specific case we assumed that came from the biology offspring issue and not due to the incest itself. How could we deal with that?

If God stated that incest was an abominable custom practised before the Law, it’s very difficult for me to assert that there were no inherent problem with incest itself. But this assumption (incest is a GOD-immoral standard) leads us to have problems to understand His intention to create us from Adam and Eve only.

I appreciate your attention,


God Bless you

One of the most basic principles for understanding God’s word is called Analogia Scripturae (analogy of Scripture), or “Scripture interprets Scripture”. That is, we must not interpret Scripture in a way that contradicts another scriptural teaching. This latter is not only the explicit propositions of Scripture, but also that which is logically deducible from them (see Loving God with all your mind: logic and creation). This means, for example, that we should not interpret Scripture in a way to contradict the doctrine of the Trinity, a logical deduction from Scripture.

Similarly, I showed in Cain’s wife explanation ‘gross and disgusting’? that Scripture logically implies that there was brother-sister intermarriage at first. This follows from the clear teaching that all people come from a single first human couple, which is also the basis for the Kinsman-Redeemer and Last Adam roles of Christ (see The Incarnation: Why did God become Man?). As Greg Demme succinctly commented on Cain’s wife and brother-sister intermarriage:

Let us also remember the simple scriptural truth proclaimed in Gen. 3:20—“The man called his wife’s name Eve, because she was the mother of all living.” That statement by itself disallows any other special creations after Adam and Eve.

Also, Abraham’s marriage to his half-sister was never condemned, but clearly God blessed this marriage, promising that through this line, not through Hagar or Keturah, the Seed would come to bless all nations (see A brief history of the Jews). Certainly it’s hard to regard the Jews, who came from that union, as a defective race. Even the persistently anti-biblical New York Times had to admit:

Jews are a famously accomplished group. They make up 0.2 percent of the world population, but 54 percent of the world chess champions, 27 percent of the Nobel physics laureates and 31 percent of the medicine laureates.1

Therefore we must find another explanation for the biblical passages you raised. One comes from the biblical fact of progressive revelation. Before God’s laws were inscripturated, they were written on the human heart. The article Cain’s wife and brother-sister intermarriage continues:

Finally, why was Cain guilty of murder although the Mosaic Commandment had not been given (and why were the Sodomites guilty of sexual sin before the Mosaic commands against homosexual behaviour)? Answer: before Moses, God had written the moral law on people’s hearts. This is not directly stated in Genesis, but is logically deducible from Romans 2:14–15:

(Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. They show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts sometimes accusing them and at other times even defending them.)

Clearly God had not written any laws against sibling intermarriage on the human heart at the times of Cain and Moses.

Also, there are other reasons for God imposing the laws against brother-sister intermarriage at the time of Moses, as mentioned in the article:

A reason might be that by this stage of the Messianic program, the family was regarded as an extension of one’s own flesh (cf. Genesis 2:24), and violating “any flesh of his flesh” was then a sin (Leviticus 18:6). But as the scholarly 19th-century commentary by Jewish Christian scholars Keil and Delitzsch points out:

There Cain knew his wife. The text assumes it as self-evident that she accompanied him in his exile; also, that she was a daughter of Adam, and consequently a sister of Cain. The marriage of brothers and sisters was inevitable in the case of the children of the first men, if the human race was actually to descend from a single pair, and may therefore be justified in the face of the Mosaic prohibition of such marriages, on the ground that the sons and daughters of Adam represented not merely the family but the genus, and that it was not till after the rise of several families that the bands of fraternal and conjugal love became distinct from one another, and assumed fixed and mutually exclusive forms, the violation of which is sin.

The progressive revelation of Scripture implies that it was not until well after Abraham’s time that “fraternal and conjugal love became distinct”. Thus sometime between Abraham and Moses, God was writing on human hearts that brother-sister intermarriage broke these now-set boundaries, so was now immoral. So God inscripturated His new standard in the Mosaic Law.

Thus as shown, the scientific reasons we adduced for the prohibition—reducing the possibility of offspring inheriting two mutant alleles in a locus—is not the only reason for it. Rather, this is a case of science catching up to the Bible, further elucidating the fact that God makes laws for our good. See also Biblical history and the role of science.

This is why your thought experiment in your second-last paragraph doesn’t work. Rather, this presupposes that the only reason for the law was genetic. To give two analogies:

  1. Homosexual behaviour: There are many well documented health risks to homosexual behaviour—see for example the section “Disease and homosexuality” in Creationism and the problem of homosexual behaviour. Christians can point to avoiding these diseases as some of the benefits of obeying God’s laws on marital purity between a man and a woman. But even if these diseases could be avoided somehow, this would not mean that homosexual behaviour would be OK. It is wrong primarily because God has forbidden it, and it misses the mark (a biblical meaning of sin) of God’s marriage standard.
  2. Abortion: studies show that abortion is harmful for the mother as well, often resulting in uterine damage and emotional trauma,2 and increased rate of breast cancer.3 But even if abortion were somehow performed to improve women’s mental health and reduce the incidence of breast cancer, it still would be very wrong because it kills an unborn baby.

God bless you too.

Jonathan Sarfati

Published: 29 September 2012


  1. Brooks, D., The Tel Aviv Cluster, New York Times, 11 January 2011. Return to text.
  2. E.g. “Abortion is also bad for women: Evidence suggests that abortion can be potentially damaging for women’s mental wellbeing”, writes Ruth Cullen, Irish Examiner, 20 December 2011; from prolifecampaign.ie Return to text.
  3. Baklinski, T.M., National Cancer Institute Researcher Admits Abortion Breast Cancer Link, LifeSiteNews.com, 7 January 2012. Return to text.

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Christianity for Skeptics
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