Cain’s Wife & Why the Pure Genetics Defense Fails
The Problem Stated
The question of where Cain got his wife is an age old question. On the one hand, one could posit that Cain married one of his sisters ; however, invoking incest to solve this problem is an uncomfortable option for many people, especially given the prohibition on incest in the Book of Leviticus. On the other hand, one could posit that Cain married a woman outside of his family; however, this entails that Adam and Eve are not the sole biological progenitors of the entire human race. In a cursory investigation, it appears that the majority of Christians think the biological primacy of Adam and Eve is too high a price to pay to resolve this issue and accept the first option: that Cain married his sister. In response, there have been efforts to resolve the tension generated by the incest problem. One popular solution is what I will call the “pure genetics defense”. This solution posits that incest was originally acceptable because Adam and Eve had pure genes and no substantial mutations occurred as a result of incest. As time went on, the mutations accumulated, so, God put a restriction on incest in Leviticus as a result. As an example, check out this explanation by Ken Ham. I think that this argument fails on two counts.
This objection applies primarily to old earth creationists who hold that a sole-biological-progenitor lived about 150,000 – 200,000 years ago. It seems that if God imposed the incest restriction to curb genetic mutations, He would’ve done it much earlier on this view. In other words, it seems rather unlikely that these mutations only became a problem in the last 3500 years and not the preceding 146,000. I suppose one could posit that God providentially preserved genetic integrity for the first 98% of human history and then let decay take over; it just seems rather ad hoc. This defense simply does not seem to be simultaneously available if the OEC and YEC are defending mutation rates that vary by several orders of magnitude. Thus, the plausibility of the pure genetics defense appears to be inversely proportional to how old one thinks humanity is.
This objection is universally applicable and more problematic. The pure genetics defense appears to compromise the logic of Romans 5. Consider the following
We employ the standard division of the Law: moral, ceremonial, and civil. Briefly, the civil law consisted of the laws for Israel as a theocratic nation, ceremonial laws were the laws concerning sacrifices/holy days, and the moral laws are the revelation of God’s character which are binding still today. Where do the incest laws fall? Leviticus 18 has traditionally been categorized as being a part of the moral law and thus binding for all time. This makes sense since statutes on improper sexual relationships don’t seem to be the type of thing to be categorized with holy days and government operations. Moreover, the conclusion of Leviticus 18 says that God is judging the other nations by this standard, implying that the chapter is applicable to all humans and not just Israel . Let’s leave that aside for just a second and hop over to Romans.
Look at the argument in Romans 5:12-14. Paul argues that sin came into the world through one man, death came through sin, and death spread to all men through their sin. In other words, everyone dies because everyone is under the curse of sin. Notice what he says: sin was in the world before the law. Traditionally, this is taken to mean that all men are subject to the moral law that is written upon their hearts (as described in Romans 1) and that for trespassing that law, the punishment is death.
Now, let’s put these two thoughts together. On the one hand, we have that while the law was not given, sin was still in the world (this sin being violation of the moral law). On the other hand, we have that incest (along with murder, theft, etc) is a part of the moral law. What happens with the genetics defense? The moral law is relegated to pure practicality . Incest isn’t *actually* a moral violation; it is something that has unfortunate effects, so, God decided to put an end to it for practical reasons. Here are the implications:
(1) In order to preserve the genetics defense, we have to kick Leviticus 18 to another category of law (either ceremonial or civil); however, I do not think that the text will allow this.
(2) If we keep the genetics argument and keep Leviticus 18 in the moral law category, Romans 5 falls apart because there would be no sin before the giving of the Law; alternatively, one could say that everything but incest was in the moral law but that seems capricious.
The pure genetics defense fails as it redefines the moral commandments of God as mere practical measures. But, is there a solution to this conundrum? I am agnostic on a specific response; however, there are a few directions that I think may be fruitful to explore.
First, one could accept that there were other humans besides Adam and his family. This is a fairly technical response and much more detail could be offered, but I will be brief here. Many shirk away from this response because they thing that it undermines the Gospel; however, I think that this is allowable within the Genesis text (a) given the ambiguity of ‘adahm’ meaning both ‘mankind’ and ‘the man Adam’ and (b) it does not seem to contradict Adam being the first man. As for preserving the Gospel message, it seems to me that the federal headship of Adam provides adequate theological categories to preserve the logic of Christ as the Second Adam. If the connection that we have with Christ is a covenantal connection, then, I do not see how our biological relationship with Adam would change matters. Moreover, it does not seem apparent that a biological connection is required to preserve Original Sin. Thus, all die covenantally in Adam. This route is not perfect as there are other, secondary doctrines that become difficult to square with an “en masse” creation of multiple humans .
Second, one could accept that incest was required to populate the earth and that it was sinful. God sometimes accomplishes His purposes through the sinful acts of humanity; there may not need to be an exception here. What I do think is a problem is removing the sinfulness of incest at the expense of Romans 5 and Leviticus 18.
Third, one could nuance Leviticus 18. For example, verse 19, which places a prohibition on sexual relations with one’s wife during her menstrual period, is often considered a purity law for national Israel instead of a moral law binding for all times, persons, etc. If this is true, then, a hermeneutical case could be made for Leviticus 18 as a mixture of different kinds of laws that possibly double dip in the different categories. The difficulty for this line of thought is overcoming vv. 24-25  which judge the nations outside of Israel for doing all of the actions mentioned.
Fourth, the genetic response may be somewhat salvageable if couched in different terms. For example, rather than positing that Adam & Eve had perfect genes, one could posit that Adam & Eve were imbued with higher genetic potential: enough to (1) appear as though the genetic bottleneck was larger than two people and (2) allow for their children to have enough genetic diversity so as to not be biologically related. This line of thought has been developed by Lydia McGrew . This solution at least appears to weaken the problem since it would not technically be incest, but, does not seem to fully rebut the textual considerations. This is a fruitful direction for maintaining internal consistency; however, an additional hermeneutical argument will be needed as supplement.
 Or one of his cousins, but, this doesn’t solve the problem as Cain’s hypothetical brother would’ve married his sister.
 vv. 24-25 NASB -“Do not defile yourselves by any of these things; for by all these the nations which I am casting out before you have become defiled. For the land has become defiled, therefore I have brought its punishment upon it, so the land has spewed out its inhabitants.”
 This is not to say that there are not practical benefits to the moral law. There are; however, practical benefits are not the primary purpose of the law. The genetics defense changes practicality from a benefit of the law to the purpose of the law.
 Acts 17:26, for example: “and He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation” (NASB)
 Read her article here.