“Where Did Cain Get His Wife?”—And Why the Question Still Matters

In the summer of 1925, the Scopes “Monkey” Trial turned a Tennessee courtroom into a national stage for debating the Bible and origins. One of the most memorable barbs came when Clarence Darrow pressed William Jennings Bryan with a line that has echoed through popular culture ever since: “Where did Cain get his wife?” The insinuation—delivered to laughter and headlines—was that anyone who believes Genesis must accept an absurdity (or worse, a moral outrage). That exchange helped cement the cultural narrative that Christianity is anti-intellectual and ethically inconsistent. A century later, the same argument still circulates on social media and in classrooms. Do Christians have a good answer to the question of where Cain got his wife?

This is a legitimate question. If Adam and Eve are the fountainhead of humanity (Gen 3:20; Acts 17:26), whom did their son Cain marry (Gen 4:17)? If the only available partners were siblings or close relatives, doesn’t that entail incest? And if so, how could that be compatible with God’s design for “one flesh” marriage (Gen 2:24)? Abraham’s marriage to his half-sister (Gen 20:12) can seem to make matters worse.

What is Incest Biblically?

Incest can be simply defined as a union between those who are related. Or, to put it another way, a union between two individuals who share certain degree of “sameness.” Scripture defines prohibited degrees of kinship in detail in Leviticus 18 and 20. Those laws forbid sexual relations and marriage among close relatives (e.g., parents, siblings, grandchildren, a wife’s sister as a rival, etc.; see Lev 18:6–18). If two people share a certain degree of sameness, then they cannot create a new “one flesh” relationship (cf. Gen 2:24) since they already share sameness.

However, at the beginning of the human race, it is possible that even those of the same family still had a significant degree of difference (i.e., the family of Adam and Eve). Therefore, brother and sister could be married because they were still significantly different from one another and did not share a significant amount of “sameness.”

Incest, Sameness, and Biblical History

The above line of reasoning seems to be confirmed if we compare the early history of Scripture with what we know about genetics today. When Adam and Eve were created by God, they contained the perfect, undamaged genetic information for the entire human race. All of the heights, skin tones, hair colors, eye colors, etc., were all possible combinations of their genes. However, because of sin, genetic mutations entered the human genome and gradually developed into pronounced traits over time.

After the flood, the human genome was completely contained in Noah and his family. As humanity spread out and segregated into various groupings (post-Babel), the genetic information became more and more diluted through mutations and isolation. By the time Israel became a nation, enough time and genetic mutations had occurred that there was significant “sameness” that existed between family units.

Therefore, in order to preserve the uniqueness of new “one flesh” relationships, as well as to preserve humanity from the deleterious genetic effects of intermarrying into the same genetic mutation patterns, God made clear that marrying close relatives was now forbidden. This would explain why marriage between family members in the pre-Israel time period was not taboo, but by the time God gave the Law to Moses (ca. 1446 BC), there was a significant amount of sameness that existed in people groups, and thus incest was a real possibility as described in Leviticus 18.

Another Possible Variation

Some people could be skeptical of the definition of incest that I’ve given above, so they might prefer to adopt the view this way: Cain marrying his sister should be considered incest, but because there were not biological problems during that time, God allowed it. Incest was not to be viewed as taboo until God outlawed it in Leviticus 18 and 20 (ca. 1446 BC). This is similar, but with the variation that Cain marrying his sister would still qualify as incest.

Conclusion

Although I strongly prefer the first variation, the point is that there are reasonable answers to those who would scoff at the Bible’s implication that Cain married his sister. Far from dragging Christianity through the mud, the old Scopes-era taunt gives Christians an opportunity to show how the biblical story makes sense when we think carefully about creation, law, and the way God has revealed himself.