"Who was Cain's wife?" has been a question many biblical scholars have tried to answer for ages. While we don't know exactly who Cain's wife was, she was most likely his sister, niece, great-niece, or another female relative (Genesis 4:17). Intermarrying relatives is not right today, but during the time of Adam and Eve, there were no other people, so there was no other way for the human population to increase. This meant that Adam and Eve's children needed to marry each other and one another's nieces and nephews.

God did not forbid interfamily marriage until the world became populated enough that it was no longer needed (Leviticus 18:6–18). In our modern day, intermarriage between family members would likely result in health issues for their kids. However, during the days of Adam and Eve, their genes would not yet have been damaged by the results of sin. While it may be strange to think about, Cain and his wife would have been related.

Cain and his wife left for the Land of Nod after Cain killed his brother, Abel (Genesis 4:16). God placed a mark on Cain's head to protect him, "But the Lord said to him, 'No. Anyone who kills you will be paid back seven times.' The Lord put a mark on Cain. Then anyone who found him wouldn't kill him" (Genesis 4:15).