Answer: Cain married a sister, or a niece, or perhaps a grandniece.
This question was popularized by the famous Scopes-Monkey Trial in Dayton, Tennessee in 1925, which pitted Clarence Darrow against William Jennings Bryan. Bryan failed to answer the question about Cain’s wife posed by Darrow. Ever since skeptics have accused the Bible of error, because it says that Cain had a wife, who bore children (Gen 4:17), when it is assumed that no other women existed besides Eve at this point.
There is a very obvious and reasonable answer to the question: “Where did Cain get his wife?” Cain obviously married a sister, or a niece, if Abel had also married a sister. Someone might object by saying, “But the Bible doesn’t mention Adam and Eve having any daughters.”
The Bible, however, does state that Adam and Eve had some daughters. In Gen 5:4, we read, “After Seth was born, Adam lived 800 years and had sons and daughters.” It is reasonable to conclude that he also had some daughters in the time period related to Cain and Abel. So Cain and Abel would have been able to marry sisters. That the Bible does not specifically mention them by name is not unusual, as the Bible often only mentions key players in a narrative.
Something that is often overlooked is the age of Cain and Abel when Cain murdered Abel. They must have been at least 100 years old, for Adam was 130 years old when Seth was born shortly after Abel’s death (Gen 4:25). So there was plenty of time for Cain and Abel’s sisters to grow up and marry. It would have been very unusual for Adam and Eve, probably the most fertile couple ever to live, to only have two boys over the course of 100 years.
Another objection to Cain marrying his sister is the danger of marrying a close relative. Such marriages usually produce serious birth defects. However, in the beginning days of the human race, there were no defective genes. Defective genes gradually polluted the human race over thousands and thousands of years. Today when close relatives marry, the chances of them having common mutant genes is great, but in the beginning there was no such risk because the gene pool was still perfect.
According to Gen 20:12, Abraham married Sarah, who was his half sister. It was only later that marriage to close relatives was forbidden, after defective genes had significantly polluted the gene pool of the human race.
We also need to remember that Moses had a Ph.D. from the University of Egypt (Acts 7:22). He was clearly an intellectual giant. If he, the author of Genesis, said that Cain married a woman and had children, and if the only person he could have married was his sister, then that’s what happened. Surely Moses wasn’t so dull that he goofed on such a basic fact!