Where did Cain’s wife come from in Genesis 4?

This is a fairly common question. I have only seen only one reasonable
explanation which does not require bending the biblical text. That is to
assume that Adam and Eve had a great number of children and that Cain
married one of them. Clearly if there were only one man and woman created
by God, then their children would have to intermarry.

This explanation may bother a person who is used to the idea that incest
(which would clearly include sex with a sibling) is bad. Attempts to
explain why God does what he does can cause one to skate on thin ice, but
let me speculate why God “changed his mind” on the incest issue. The fact
is that there is a significantly increased risk of a great number of
genetically-linked diseases for children of closely related parents. This
may explain God’s admonition that incest is a sin. God is protecting us
from serious disease. One can assume that Adam and Eve were created
without genetic defects, which would allow their children to intermarry
without fear of inducing genetic-related diseases in their children. Over
the thousands of years since the original couple, a good number of
mutations which have led to genetic diseases have entered the human
population, making it quite dangerous to marry close relatives.

So the answer to the question you ask is that Cain married one of his
sisters. One possible answer to the question you did not ask, but which
you surely have in mind is that what applies to us did not apply to the
children of Adam and Eve. Incest is dangerous for us but was not for them.

John Oakes, PhD