The question is almost always posed in a topic of biblical discussion concerning the Book of Genesis. 

We have only a record of three sons in Genesis, Cain, Abel, and Seth. We know Cain, the firstborn, murdered Abel and was banished to a land towards the east called Nod.  Since the bible tells us we are all descended from Adam and Eve, where could Cain have found a wife?

According to Genesis 4, Cain not only found a wife, but by the time of the birth of his son Enoch, he found enough people to help him build and populate a city!

We're also told that Adam and Eve did not give birth to Seth until after the murder of Able.

How could this have possibly happened from only 2 original people on the earth? 

Resolving the Population Problem

The first step in the solution of this problem is to recognize that Adam and Eve had many more children than Cain, Abel, and Seth.

Genesis 5:4  "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters"

The genealogy of Genesis 5 records that every descendant of Adam all the way down to Lamech, had "sons and daughters."

These other sons and daughters were born to some of the men who, at the time, were more than 180 years old! Genesis 5:28 records:

"And Lamech lived an hundred eighty and two years, and begat a son:..."

Considering the long life spans recorded in Genesis 5 and assuming that couples remained reproductive for about half their lifetime (as it is now), the possibility existed for a huge population in a relatively short time.

Mathematically, The world's population could have approached a few billion by the time Adam's died at the age of 930. 

Examine the following table to see the possibility. 

According to simple mathematics, if Cain waited to marry until he was about 200 years old, he probably had several women to choose from.

 If Cain waited another 200 years to build a city, he could have had a a few thousand people to help him.

It is entirely possible that Cain had sisters from whom to take a wife even before his banishment and before the birth of Seth. Since the bible does not list the "daughters" who were born nor the timing of their birth and since, the text does not tell us, we don't have any way to know how many sisters he may have had on the day he was banished.   

Resolving the "Incest" Problem

Wait a minute you say!  He married a sister? It is quite possible! 

Doesn't that violate the commands of God?  The plain answer is no, it does not.   The Levitical laws had not yet been given and the only condemnation or forbidden marriage was that between children and their parents, not between brothers and sisters.   There were also no laws prohibiting a man from marrying a niece.   Abraham, for example, married his half-sister.

There was absolutely no prohibition against intermarriage at the time and the blood, genetic structure and physical characteristics of mankind would not have been corrupted as it is today.

 Not until the time of Moses were laws established forbidding a man from marrying a sister or niece. The timing of this command makes perfect sense biologically, for genetic mutations and defects as a result of intra-family marriage would not begin to crop up until after the first few dozen generations. 

Archeological evidence indicates that population growth did not take off explosively in the pre-Flood era. What happened? I believe it is quite possible that the sin of Cain eventually grew out of control.  Cain's greatest fear was that someone would kill him.

Murder may well have become the number one cause of death among mankind and considering the very strong language God used with Noah in Genesis 9:6:  "Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man", it is entirely possible that this was the case. 

Noah's descendants were to exercise whatever means necessary; up to the death penalty to contain and restrain the sin of murder.  

The Mystery of Cain's Mark

Genesis 4 tells us that God found it necessary to put a mark on Cain as a warning to other humans not to take vengeance upon him. The necessity of this mark indicates that the population of the human race had reached (or would shortly reach) such a level that mistaking Cain for someone else would be a problem.

The text says nothing about what kind of mark Cain received or about its being passed on to his descendants.

There are some very vile teachings that certain races of man today came from Cain and some have attempted to use 1 John 3:12 to imply that Cain was the physical "love child" of an unholy union between Satan and Eve.  This is pure nonsense and is in itself a demonic doctrine.

1 John 3:12:  "Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous."

The phrase "who was of that wicked one" simply means that Cain followed the ways of Satan and his works were evil.  This is much like the statement Christians make saying we are "in the world" but not "of the world".  It would be ludicrous to apply a literal meaning to the phrase "of the world" after having been born again.  It is likewise, ludicrous, to state Cain was born of fornication between Eve and Satan.  The meaning, folks, is SPIRITUAL.  Cain was "of the ways of" that wicked one: Satan not "fathered by" Satan.

There is absolutely no Biblical basis for believing that any one of the races of man carries the mark of Cain, came from Cain or have any defect because of Cain. 

Where did Cain find a wife?  Depending upon the time frame of his marriage, it is quite possible he had anywhere from 10-500 women from which to choose depending upon his age at the time of marriage. 

The Word of God is True!  Do you Believe it?