It's probably best to distinguish between incest and inbreeding, but there is not a clear distinction. Incest, sexual relations between close relatives, has been frowned on by most societies. The Old Testament bans some specific sexual relationships, quite a few, in fact, in Leviticus 18. (At least in the NIV, it doesn't say it's banning marriage, but sex. I suppose that banning sex between, say, brother and sister would also prohibit marriage between them, but I'm not certain of that.) Inbreeding is a more general term. Inbreeding, to a geneticist, occurs when offspring are born to parents who are more closely related to each other than the general population is. There is a statistic, known as the inbreeding coefficient, which can be calculated.
Presumably, Adam and Eve were created with a perfect genotype. The Fall probably would have allowed mutations, gradually putting bad genes into the gene pool. This is consistent with the ages given in Genesis--they gradually got shorter and shorter, as would be expected as more deleterious genes came into the gene pool.
If they had perfect genes (no recessive bad ones) then inbreeding wouldn't matter, genetically, at least.
There are reasons other than genetics that make incest (or close inbreeding) bad, but the first humans would have had no choice, if they were the only possible mates for each other. Cain, and Seth, and maybe Abel, would have had to marry their sisters.
Were the first humans the only possible mates for each other? Probably.
However, there is the legend of Lilith (which is extra-biblical) who was another wife to Adam. George MacDonald wrote fantasy in the 19th century. C. S. Lewis was influenced by his writing. In MacDonald's Lilith, Adam, Eve, and Lilith are all characters. If there was another wife, then it would be possible to marry half-siblings, rather than siblings.
Genesis says:
Gen 6:1 And it came to pass, when men began to multiply on the face of the ground, and daughters were born unto them, Gen 6:2 that the sons of God saw the daughters of men that they were fair; and they took them wives of all that they chose. Gen 6:4 The Nephilim were in the earth in those days, and also after that, when the sons of God came unto the daughters of men, and they bare children to them: the same were the mighty men that were of old, the men of renown. (ASV, which is public domain)
Some interpret this as meaning that angels and humans married, although most seem to doubt this, for a number of reasons. This verse, of course, is not until Genesis 6, by which time Cain, Seth, and other generations of humans would have already had to marry somebody. There are other interpretations, such as that Seth's descendants married Cain's, and no one seems to know for sure what it means.
Madeleine L'Engle, Christian fantasy writer, wrote Many Waters (I don't usually link to Amazon, but they allow searching inside the book.) which supposes that humans of Noah's time could mate with demons, apparently basing this idea on the text above.
In Tolkien's Middle-Earth, there were three marriages (maybe four--see the article on Half-elven in the Encyclopedia of Arda) between elves and humans, and Eärendil, Elrond, Arwen and Aragorn were among the descendants of such unions. Tolkien didn't think of elves as like angels, but they were clearly different than humans. Maiar were more like angels. Melian (See entry under her name in the Encyclopedia of Arda) was of the Maiar, and married Thingol, an elf. They had a daughter, Lúthien. In C. S. Lewis' Narnia books, King Caspian married a star's daughter, and they had a son, Rilian.
Some believers think that God took pre-humans already in existence (Neanderthals?) and gave them souls. If that is what Genesis means, then presumably there could have been matings between humans and pre-humans.
Some ancient societies apparently did allow, and encourage, incest, at least in their royal families.
Abraham and Sarah were half-sibs, and Isaac and Jacob both married relatives. See the chart below (It should be possible to see larger versions of these charts by using either of them as a link, which will transfer you to Flickr. In Flickr, which doesn't require any password for such uses, you can click on the Down Arrow, to the right of the page, to download a graphic. That lets you see, or download, larger versions of the graphic.
There was marriage between relatives in David's family, also:
As to incest being evil, it is now. That doesn't mean that it was always evil. God had a different set of expectations for the Jews than He does for Christians. He at least allowed polygamy in some of His closest followers (although it often led to serious family trouble) in the Old Testament, but there doesn't seem to be even a mention of it in the New.
What about the question of inbreeding
A God who is capable of creation from nothing shouldn't have had any problems with giving Adam and Eve perfect genes, so that if Seth married a sister, it wouldn't have had bad genetic consequences. Even if Eve was created literally from Adam's flesh, He could have given them different sets of genes, so that apparent inbreeding wouldn't have mattered. I would guess that Eve at least had a second X chromosome, whereas Adam had only one, so that their genes were partly different.
This is all speculation, of course. The Bible doesn't name Cain's wife, Seth's wife, or Abel's wife (if he had one) let alone where they came from.
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This material was modified or added on April 6th, 2005:
The charts above are free for use, provided that any republication also allows free use by others. You can probably get a larger size through Flickr -- the two charts are links to the original charts, on my Flickr photostream. If I have made any errors in the charts, I'd appreciate knowing it. Leave a comment. Thanks.
October 17, 2007: I modified the charts, mostly adding more people, and making the chart of David's family more useful for general Bible study. I haven't checked to see if April's and Ish's web pages, that I referred to in the beginning of this post, are still available.
February 1, 2014: I modified the chart of David's family slightly, and changed the font size of the post.
October 20, 2014: I modified the chart of Terah's family.
Thanks for reading and looking!
