From The Art and Popular Culture Encyclopedia
Incest is sexual activity between family members who would be forbidden to marry. Incest constitutes a cultural taboo in all current nations and many past societies. In many areas, incest is also prohibited by law.
Livestock breeders often practice controlled breeding to eliminate undesirable characteristics within a population, which is also coupled with culling of what is considered unfit offspring, especially when trying to establish a new and desirable trait in the stock.