QUOTE : The ultimate cause suggested by the biological hypothesis is the loss of genetic fitness that results from incest. It is a fact that incestuously produced children leave fewer descendants. The biological hypothesis states that individuals with a genetic predisposition for bond exclusion and incest avoidance contribute more genes to the next generation. Natural selection has probably ground away along these lines for thousands of generations, and for that reason human beings intuitively avoid incest through the simple, automatic rule of bond exclusion. To put the idea in its starkest form, one that acknowledges but temporarily bypasses the intervening developmental process, human beings are guided by an instinct based on genes. Such a process is indicated in the case of brother-sister intercourse, and it is a strong possibility in the other categories of incest taboo.