Julie and Mark are brother and sister. They are travelling together in France on a summer vacation from college. One night they are staying alone in a cabin near the beach. They decide that it would be interesting and fun if they tried making love. At the very least it would be a new experience for each of them. Julie was already taking birth control pills, but Mark uses a condom too, just to be safe. They both enjoy making love, but they decide not to do it again. They keep that night a special secret, which makes them feel even closer to each other. Was it okay for Mark and Julie to make love? [Out of Character, 41]
Almost everyone posed with this question answers with a resounding no. Yet when asked to explain their rationale for their answer, no logical answer can be provided. Since there are no objective consequences to this sort of incest, how can anyone, on rational grounds, possibly object to it?
Objections to incest clearly arise from strong innate predispositions against it. It is not a consequence of philosophical or moral premises imbibed in childhood. While the horror of incest may be rational in the sense that it helps prevent problems associated with inbreeding, the emotions one feels are not a consequence of such careful, thoroughly researched and peer reviewed scientific calculations. The science came only much later, long after the predispositions arose.
Rand claims that there is no such thing as an innate predisposition (or tendency), since that would contradict free will:
A free will saddled with a tendency is like a game with loaded dice. It forces man to struggle through the effort of playing, to bear responsibility and pay for the game, but the decision is weighted in favor of a tendency that he had no power to escape. If the tendency is of his choice, he cannot possess it at birth; if it is not of his choice, his will is not free.
This being the case, how would Rand explain the strong predisposition, nearly universal, against incest? Why does nearly everyone make the same choice about incest, but not the same choice regarding which political or economic system they prefer? Why does there exist a near universal horror of incest, but not a near universal horror of eating broccoli, or folk dancing, or socialism, or many of the other irrational follies of mankind?
Not only is Rand unable to adequately explain the predisposition against incest, she would fail at providing a moral rationale against it as well. Yes, she could, like everyone else, invoke the horrors of inbreeding. But if the participants used birth control (or were sterile), that reason no longer applies. So what other "rational" reason can be invoked? Could psychological objections be raised? But how can they be when one assumes that a person's psychology is a product of his premises? If psychological problems arise, why not just change the person's premises, thus adapted their psychology to specific conduct?
These difficulties aren't noticed because Rand never allowed herself to be challenged on such issues. Without effective criticism, human beings tend to slip into rationalization, which is the default form of thinking in ethics (and philosophy).