societies during historical times—ancient Egyptians, Bunyoro,nNyanza, and so on—institutionalized brother-sisternincest for royalty and a few other groups of high status. Butnthe practice is (or was) hedged with ritual, and in everyninstance the incestuous males are (or were) permitted tontake additional wives and hence to practice outbreeding.nThe avoidance of brother-sister incest originates in whatnpsychologists hae called prepared learning. This meansnthat people are innately prone to learn one alternative asnopposed to another. They pick it up more readily, theynenjoy it more, or both. The avoidance of sibling- incestncomes from the “potty rule” in mental development:nindividuals reared in close domestic proximity during thenfirst six years of life (they share the same potty) arenautomatically inhibited from strong sexual attraction andnbonding when they reach sexual maturity. The rule worksneen uhen the children reared together are biologicallynunrelated and later encouraged to marry and have children,nas in the Israel kibbutzim and traditional minor marriagesn(simpua) of prerevolutionary China. Those affected arenusually quite unable to offer a rational explanation of whynthe)’ feel no attraction. Some unconscious process tickednoer in the brain, and the urge, they explain, never came.nThe inhibitory rule is an example not only of preparednlearning but also of “proximate causation” as it is conceptualizednby evolutionary biologists. This means that it channelsna response of importance to the survixal or reproductionnof the organism. Proximate causes are put into place bynthe assembly of. genes through the process of naturalnselection. The ultimate causation, in other words thenparticular selection regime that enabled certain genes tonpredominate in the first place, is the well-documentedneffect of inbreeding depression. When mating occurs betweennbrother and sister, father and daughter, or mothernand son, the probability of matching debilitating genes innboth homologous chromosomes of the offspring is greatlynincreased. The end result is a rise in abortion, birth defects,nand genetic disease. Hence genes prescribing a biologicalnpropensity’ to avoid incest will be favored over those that donnot. Most animal and plant species display proximatende’ices of one kind or another, and it does indeed protectnthem from inbreeding depression. In some, the response isnrigidly determined. In others, especially the brighter mammals,nit is based on prepared learning. Interestingly enough,nthe human proximate form is nearly identical to that of thenchimpanzee.nIt is exquisitely human to semanticize innate tendencies.nIn many societies incest a’oidance is underwritten bynS}’mbolically transmitted taboos, myths, and laws. These,nnot the emotions and programs of prepared learning, are thenalues we percei’e by casual obserxation. They are easilyntransmitted from one person to the next, and the mostnreadily studied by scholars. But the phenomenon of greatestninterest is their etiology: the chain of events leading fromnultimate cause in natural selection to proximate cause innprepared learning to reification and legitimation in culture.nIf the terminal cultural form were somehow to be strippednaway by a collective loss of memory, people would stillnavoid sibling incest. Given enough time, they would mostnlikely invent religious and ethical rationalizations to justifyntheir feelings.nCrude genetic determinism has no part in this process.nThe existence of the three-step etiology in mental developmentnin no way constricts free will. Individual choicenpersists even when learning is strongly prepared by heredity.nIf some future society decides to encourage brother-sisternincest, for whatever bizarre reason, it now has the knowledgento do so efficiently. The possibility, however, isnvanishingly remote, because the same knowledge tells usnthat incest avoidance is programmed as a powerful rule andnprotects families from genetic damage. We are likely tonagree still more firmly that the avoidance is a part of humannnature to be fostered. In other words, incest avoidance isnand will continue to be one of our common values.nThis approach to moral reasoning is consistent with thenrelatively new discipline of sociobiology, but it is not annessential part—or even necessarily correct! Sociobiologynstrictiy defined is the systematic study of the biological basisnof social behavior. It is part of the mainstream of thenbiological sciences. But as its practitioners have extendedntheir modes of analysis into the domain for human behavior,nthey have run into trouble precisely because of thensometimes unwelcome implications for social theory. Innmy opinion, shared by some but far from all other scientists,nthis is the way to go; the materialistic metaethic is essentiallyncorrect. However, applications to specific moral problems arenalways subject to additional evidence and new challenges.nThey are also subject to questions about the generality ofntheir importance. It will immediately occur to the readernthat incest avoidance might rank as nothing more than anspecial case. There is a vast difference between this relativelynsimple phenomenon and economic cycles, religiousnrites, and Presidential elections. Might such particularitiesnfall within a wholly different domain of explanation andnrequire a different metaethic? Perhaps, but I don’t think so.nThe e’idence favoring the evolutionary approach tonmoral reasoning is as follows. By mid-1985 no fewer thann3,577 human genes had been identified, of which aboutn600 had been placed on one or another of the 23 pairs ofnchromosomes. This is a respectable fraction of the entirenhuman complement, since Homo sapiens is a Class IIInspecies with no more than several hundred thousand genesn(it is what the genes do, not their absolute number, thatncounts). New techniques for separation and identificationnmake it possible eventually to map all of the genes, perhapsnby early in the next century. Hundreds of the genes alreadynknown alter behavior in one way or another. In most casesnthe effect is crude or indirect. But a few change behavior inna precise manner, as for example those modulating depression,nreading ability, and performance on spatial tests. Twinnand adoption studies have implicated other genes, as yetnunmapped and probably working in complex multiples, innschizophrenia, propensity toward homosexuality, performancenon tests measuring empathy, and a wide range ofnpersonality traits from introversion-extroversion to athleticismnand proneness to alcoholism. Moreover, preparednlearning and biases in perception have been discovered innvirtually every category of behavior thus far studied.nOf equal importance, people often behave as though theynwere acting in their genetic self-interest, even when this endnresult is not easily deduced during everyday life. Fornexample, men tend to favor the children of their sistersnnnAUGVST 198B /13n