Longevity factor in hominoid social organization

In most primate groups emigration of the maturing young of one or the other sex tends to serve as an incest avoidance mechanism. Among most primate species it is the males who change groups. This supports the theory that, in terms of reproductive success, males should compete for mates and females should compete for resources. In hominoids the combination of increased longevity and greater female discrimination in mate selection seems responsible for female emigration. This may relate to the high frequency of patrilocality and male control of resources among human groups.     

Avoidance and prohibition of brother–sister sex in humans

Sex between full sibs is unusual in birds, mammals, and humans. These species likely possess an innate avoidance mechanism based on early proximity (i.e., the Westermarck hypothesis), and the rare occurrences may be attributable to error. Alternatively, an inclusive fitness argument shows that a low rate of sib mating may be an adaptation. The widespread occurrence of a prohibition against brother–sister sex in human societies is often invoked as evidence against an innate avoidance mechanism, since if the latter were to exist the former would be superfluous. However, given that punishing violators is costly, a prohibition is more likely to spread through an egalitarian society when the prohibited behavior is already avoided. I describe a model of the cultural dynamics of the sibling incest taboo which we have used to investigate this possibility. The predictions derived from this model are consistent with, and add rigor to, Westermarcks theory of the origin of the incest taboo.     

Male mating competition,female choice and dominance in a free ranging group of Japanese macaques

Focal and ad libitum samples of high and middle ranked males in a group of free ranging Japanese macques were taken in order to examine rank related differences in male mating strategies. Males tended to have like ranked females as consort partners, with high rank males showing more consort activity, over all, than middle rank males. High rank males tended to interfere in consorts and middle rank males tended to have their consorts disrupted. Consorts involving high rank females were most subject to interference. With one exception, proximity between partners in consorts involving high rank males was due to male actions while proximity in consorts involving middle ranked males was due to female actions. The two highest ranked males were never observed copulating. Their mating failure may have been due to avoidance by females who had known them since immaturity. High rank males were somewhat more likely than middle rank males to have consorted with females during the period of likely conception. There was some evidence that frequent consort partners joined the same subgroup during a group fission. Males appeared to use the advantage conferred by high rank mainly in competition for high rank females. Females showed some indications of preference for mates likely to retain or attain high rank in the future.     

Kinship on the Kibbutz: coresidence duration predicts altruism, personal sexual aversions and moral attitudes among communally reared peers

The natural experiments created by the Israeli Kibbutzim and Taiwanese minor marriages provide unique testing grounds for investigating the mechanisms governing sibling detection, inbreeding avoidance and kin-selected altruism. Here we present two studies conducted on the coreared peers of Israeli Kibbutzim. We examined how coresidence duration — a cue that would have indicated genetic relatedness in ancestral environments — impacts the development of kin-directed behaviors. In both studies, we found that coresidence duration predicts levels of altruism and sexual aversions directed toward peers. We also investigated the relationship between personal sexual aversions and moral attitudes relating to peer sexual behavior. The absence of norms proscribing sex between peers on the Kibbutz allows for a more tightly controlled investigation of this relationship. We found that total coresidence duration with opposite-sex peers predicts the intensity of moral wrongness associated with third-party peer sexual behavior, but not other behaviors, including sibling incest. More directly, we found that the summed sexual aversion felt toward all opposite-sex peers predicts levels of moral wrongness associated with third-party peer sex. Mediation analyses confirmed that personal sexual aversions mediate the relationship between coresidence duration and moral attitudes regarding peer sex. These results bolster Westermarck's original claims that childhood coresidence serves as a kinship cue, leading to greater sexual aversions and altruistic motivations, and that personal sexual aversions shape attitudes relating to third-party sexual behavior.     

Does evolutionary biology contribute to ethics?

Human propensities that are the products of Darwinian evolution may combine to generate a form of social behavior that is not itself a direct result of such pressure. This possibility may provide a satisfying explanation for the origin of socially transmitted rules such as the incest taboo. Similarly, the regulatory processes of development that generated adaptations to the environment in the circumstances in which they evolved can produce surprising and sometimes maladaptive consequences for the individual in modern conditions. These combinatorial aspects of social and developmental dynamics leave a subtle but not wholly uninteresting role for evolutionary biology in explaining the origins of human morality.     

Olfactory sexual inhibition and the westermarck effect

The Westermarck effect (sexual inhibition among individuals raised together) is argued to be mediated olfactorily. Various animals, including humans, distinguish among individuals by scent (significantly determined by MHC genotype), and some avoid cosocialized associates on this basis. Possible models of olfactory mechanisms in humans are evaluated. Evidence suggests aversions develop during an early sensitizing period, attach to persons as much as to their scents, and are more powerful among females than among males. Adult to child aversions may develop similarly, but more likely result from stimulus generalization. This hypothesis accords with current evidence and yields testable predictions (e.g., anosmia will prevent inhibition) that, should they be supported, will conclusively ground the Westermarck effect in a biological mechanism. A version of this paper was originally presented at the 92nd annual meeting of the American Sociological Association in Toronto, Canada. Mark A. Schneider is a sociologist of science and author of “Sacredness, Status and Bodily Violation” (Body and Society, 1996) as well as Culture and Enchantment (Chicago, 1993). Lewellyn Hendrix has written extensively on families in cross-cultural perspective and is the author of Illegitimacy and Social Structures (Bergin and Garvey, 1996). They recently coauthored “Assumptions on Sex and Society in the Biosocial Theory of Incest” (Cross-Cultural Research, 1999).