The term incest refers to the marriage and/or sexual intercourse between two individuals considered to be close kin according to local cultural norms. Although incest typically applies to cases when couples are in fact genetic relatives, it can also apply when couples are genetically unrelated, yet are categorized as kin according to local customs. In one sense, then, rules against incest can be seen as a way to regulate who one marries (and has sex with) within a particular culture. Although incest is typically a term used to understand norms relating to marriage patterns, the related term inbreeding is used to mark the degree of genetic relatedness between mating partners. Both incest and inbreeding are used interchangeably, yet they refer to slightly different concepts, with incest being a topic of greater interest in anthropology and cultural psychology and inbreeding a topic of greater interest in biology and cognitive science. This entry focuses on the aspect of incest that overlaps with inbreeding: the mating of individuals who are genetically related by virtue of sharing a recent common ancestor.
There are sound biological reasons that natural selection would have led to the evolution of mechanisms to reduce the probability of mating with a close genetic relative. Throughout the evolutionary history of our species, the selection pressures posed by harmful genetic mutations and disease-causing organisms would have severely negatively affected the health and viability of offspring of close genetic relatives. All else being equal, individuals who avoided mating with a close genetic relative and instead mated with someone who did not share an immediate common ancestor would have left a greater number of healthier offspring. Importantly, the negative consequences of incest are enhanced the more closely related two individuals are, with the most severe consequences occurring between individuals who have a probability of .5 of sharing particular genes (i.e., brother and sister, mother and son, or father and daughter). The deleterious effects drop off as two partners become less closely related. Interestingly, in most (if not all) societies, incest between nuclear family members is forbidden or simply absent. In the United States, incest laws vary by state, but most have sanctions targeting marriage and sexual intercourse with a parent, child, sibling, grandparent, grandchild, niece, nephew, uncle, and aunt. Although some states even sanction first-cousin marriage, none sanctions second-cousin marriage. A variety of studies have documented the negative fitness consequences associated with incest. For instance, in both humans and nonhuman animal species, incest is associated with an increased risk of mortality, mental deficiencies, congenital malformations, and disease. Given these negative consequences, it is likely that evolution engineered mechanisms to prevent individuals from choosing close relatives as sexual partners. But what might such mechanisms look like?
To avoid close genetic relatives as sexual partners, a well-designed mechanism would require (at least) two types of procedures: (1) procedures that categorize individuals according to their probability of relatedness (i.e., procedures for detecting kin), and (2) procedures that use information regarding kinship to regulate sexual attraction. With respect to kin detection, what cues do humans use? Because we cannot see another person's DNA, the best evolution could do is to use cues that were reliably correlated with genetic relatedness in the ancestral past to compute a probability of relatedness. To the extent that different cues identified different categories of kin (e.g., mother, father, sibling, offspring), different detection mechanisms likely exist.
One cue found to mediate the detection of a particular type of relative, siblings, is childhood coresidence duration. The longer one lives with another person starting from birth, the greater the sexual aversion that develops toward that individual later during adulthood. Furthermore, longer periods of childhood coresidence are associated with lower incidents of adult sexual behavior and greater moral opposition to sibling incest. The effect of childhood coresidence on sexual attraction is known as the Westermarck Effect after the 19th-century Finnish social scientist, Edward Westermarck, who first proposed that early childhood association leads to the development of a sexual aversion later during adulthood.
Two well-known natural experiments provide compelling evidence for the Westermarck Effect. The first is the communal childrearing practices of the Israeli kibbutzim, where unrelated children were put into children's houses starting from a few weeks after birth and raised together under siblinglike conditions. In these communities, individuals raised together in the same children's house rarely married one another despite the absence of any rules forbidding such unions. This pattern suggests that early childhood exposure influences later sexual attraction.
The second natural experiment testing the Westermarck Effect is the case of Tawainese minor marriages. In this form of marriage, a young bride is adopted into her future husband's family as a newborn and raised alongside him until one day, during adulthood, the parents determine it is time for them to marry. Compared with marriages in which the husband and wife met for the first time as adults, in minor marriages, there were lower rates of fertility and greater rates of divorces and extramarital affairs.
These two experiments point to childhood cores-idence duration as one cue that the human mind uses to detect kin and mediate incest-avoidance behaviors. As they suggest, individuals do not have to be genetically related to develop a sexual aversion toward one another. This can be seen in coreared adopted and stepsiblings who also develop intense sexual aversions toward one another despite knowing they are not genetic relatives.
But cues other than childhood coresidence might also play a role in incest avoidance. For instance, seeing one's mother caring for (e.g., breastfeeding) a newborn might serve as a potent cue to kinship. This cue would have only been available for older siblings already present in the social environment and would have been reliable regardless of coresidence duration. But younger children, who are not around to see their mother pregnant and caring for a newborn, might rely on coresidence duration or other possible cues such as facial similarity or olfactory recognition.
Certainly there is much to be learned about the processes mediating how humans avoid incest. The prior discussion illustrates that particular social cues might govern kin detection and the development of sexual aversions. But many questions remain. For instance, what cues do humans use to detect other types of close genetic relatives: Are they the same as those found for siblings or do they differ? What emotions regulate incest avoidance? What contextual factors influence opinions about incest? How can scholarly understanding of incest-avoidance mechanisms inform the field of child abuse and neglect? If kinship cues are required to activate sexual aversions toward close genetic relatives, it is important to identify which cues operate for each type of family member. Circumstances in which the evolved cues indicating relatedness are absent might lead to greater risks of incestuous unions (e.g., as can occur when brothers and sisters are reared separately or when men marry women with children from another marriage). Last, how does our evolved psychology influence legal codes related to incest? The next few decades promise to shed light on this culturally universal yet underexplored behavior.
Abuse and Violence in Relationships, Evolutionary Perspectives on Women's Romantic Interests, Evolutionary Psychology and Human Relationships, Fictive Kinship, Interpersonal Attraction, Kin Relationships, Kin Selection, Mate Preferences
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