Evidence for frequent incest in a cooperatively breeding mammal

As breeding between relatives often results in inbreeding depression, inbreeding avoidance is widespread in the animal kingdom. However, inbreeding avoidance may entail fitness costs. For example, dispersal away from relatives may reduce survival. How these conflicting selection pressures are resolved is challenging to investigate, but theoretical models predict that inbreeding should occur frequently in some systems. Despite this, few studies have found evidence of regular incest in mammals, even in social species where relatives are spatio-temporally clustered and opportunities for inbreeding frequently arise. We used genetic parentage assignments together with relatedness data to quantify inbreeding rates in a wild population of banded mongooses, a cooperatively breeding carnivore. We show that females regularly conceive to close relatives, including fathers and brothers. We suggest that the costs of inbreeding avoidance may sometimes outweigh the benefits, even in cooperatively breeding species where strong within-group incest avoidance is considered to be the norm.     

Does morality have a biological basis? An empirical test of the factors governing moral sentiments relating to incest.

Kin-recognition systems have been hypothesized to exist in humans, and adaptively to regulate altruism and incest avoidance among close genetic kin. This latter function allows the architecture of the kin recognition system to be mapped by quantitatively matching individual variation in opposition to incest to individual variation in developmental parameters, such as family structure and co-residence patterns. Methodological difficulties that appear when subjects are asked to disclose incestuous inclinations can be circumvented by measuring their opposition to incest in third parties, i.e. morality. This method allows a direct test of Westermarck's original hypothesis that childhood co-residence with an opposite-sex individual predicts the strength of moral sentiments regarding third-party sibling incest. Results support Westermarck's hypothesis and the model of kin recognition that it implies. Co-residence duration objectively predicts genetic relatedness, making it a reliable cue to kinship. Co-residence duration predicts the strength of opposition to incest, even after controlling for relatedness and even when co-residing individuals are genetically unrelated. This undercuts kin-recognition models requiring matching to self (through, for example, major histocompatibility complex or phenotypic markers). Subjects' beliefs about relatedness had no effect after controlling for co-residence, indicating that systems regulating kin-relevant behaviours are non-conscious, and calibrated by co-residence, not belief.     

The Westermarck effect revisited: a psychophysiological study of sibling incest aversion in young female adults

Research on human inbreeding avoidance has uncovered at least two central cues to the detection of siblings: witnessing an infant being taken care of by one's mother (i.e. maternal perinatal association) and growing up in close proximity to a child (the so-called Westermarck effect). Both cues have been supported by fieldwork in populations under specific cultural conditions, and by survey research mainly in student populations. Both types of research have relied often on self-reporting. Unfortunately, this method is frequently colored by ceiling effects and notions of social desirability. In order to circumvent this problem, we explored a complementary method for investigating incest aversion that involved measuring psychophysiological responses during an imagery task. As such, we analyzed data on 63 heterosexual female students who viewed pictures of sexual and non-sexual activities while imagining performing these activities with either their partner or their brother. In female subjects with only (one or more) older brothers—a proxy for lacking maternal perinatal association with an opposite-sex sibling—the duration of coresidence with brother(s) predicted activity in the mm. levator labii superioris and alaeque nasi, facial muscles that are highly active when a subject expresses facial disgust. The strength of these responses was also predicted by the frequency of having bathed and shared a bedroom with a brother in early childhood; two activities that may serve as additional cues for relatedness as it can be expected that they are typically performed with genetically related children. As a result, the psychophysiological approach not only complements the use of self-reports in recent research on incest aversion, but also has the potential to fine-tune well-established cues for sibling detection, or to uncover additional ones.     

SENSUAL ATTACHMENT AND INCEST AVOIDANCE IN HUMAN EVOLUTION AND CHILD DEVELOPMENT

The authors take issue with the critique of psychoanalysis and the depiction of human sexuality and incest avoidance in evolutionary psychology. Drawing on human neurobiology and evolutionary anthropology, they show that human beings have an evolved disposition toward pair-bonding and evolved capacities for self-regulation of sexual and aggressive impulses. The realization that these characteristics are not only important but also interrelated leads to a reassessment of the Oedipus complex, a new model of incest avoidance in humans, and a fresh perspective on the relation between reproductive behaviour and environmental conditions.     

Causes and consequences of incest avoidance in the cooperatively breeding mole-rat, Cryptomys darlingi (Bathyergidae)

A number of social mole-rat species maintain a strong reproductive skew (only one breeding pair in the group) solely through incest avoidance. Incest avoidance probably evolved for one of two reasons, namely for actually maintaining a reproductive skew or, alternatively, to avoid high inbreeding depression. In the latter case a strong reproductive skew would result as a fortuitous by-product of the combination of a cloistral family life style of mole-rats and incest avoidance. We undertook breeding experiments in which the fertility of pairs of unrelated individuals were compared with that of pairs of double first cousins. Inbreeding depression was remarkably high and an accompanying model suggests that it may be sufficient to support the idea that strong incest avoidance evolved primarily to eliminate the costs of inbreeding and subsequently facilitated the evolution of reproductive skew.     

Is friendship akin to kinship?

Although unrelated friends are genetically equivalent to strangers, several lines of reasoning suggest that close friendship may sometimes activate processes more relevant to kinship and that this may be especially true for women. We compared responses to strangers, friends, and kin in two studies designed to address distinct domains for which kinship is known to have functional significance: incest avoidance and nepotism. Study 1 examined emotional responses to imagined sexual contact with kin, friends, and strangers. Results revealed that women, compared to men, treated friends more like kin. Study 2 examined benevolent attributions to actual kin, friends, and strangers. Results revealed that women treated friends very much like kin, whereas men treated friends very much like strangers. The current findings support a domain-specific over a domain-general approach to understanding intimate relationships and raise a number of interesting questions about the modular structure of cognitive and affective processes involved in these relationships.     

Reproductive roles in the cooperatively breeding acorn woodpecker: incest avoidance versus reproductive competition

Incest is rare in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) despite a polygynandrous mating system in which nearly all group members are close relatives. Here we test the relative importance of avoiding matings between close relatives (incest avoidance) and within-sex competition for breeding opportunities (reproductive competition) in determining the mating system of acorn woodpeckers by examining how reproductive roles change following breeding vacancies. In 83% of cases in which helpers of the same sex were present in the group, reproductive vacancies were resolved when new unrelated immigrants filled the vacancy to the exclusion of resident same-sex helpers, who generally emigrated or did not breed while they remained in the group. Helpers of the opposite sex, especially when male, were significantly more likely to remain in their natal group and in about half the cases inherited and bred following reproductive vacancies. This result was not explainable by reproductive competition, since the number of immigrants was often less than or equal to the number of same-sex helpers in the group. Apparent incest resulted in 5% of cases. The time required to resolve reproductive vacancies was significantly longer for groups with helpers of the same sex as the vacancy. These results confirm that both incest avoidance and reproductive competition are important factors determining reproductive roles within groups of this highly social species.     

Intergenerational incest aversion: self-reported sexual arousal and disgust to hypothetical sexual contact with family members

The biological costs of inbreeding are expected to have shaped human incest aversion. These costs depend on biological sex, relatedness, and age. Whereas previous studies have focused on investigating how these factors modulate incest aversion in siblings and cousins—family members of the same generation— we examined relatives of different generations. In a population-based sample, 2499 respondents reported reactions to imagined sexual behaviors with either a biological child or parent, a niece/nephew or aunt/uncle, or a stepchild or stepparent; these responses were compared to reactions to imagined sexual behaviors involving a friend's child or parent. Replicating prior results, women report stronger incest aversions than do men. We extend previous findings by showing that incest aversions tended to be stronger between close (vs. more distant) intergenerational relatives. Indeed, for biological relatives, decreased degree of relatedness was associated with decreased incest aversion, and for biological relatives, the certainty in relatedness was also positively associated with incest aversion. As expected, age modulated sexual aversion for unrelated, but not related, target individuals. Sexual aversions towards step-relatives did not differ from sexual aversions to biological relatives.     

The evolutionary significance of incest rules

Evolutionary theory predicts that humans should avoid incest because the behavior depresses individual fitness through production of defective offspring. Selection for avoidance of close-kin mating has resulted in a developmental mechanism that promotes voluntary incest avoidance. If avoided, why are social rules constructed in most cultures to regulate incest? The suggestion in this article is that incest do not regulate close-kin mating, but instead regulate inbreeding between more distant kin and sexual relations between nonkin. Inbreeding (e.g., cousin marriage) is hypothesized to be regulated because if it occurs, it can concentrate wealth and power within families threatening the powerful positions of rulers in the society. The hypothesis was supported using a worldwide sample of 129 societies, whereas two other alternative hypotheses (one dealing with coadapted genomes, and the other with sexual reproduction and host/ parasite coevolution), were not.     

Same-Sex Avoidance Relations and What They Reveal about Male Competition

Avoidance relations between male kin are a pervasive social phenomenon, yet the subject has received comparatively little treatment in the anthropological literature. When anthropologists have addressed it, they have usually done so indirectly, or put forward theories better suited to explaining other social phenomena. The most common explanations one comes across in the anthropological literature to account for avoidance relationships between male kin, or what I also describe as same-sex avoidance relations in the paper, are the incest taboo and Radcliffe-Brown's theory of respect. In contrast to these explanations, I propose to demonstrate that the reason male kin avoid each other in certain types of settings is not just to maintain a sense of authority and precedence as Radcliffe-Brown's theory implies, or to avoid contravening incest prohibitions, which as Robert Lowie pointed out long ago is incorrect. Rather, because closely related male kin should not compete with each other, as this would contravene the ideology of descent which demands loyalty to one's kin.     

The Precultural Basis of the Incest Taboo: Toward a Biosocial Theory

The literature on the origins of the incest taboo is characterized by controversy over the nature/nurture issue, and fears of reductionism. In recent years work emanating from such diverse disciplines as cultuml and physical anthropology, ethology, and neuropsychology warmnts a new look at this intriguing issue. It is probable that incest avoidance is widespread among the vertebrata and is "built into the wiring." As learned behavior becomes more important phylogenetically, curiosity and exploration plays a larger role in adaptation and has manifest survival admntages. Incest awidance functions as a mechanism to propel the individual into new relationships and "social territory." For humans, incest avoidance and its later elaboration into a cultuml taboo serve to motivate exploration of and attachment to a wider social nexus than the family. It also prevents fixation at a relatively undifferentiated psychological stage of development     

Sibling recognition in the prairie vole, Microtus ochrogaster

Since mating is seldom observed between sibling prairie voles, Microtus ochrogaster, this, behavioural discrimination was used to investigate sibling recognition in this species. Cross-fostering of 1–3-day-old pups demonstrated that unrelated pups reared together did not breed, whereas siblings reared apart bred readily when paired at weaning. When unrelated voles were paired at 14 days of age, prior to sexual maturity, significantly fewer pairs bred than when strangers were paired at 21 days of age. Separation of 21-day-old siblings for 8 days before pairing overcame incest avoidance; a 15-day separation was required for breeding by siblings that had remained together until they were 50 days old. These results indicate that sibling recognition and, consequently, incest avoidance depend on association prior to weaning.     

Brother-sister and parent-child marriage outside royal families in ancient egypt and iran: A challenge to the sociobiological view of incest avoidance?

Copious and unequivocal evidence of legally condoned and socially favored brother-sister and parent-child marriage among common people from Roman Egypt (first to third centuries, A.D.) and Zoroastrian Iran (fifth century, B.C. to 11th century, A.D.) can be taken to pose a challenge to the sociobiological case for universal evolved incest avoidance within the nuclear family, triggered by early childhood proximity (the Westermarck effect). Official census documents from Roman Egypt show a high incidence of full sibling unions with relatively small age gaps between the spouses and no indication of reduced marital fertility, sexual aversion, or increased infant and child mortality. Zoroastrian religious tracts actively encourage nuclear family incest and extol its meritorious nature and supernatural benefits. A schematic assessment of the likely extent of inbreeding depression in such families under conditions of very high mortality through other causes makes reproduction at replacement level seem difficult to accomplish. But given the lack of information on the frequency of deleterious recessive genes in these populations, this reconstruction is fraught with uncertainty; pertinent ancient evidence is suggestive of some incidence of inbreeding depression but remains inconclusive. Aversion and revulsion between incestuous spouses proves a similarly elusive issue. Although these cases from antiquity do not clearly contradict the view of incest avoidance as an evolved mechanism that engenders sexual indifference and normally translates into corresponding cultural norms, they demonstrate the need for a more comprehensive consideration of the available historical record in the testing of evolutionary rules.     

Beyond the Taboo: Imagining Incest

This article provides an overview of anthropology's 150-year discussion of the incest taboo in light of the last 30 years of feminist and psychoanalytic discoveries about the incestuous abuse of children, it invites anthropologists to explore incest ethnographically and offers three suggested ways: one biosocial, a second social relational, and a third psychoanalytic, focusing on a connection between what psychologists call dissociation and what anthropologists call trance or possession. [Key words: incest taboo, childhood sexual abuse, dissociation, trance/possession, innate avoidance mechanism]     

Disappearance of the Incest Taboo: A Cross-Cultural Test of General Evolutionary Hypotheses

Employing a cross-cultural sample of 121 societies, this research tests and supports three hypotheses offered by Yehudi Cohen (1978) concerning the relationship between the general evolution of society and the extension of the incest taboo. Cohen generally proposes that the number of relatives included in the incest taboo will be reduced as societies become technologically and socially more complex. More specifically, Cohen identifies the development of trade institutions as eliminating the need for extended incest regulations. Furthermore, Cohen proposes that as the incest taboo contracts and becomes less important, violations of this taboo are treated less severely.     

Homosexual incest avoidance among females in captive Japanese macaques

This paper reports a systematic pattern of homosexual incest avoidance among females in a captive group of Japanese macaques (Macaca fuscata) culled from the Arashiyama-West troop known for its high rates of female homosexuality. The study group included three matrilines and two generations. Between eight and 11 females were sexually active over four consecutive mating seasons, and all engaged in both heterosexual and homosexual activity. While all females performed homosexual acts with almost all possible non-kin partners, they systematically avoided homosexual interactions with their mother, daughters, and sisters. This pattern could not be explained either in terms of kin not being simultaneously in estrus, kin avoiding affiliative interactions in general, or non-kin utilizing the tension-reducing effect of estrus to affiliate exclusively with each other. In contrast to homosexual females, heterosexual pairs of relatives (brother-sister, mother-son) were sometimes incestuous. Assuming that female homosexuality expresses the reproductive strategy of females unconstrained by male influence, the present results point to the strong tendency of females to avoid incest and suggest that males are primarily responsible for the reported exceptions to incest avoidance.     

Coming too Close,Going too Far: Theoretical and Cross‐cultural Approaches to Incest and its Prohibitions

Although interest in cultural explanations of topics like incest waned among Anglo anthropologists along with the abandonment of kinship theory in general, other paradigms provided a renewed interest in some older theories about the prohibition of incest. The approach presented in this paper gives due critical consideration to both evolutionary and ‘cultural’ explanations of incest and its prohibition suggesting their mutual involvement, but the necessity of a close analysis of the specificity of the cultural forms in which shared substance, connection, difference and nurture are expressed as reproductive ontology. Particular cases are discussed in the light of recent suggestions that the cross‐cultural incidence of incest and its prohibition is to be understood via theories of dissociative disorders in the West that allegedly illuminate a wide range of practices claimed to be linked to sexual abuse in childhood.     

Inbreeding avoidance through non-random mating in sticklebacks

Negative effects of inbreeding are well documented in a wide range of animal taxa. Hatching success and survival of inbred offspring is reduced in many species and inbred progeny are often less attractive to potential mates. Thus, individuals should avoid mating with close kin. However, experimental evidence for inbreeding avoidance through non-random mating in vertebrates is scarce. Here, we show that gravid female three-spined sticklebacks (Gasterosteus aculeatus) when given the choice between a courting familiar brother and a courting unfamiliar non-sib prefer to mate with the non-sib and thus avoid the disadvantages of incest. We controlled for differences in males' body size and red intensity of nuptial coloration. Thus, females adjust their courting behaviour to the risk of inbreeding.     

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.