خویشاوندی در کیبوتس: پیش بینی نوع دوستی، بیزاری جنسی شخصی و نگرش اخلاقی در میان همسالان با مدت زمان اقامت مشترک

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.

Research on the cognitive architecture of human kin detection has started to uncover the cues used to identify different categories of close genetic relatives (Alvergne et al., 2007, DeBruine, 2005, Fessler and Navarrete, 2004, Kaminski et al., 2010, Lieberman et al., 2003 and Lieberman et al., 2007; for a review, see Park, Schaller, & Van Vugt, 2008). According to the model of kin detection proposed by Lieberman et al. (2007), cues indicative of kinship in ancestral environments are taken as input by two separate motivational systems: one that regulates altruistic behaviors as detailed by inclusive fitness theory (Hamilton, 1964) and another that regulates sexual attraction and aversion, thereby avoiding the deleterious consequences associated with close inbreeding (Bittles and Neel, 1994 and Tooby, 1982). With respect to siblings, recent findings suggest that there are at least two ecologically valid cues that aid detection: prolonged periods of shared parental investment — operationalized as childhood coresidence duration — and exposure to maternal–infant associations. Specifically, Lieberman et al. (2007) found that when an individual did not observe his or her mother caring for a particular sibling during infancy (as would have been the case for the younger sibling in a sibpair and paternal half siblings), coresidence duration with that sibling predicts opposition to sexual behaviors with that sibling and sibling-directed altruism. In contrast, when the maternal–infant association cue was present (as would have typically occurred for the older sibling in a sibpair), coresidence duration does not predict aversions or altruism. Rather, the presence of this cue is associated with elevated levels of both types of kin-directed behavior. The nonadditive effects of these kinship cues led Lieberman et al. (2007) to postulate the existence of a kinship estimator that, in the case of siblings, uses coresidence duration to compute estimates of relatedness conditional on the presence of the alternate, more reliable cue of maternal–infant association. Despite these recent findings, a number of questions regarding human kin detection remain unanswered. One question is whether coresidence duration, absent other kinship cues, is really a driver of sexual aversions and altruism. To date, in the populations used to psychologically investigate kinship cues and the development of kin-directed behaviors, multiple potential cues co-occur, making it difficult to assess whether the actual kinship cue is, say, coresidence duration, or another parameter such as culturally shared kinship terms (Jones, 2004). This leaves open the question of whether childhood coresidence duration predicts sexual aversions and altruistic tendencies in populations where linguistic confounds and confounds such as actual genetic relatedness, facial similarity and olfactory similarity are absent. A second question relates to morality. Previous studies have used moral and punitive reactions toward third-party sibling incest as a dependent measure of personal sexual aversions — one solution to the ceiling and social desirability effects inherent in asking about sexual behaviors with one's own siblings (Fessler and Navarrete, 2004 and Lieberman et al., 2003). But this raises the question of the relationship between personal sexual aversions and moral attitudes relating to incest. Westermarck (1891/1922) suggested that the personal sexual aversions that target childhood associates manifest themselves on the cultural stage as proscriptions against incest. That is, moral attitudes might be patterned after personal sexual aversions. Some research supports this claim. For instance, Lieberman et al., (2007) found that coresidence durations with opposite-sex siblings predict moral sentiments relating to third-party sibling incest in the same way they predict personal sexual aversions. Other research, however, suggests otherwise. Royzman, Leeman, and Sabini (2008) found no difference in moral attitudes relating to sibling incest between individuals with versus individuals without opposite-sex siblings. If opposite-sex sibling coresidence duration shapes personal sexual aversions and moral attitudes alike, then differences should have been observed. One problem with past studies that have examined the relationship between inbreeding avoidance and moral views relating to incest is that they have been conducted in cultures where norms proscribe behavior between genetic relatives (Fessler and Navarrete, 2004, Lieberman et al., 2003, Lieberman et al., 2007 and Royzman et al., 2008). It is therefore difficult to assess the extent to which moral views are shaped by inbreeding avoidance mechanisms, cultural norms or perhaps both. What is needed to disentangle the causal contributions of kinship cues, which regulate personal sexual aversions, and societal norms is a population where sanctions against sexual relations between individuals raised as siblings are absent. Here we present a novel methodology that allows us to empirically investigate both lines of inquiry: the validity of coresidence duration as a cue to siblingship and the relationship between personal sexual aversions and moral attitudes. Specifically, we take advantage of the cultural institutions of Israeli Kibbutzim where children were raised outside the nuclear family household in communal children's houses (“bet yeladim”) with approximately 6–20 other unrelated peers born around the same time (Talmon, 1972). The children's house is where peers slept, ate and bathed. Aside from returning to their parent's home for a few hours each afternoon, children spent most of their time with their peers in their own children's house. This living arrangement led to the coresidence of genetically unrelated peers starting from birth and, in many cases, extended until age 18 years when individuals went into the military. What makes the Kibbutz a unique population to investigate moral attitudes is that there were no explicit prohibitions against marrying a coreared peer group member (Talmon, 1964). In fact, parents tended to prefer peers as marriage partners for their children (Shepher, 1983). 1.1. Research questions and predictions The Kibbutz rearing environment offers a special and historically rare opportunity to address fundamental questions regarding human kin detection, inbreeding avoidance and morality. First, does coresidence duration serve as a kinship cue and predict levels of sexual aversions and altruistic motivations among coreared peers? According to the living arrangement of the Kibbutz, coresidence duration would have been a readily available kinship cue. Peers would not have seen their own mother caring for (e.g., breast-feeding) other peers, suggesting that maternal–infant perinatal association would not have been used as a primary kinship cue. If, as previous research suggests (Shepher, 1971), coresidence duration is used as a cue to siblingship in the children's houses of the Kibbutz, then it should predict psychological aversions toward opposite-sex peers as well as peer-directed altruism. Second, are moral views relating to third-party sexual behavior a derivative of personal sexual aversions that develop toward those categorized as kin? If so, then the same kinship cue predicting personal sexual aversions should also predict these moral attitudes. Applied to the case of the Kibbutz, coresidence durations with opposite-sex peers should predict not only personal sexual aversions toward these peers but also moral sentiments relating to the sexual behavior between peers in general. However, coresidence duration with peers should not necessarily predict other types of moral sentiments (e.g., see Lieberman et al., 2007), including sexual behavior between actual siblings. That is, if the by-product explanation is correct, then the type of individual categorized as kin should form the class of individuals about whom moral rules regarding sex apply. With these questions in mind, we designed the following study.