Evolution and Human Behavior最新文献

Considered a part of the behavioral immune system, disgust functions as a protective mechanism against potential pathogen threat. There is evidence that disgust sensitivity varies depending on immunological and hormonal changes, including those occurring during the menstrual cycle or pregnancy. Although some studies indicate that disgust is elevated in early pregnancy, no study has yet compared disgust sensitivity in pregnant and non-pregnant women. This study aimed to examine differences in disgust sensitivity in pregnant versus non-pregnant women, while investigating whether disgust sensitivity differs depending on the phase of the menstrual cycle in non-pregnant women. The sample included 172 women (aged 21–40) in the first trimester of pregnancy and 354 non-pregnant, naturally cycling women (aged 20–40), out of whom 218 (61.6%) were in the luteal phase. All women filled out the Disgust Scale-Revised and the Three Domains of Disgust Scale. Non-pregnant women also completed the Culpepper Disgust Image Set. We observed that pregnant women had significantly higher pathogen-related and sexual disgust sensitivity than their non-pregnant counterparts (in both the follicular and luteal cycle phases). In non-pregnant women, there was no difference in disgust sensitivity between women in the follicular and luteal phases. When comparing pregnant women, women in the follicular phase, and those in the luteal phase, pathogen-related disgust sensitivity was lowest in the follicular, then in the luteal phase, and the highest in early pregnancy, although the difference between women in the follicular and luteal phase was not significant. Our results provide further evidence for the hypothesis that disgust is elevated when there is a need for increased protection, such as in the critical period of organogenesis in the first trimester of pregnancy.

Two well-supported generalizations from aggression research are that: a) people are less likely to commit homicide against close kin compared to non-kin, and b) females are less likely to engage in direct aggression than are males. Aggression between siblings, however, is somewhat more complicated than one might surmise from those two generalizations. Data from 3 studies collected using undergraduate and Prolific samples (N = 1640) reveal classic sex differences in direct aggression between non-relatives, but not between sisters and brothers. Whereas only a small minority of females have hit a friend or an acquaintance, the majority of females, like the majority of males, have hit a sibling. Although reputational aggression is substantially less likely between siblings than between friends or acquaintances, mild forms of direct aggression (such as hitting) are quite frequent between siblings. Discussion considers several possible limitations of the findings reported here and considers results in light of Trivers' theory of parent-offspring conflict.

Young children sometimes help others at a cost to themselves, but little is known about the emotional mechanisms underlying this behaviour. Here, 5-year-old children (n = 96, 45 girls, mean age = 5.57 years, SD = 1.79 months, range = 5.19 years to 5.9 years, families recruited from a local database based in a medium-sized German city) were engaged in one task and then asked either to help (child-helps) or watch (child-watches) an adult complete another task. Children would lose (cost) or not lose (no-cost) the progress they had made on their own task if they engaged with the adult. Children were more likely to interrupt their own task in the helping condition and were overall faster to do so when helping was not costly. Children who chose to incur a cost to help showed more positive emotions after helping—as measured via changes in their postural elevation—compared to helping at no cost. This pattern was not found in the child-watches condition. This suggests that costly helping holds emotional rewards for children in ways that non-costly helping does not.

Research on close relationships often focuses on the dyad (e.g., dyads of romantic partners) and on how dyad members affect each other's welfare. But dyads exist embedded in broader, densely-interconnected social networks, and less research attention has been paid to the myriad ways in which people outside the dyad impact one's welfare through their interactions with, or even their attitudes about, the other member of the dyad. What drives our feelings toward such extra-dyadic individuals? Balance Theory, an influential formalist theory in social psychology, suggests that our feelings are driven by the need for affective balance, achieved by, for example, liking strangers who share our feelings toward our existing partners or by disliking strangers who do not. We propose an alternative theory, the Embedded Dyad Framework, which foregrounds the substantive effects that strangers can have on our welfare through their interactions with our dyadic partners. Across four experiments (N = 1589) with U.S.-residing participants we predict and find, consistent with the Embedded Dyad Framework, that we like strangers who share our hatred for our rivals and our love for our friends (consistent with Balance Theory); but we dislike strangers who share our love for our spouses (contradicting Balance Theory). Further supporting predictions from an Embedded Dyad Framework, (a) greater perceived exclusivity in welfare-enhancing dyadic relationships (e.g., friendships) drives our lesser liking of strangers who share our love for our partners, and (b) greater perceived welfare suppression by our antagonistic partners (e.g., rivals) drives our liking of strangers who share our hatred of our antagonists. This framework outpredicts cognitive consistency views by emphasizing the real threats and opportunities that dyadic relationships afford people when dyads are embedded in social networks.

Many primates produce copulation calls, but we have surprisingly little data on what human sex sounds like. I present 34 h of audio recordings from 2239 authentic sexual episodes shared online. These include partnered sex or masturbation, but each recording has only one main vocalizer (1950 female, 289 male). Both acoustic features and arousal ratings from an online perceptual experiment with 109 listeners recruited on Prolific follow an inverted-U curve, revealing the likely time of orgasm. Sexual vocalizations become longer, louder, more high-pitched, voiced, and unpredictable at orgasm in both men and women. Men are not less vocal overall in this sample, but women start moaning at an earlier stage; speech or even minimally verbalized exclamations are uncommon. While excessive vocalizing sounds inauthentic to listeners, vocal bursts at peak arousal are ubiquitous and less verbalized than in the build-up phase, suggesting limited volitional control. Human sexual vocalizations likely include both consciously controlled and spontaneous moans of pleasure, which are perhaps best understood as sounds of liking rather than signals specific to copulation.

Why do people blame, devalue or derogate the victims of misfortune? The literature suggests general factors like a belief in a just world or a desire to distance oneself from misfortune, but the empirical results are often unclear. Here we suggest another potential factor in victim-devaluation in particular. Attitudes to victims should be seen in the context of human cooperation, as victims can be a source of costs for others and, therefore, may constitute poor potential cooperation partners. If that is the case, devaluation should be associated with a reluctance to offer help to victims. As predicted, across six pre-registered studies, we found that participants' reluctance to donate their own money (their bonus for participation), or allocate other people's money to a victim predicted the devaluation of the victim's character. Both devaluation and willingness to help were influenced by manipulating the victim's apparent competence, and the victim's concern for other people's possible costs, two crucial dimensions of cooperative potential. These results are consistent with the overall hypothesis that people's intuitions about a victim's cooperation potential are relevant to victim-devaluation.

Social scientists have often claimed or implied that hazing selects out uncommitted newcomers in voluntary associations. Because groups that engage in hazing are generally secretive about their practices, there has never been a real-world, in situ test of this claim. Using an American social fraternity, we report the first real-world, longitudinal test of hazing's relationship with selective newcomer attrition. Our data are derived from six sets of fraternity inductees who experienced the fraternity's hazing induction process (N = 126). Our analyses suggest that experienced hazing severity is a predictor of attrition and that hazing severity differentially predicts the attrition of low-commitment newcomers. However, real-world fraternity inductions (and measurements thereof) are complex in ways that add important caveats to our findings. Our discussion focuses on the best means by which to confirm or disconfirm our results through future replications.

According to the Trivers-Willard hypothesis, mothers who give birth to sons when their general condition is good and daughters when their condition is bad have an advantage in fitness. The purpose of this study was to test this hypothesis in humans by examining sex differences in birth weight according to maternal age based on a comparison of the birth weights of twins. A total of 2138 Indian twins (668 opposite sex, 2940 same sex) from the 2015–2016 National Family Health Survey IV database were identified for analysis. In total, 3700 mothers were at low risk for having low-birth-weight babies in terms of maternal age (ages 20–34 years) and 576 mothers were at high risk in terms of maternal age (20 years younger or 35 years or older). The results of the analysis of covariance showed that: 1) the birth weight ratio of opposite-sex twins is small (female newborns are heavier) when the mothers are at a high risk age. 2) At a high risk age, female newborns in opposite-sex twins, who can receive sex-based discriminatory investments, are heavier at birth than female babies in same-sex twins. These results remained significant after controlling for variables potentially related to birth weight. This study provides potential evidence that in utero selection is retained by the mother as a countermeasure against threats in terms of fitness.

Studies investigating facial attractiveness in humans have frequently been limited to studying the effect of individual morphological factors in isolation from other facial shape components in the same population. In this study, we go beyond this approach by focusing on multiple components and populations while combining geometric morphometrics of 72 standardized frontal facial landmarks and a Bayesian statistical framework. We investigate preferences in both sexes for three structural components of other sex facial beauty that are traditionally considered indicators of biological quality: symmetry, sexual dimorphism, and distinctiveness (i.e., the opposite of averageness). Based on a large sample of faces (n = 1550) from 10 populations across the world (Brazil, Cameroon, Czechia, Colombia, India, Namibia, Romania, Turkey, UK, and Vietnam), we found that distinctiveness negatively affects the perception of attractiveness in both sexes and that this association is stable across all studied populations. We corroborated some previous results indicating both a positive effect of femininity on male assessment of female facial beauty and a null or weak effect of masculinity on female evaluation of male facial attractiveness. Facial symmetry had no effect on facial attractiveness. In concert with other recent studies, our results support the importance of facial prototypicality but cast doubt on the role of symmetry as one of the key constituents of attractiveness in the human face.

Parochial altruism (PA), or ingroup favoritism paired with outgroup hostility, is sometimes treated as a synonym for human intergroup relations. However, empirical data suggest that PA is highly variable—across individuals, across situations, and across groups. Here, we review theory and data on PA to explore the candidate sources for this variability. Along the way, we unpack assumptions (e.g., what constitutes a group?), identify precursors to PA behavior (e.g., context and internal states), and review evidence for the pairing of ingroup favoritism with outgroup hostility. We discuss phenomena with measurable impact on downstream behavior, including resource access and cultural institutions, but also flag how researcher expectations and methodological design impact reported variability in PA. We close by making recommendations for how researchers can reduce noise in the study of PA by checking assumptions and being deliberate in research design; this is key, as the PA literature is part of sensitive public discourse.