Evolution & Human Behavior - Articles in Press

Sex differences in jealousy: a meta-analytic examination - Corrected Proof

2012-05-11

Abstract: The theory of evolved sex differences in jealousy predicts sex differences in responses to sexual infidelities and emotional infidelities. Critics have argued that such differences are absent in studies that use continuous measures to assess responses to hypothetical infidelities or in studies that assess responses to real infidelities. These criticisms were tested in two random-effects meta-analyses of 40 published and unpublished papers (providing 209 effect sizes from 47 independent samples) that measured sex differences in jealousy using continuous measures. A significant, theory-supportive sex difference emerged across 45 independent samples using continuous measures of responses to hypothetical infidelities, g*=0.258, 95% confidence interval (CI) [0.188, 0.328], p<.00001. Measured emotion significantly moderated effect size. Effects were strongest when measures assessed distress/upset (g*=0.337) and jealousy (g*=0.309). Other commonly measured negative emotions yielded weaker effects, including hurt (g*=0.161), anger (g*=0.074), and disgust (g*=0.012). Across the 45 independent samples, six significant moderators emerged: random sampling, population type (student vs. nonstudent samples), age, inclusion of a forced-choice question, number of points in the response scale, and year of publication. A significant, theory-supportive effect also emerged across seven studies assessing reactions to actual infidelities, g*=0.234, 95% CI [0.020, 0.448], p=.03. Results demonstrate that the sex difference in jealousy neither is an artifact of response format nor is limited to responses to hypothetical infidelities.

Fear is readily associated with an out-group face in a minimal group context - Corrected Proof

2012-05-10

Abstract: Research on prepared learning demonstrates that fear-conditioning biases may exist to natural hazards (e.g., snakes) compared to nonnatural hazards (e.g., electrical cords) and that fear is more readily learned toward exemplars of a racial out-group than toward exemplars of one's own race. Here we push the limits of the generalizability of the mechanisms underlying race biases in a fear-conditioning paradigm by using arbitrary group categories not distinguished by race. Groups were distinguishable solely by t-shirt color, with assignment based on performance in a perceptual task. In this “minimal group paradigm,” we found that out-group exemplars were more readily associated with an aversive stimulus than exemplars of one's in-group. Our findings suggest that prepared learning in an intergroup context is not limited to contexts involving racial categories involving histories rife with cultural stereotypes and that previous findings of learning biases along racial lines may be interpreted as a by-product of a broader psychological system for prepared fear learning toward categories of agents that may have posed persistent threats over human evolutionary history.

Short stature in African pygmies is not explained by sexual selection - Corrected Proof

2012-05-10

Abstract: African pygmies' short stature has been studied for more than a century, but the evolution of this extreme phenotype remains unknown. The present study tests the hypothesis that sexual selection, through preference for short partners, may have contributed to the evolution of pygmies' stature. We gathered anthropometric and familial data from 72 Baka pygmy couples and 27 neighboring Nzimé nonpygmy couples from Cameroon. We found evidence for positive assortative mating and partial evidence for the male-taller norm in both groups. This is surprisingly close to results reported for many modern occidental populations, in which sexual selection is thought to exert a positive selective pressure on men height. Semistructured interviews of Baka pygmies concerning height and mate choice suggested that the male-taller norm matches mating preferences. Stature was also positively correlated with the number of serial marriages contracted by men of both populations, while the stature of women was not related to their mating success. Finally, we did not detect any linear or quadratic effect of height on reproductive success for either men or women. Altogether, our results demonstrate that stature influences mate choice in pygmies, and we argue that, if of any influence for sexual selection, mate choice should have favored tallness rather than shortness in our pygmy population. Consequently, this study establishes that sexual selection is a very unlikely candidate to account for the evolution of pygmies' short stature.

Childlessness drives the sex difference in the association between income and reproductive success of modern Europeans - Corrected Proof

Julia A. Barthold, Mikko Myrskylä, Owen R. Jones — 2012-05-09

Abstract: The association between reproductive success and income in economically developed societies remains a controversial and understudied topic. The commonly made statement that individuals with a higher income have fewer children defies evolutionary explanation. Here we present results from an analyses of the association between lifetime reproductive success (LRS) and income for modern Europeans from 13 countries. We examine the relationships among income, partner income, sex and LRS, and the role of childlessness in driving the relationships. For women, we find a negative association between LRS and income, while for men, we find a flat or slightly positive one. The sex difference in the association appears to be driven by income's sex-specific association with childlessness; men with a low income have a relatively high risk of childlessness, while women with a low income have a low risk of childlessness. Consequently, once childless people are excluded from the analysis, LRS is negatively associated with income for both sexes. We argue that the observed LRS–income associations may be an outcome of evolved behavioural predispositions operating in modern environments and conclude that, even though humans fail to maximise LRS at all income levels in modern settings, evolutionary theory can still help to explain sex differences in LRS.

A Bayesian approach to the evolution of social learning - Corrected Proof

Charles Perreault, Cristina Moya, Robert Boyd — 2012-04-25

Abstract: There has been much interest in understanding the evolution of social learning. Investigators have tried to understand when natural selection will favor individuals who imitate others, how imitators should deal with the fact that available models may exhibit different behaviors, and how social and individual learning should interact. In all of this work, social learning and individual learning have been treated as alternative, conceptually distinct processes. Here we present a Bayesian model in which both individual and social learning arise from a single inferential process. Individuals use Bayesian inference to combine social and nonsocial cues about the current state of the environment. This model indicates that natural selection favors individuals who place heavy weight on social cues when the environment changes slowly or when its state cannot be well predicted using nonsocial cues. It also indicates that a conformist bias should be a universal aspect of social learning.

Religious cognition down-regulates sexually selected, characteristically male behaviors in men, but not in women - Corrected Proof

Michael E. McCullough, Evan C. Carter, C. Nathan DeWall, Carolina M. Corrales — 2012-04-23

Abstract: Men are typically stronger, riskier, “showier,” and more impulsive than women. According to sexual selection theory, such behaviors may have enhanced reproductive fitness for ancestral human males. However, such behaviors are facultative, and the mechanisms that cause them respond to social and environmental cues that indicate whether outlays of strength, risk-taking, showing off, or impulsivity are likely to lead to payoffs in any given instance. Recent research based on the Reproductive Religiosity Model suggests that, in contemporary Western societies, religious beliefs and institutions are differentially espoused and promulgated by restricted sexual strategists (whose reproductive strategies focus on high fertility, monogamy, and high parental care) to limit the exercise of unrestricted sexuality, which threatens the viability of restricted sexual strategies (e.g., by reducing paternity certainty and male parental investment). On this basis, we hypothesized that experimental manipulations of religious cognition would reduce men's impulsivity and motivation to demonstrate their physical prowess. Supporting this hypothesis, three experiments revealed that priming participants with religious concepts (i.e., participants wrote essays about religion, read an essay supporting the existence of an afterlife, or were implicitly exposed to religious words) reduced men's (but not women's) impulsivity with money and their physical endurance on a hand grip task. The primes affected men's behaviors irrespectively of men's scores on a self-report measure of religious commitment.

Prepared social learning about dangerous animals in children - Corrected Proof

H. Clark Barrett, James Broesch — 2012-04-20

Abstract: Natural selection is likely to have shaped developmental systems for rapid acquisition of knowledge about environmental dangers, including dangerous animals. However, learning about dangerous animals through direct encounters can be costly and potentially fatal. In social species such as humans, the presence of stored information about danger in the minds of conspecifics might favor the evolution of prepared social learning mechanisms that cause children to preferentially attend to and remember culturally transmitted information about danger. Here we use an experimental learning task to show that children from two very different cultures exhibit prepared social learning about dangerous animals: city-dwelling children from Los Angeles, who face relatively little danger from animals, and Shuar children from the Amazon region of Ecuador, to whom dangerous animals pose a much greater threat. Both populations exhibited similar prepared learning effects. Danger information was learned in a single trial without feedback, immediately entered long-term memory, and was recalled with only minor attenuation a week later, while other information presented at the same time (animal names and diets) was immediately forgotten. We discuss the significance of these design features of prepared learning in light of the phylogeny and function of danger learning systems.

Facial attractiveness and fertility in populations with low levels of modern birth control - Corrected Proof

2012-04-19

Abstract: Evolutionary models of human mate choice generally assume that physical attractiveness has evolved through sexual selection, i.e., it has been associated with higher mating opportunities and subsequent reproductive success across our evolutionary history. Here we investigate whether facial attractiveness is related to fertility in order to understand the extent to which selection can operate on attractive traits in modern populations. We used data from two populations where the prevalence of modern birth control methods is low and thus unlikely to disconnect mating opportunities from reproductive success: men and women from contemporary rural Senegal and men from the West Point Military Academy in the USA who graduated in 1950. We found that facial attractiveness negatively predicts age-specific reproduction in both sexes in Senegal and is independent from lifetime reproductive success in men from the USA. Overall, the results suggest that facial attractiveness is not under positive selection and raise questions about methodological approaches currently used to assess attractiveness.

Judging romantic interest of others from thin slices is a cross-cultural ability - Corrected Proof

Skyler S. Place, Peter M. Todd, Jinying Zhuang, Lars Penke, Jens B. Asendorpf — 2012-04-19

Abstract: The ability to judge the romantic interest between others is an important aspect of mate choice for species living in social groups. Research has previously shown that humans can do this quickly—observers watching short clips of speed-dating videos can accurately predict the outcomes. Here we extend this work to show that observers from widely varying cultures can judge these same videos with roughly equal accuracy. Participants in the USA, China, and Germany perform similarly not only overall but also at the level of judging individual speed-daters: Some daters are easy to read by observers from all cultures, while others are consistently difficult. These cross-cultural performance similarities provide evidence for an adaptive mechanism useful for mate choice that could be resilient to cultural differences.

Male facial width is associated with death by contact violence: narrow-faced males are more likely to die from contact violence - Corrected Proof

Michael Stirrat, Gert Stulp, Thomas V. Pollet — 2012-04-19

Abstract: Male facial width-to-height ratio (bizygomatic width scaled for face height) is a testosterone-linked trait predictive of reactive aggression, exploitative behavior, cheating, deception, and dominance. We tested whether facial width was systematically related to cause of death in a forensic sample. We hypothesized that wider-faced males, being more aggressive and robust, would be less likely than narrower-faced males to die from contact violence (stabbed, strangled, or bludgeoned to death) compared with other forms of homicide. We tested this hypothesis in a forensic data sample covering 523 male and 339 female skeletons. In these data, men with narrower faces were more likely to have died as a consequence of homicides involving direct physical contact than men with wider faces. No such effect was found for women. This effect was found when considering all causes of mortality and when limiting the sample to homicides. This finding suggests that wider-faced males are less likely to die from male–male physical violence, perhaps because of their formidability. Our findings are discussed with reference to the previous literature indicating that facial width-to-height ratio is a marker for male dominance.

The evolved psychology of voice: evaluating interrelationships in listeners' assessments of the size, masculinity, and attractiveness of unseen speakers - Corrected Proof

Katarzyna Pisanski, Sandeep Mishra, Drew Rendall — 2012-04-16

Abstract: A growing body of research has examined how voice characteristics advertise personal dimensions relevant in mate competition and mate choice. This work has centered on two key voice features, namely, fundamental frequency (F0) and formants (Fn), and has consistently found that speakers with low F0, low Fn, or both are rated as being larger, more masculine, and more attractive if men but less attractive if women. However, this consistency in listeners' perceptions is not matched by an equivalent consensus in how these mate-relevant dimensions are causally related or signaled by voice characteristics. Consequently, it is critical to test whether the strong correlations in listeners' perceptions reflect reliable causal relationships between these dimensions or, alternatively, whether they reflect some perceptual or cognitive nonindependence, for example, “what is large is masculine” and “what is small is feminine.” To test this latter possibility, we report detailed analyses of interdependence in listeners' ratings of perceived size, masculinity or femininity, and attractiveness of natural and manipulated voices of the opposite sex. We found strong correlations in listeners' ratings of all three dimensions, confirming past research. Principal component analysis corroborated these interrelationships but also revealed some independence in women's ratings of men's attractiveness and additional (but weaker) independence in men's ratings of women's size. We discuss possible implications for future research on the evolved psychology of voice and whether and how it reflects adaptive functional heuristics for discriminating mates.

Evidence for a nonverbal expression of triumph - Corrected Proof

David Matsumoto, Hyi Sung Hwang — 2012-04-16

Abstract: An expression signaling victory in agonistic encounters may help to establish status and dominance in a hierarchy. We investigated the possibility that an expression of triumph is such a signal. In Study 1, spontaneous expressions of athletes who had just won a medal at the 2004 Olympic Games were shown to three groups of observers who judged the emotion portrayed. All were given a set of common response alternatives that included Anger, Contempt, Disgust, Fear, Joy, Sadness, Surprise, Neutral, and Other. Group 1 was also given Pride, Group 2 Triumph, and Group 3 both Pride and Triumph. Group 1 labeled some expressions as pride, but Group 2 labeled all the expressions as triumph. For Group 3, some expressions were consistently labeled as triumph, and others as pride. Behavioral coding indicated that the expressions labeled triumph were distinct from other expressions. In Study 2, we replicated this finding with a judgment task involving more positive emotion choices to eliminate the possibility that the previous findings occurred because of a limited number of positive emotion labels. In Study 3, we replicated the findings again in two groups of observers from South Korea using two different judgment tasks. These findings suggest that triumph may have a unique nonverbal expression.

The baby effect and young male syndrome: social influences on cooperative risk-taking in women and men - Corrected Proof

Dominic Fischer, Thomas T. Hills — 2012-04-16

Abstract: Parental investment theory predicts differences in risk-taking for females and males as a consequence of reproductive context, with females attempting to reduce risks in relation to their own offspring (here called the baby effect) and males taking more risks in competition with one another (young male syndrome). The experiment we report tests these predictions in a cooperative context by introducing the Social Balloon Analogue Risk Task—the Balloon Analogue Risk Task modified to include a social partner (adult male, adult female, or baby)—along with a commitment device in which participants choose among several possible social partners, with whom they will share their earnings. Results were consistent with the predictions of parental investment theory. Females did not change their levels of risk-taking when paired with adult males or females, but showed a strong reduction in risk when paired with babies. Consistent with previous research, males were strongly inclined to take more risks when paired with another male of the same age, but males showed no change in risk-taking when paired with a female of the same age or a child. The current work provides the first experimental evidence of gender differences in cooperative social risk-taking, as well as the first experimental evidence of a mediator of female risk-taking, i.e., babies.

The General Factor of Personality (GFP) and parental support: testing a prediction from Life History Theory - Corrected Proof

2012-04-16

Abstract: In the present study, we tested whether the General Factor of Personality (GFP) is related to the level of parental support. The GFP is assumed to occupy the apex of the hierarchy of human personality structure and is believed to reflect a socially and sexually selected aggregate of behavioral characteristics that are generally valued as “desirable” in interpersonal relationships. The relationship between the GFP and parental support tested in this study is predicted by Life History Theory, a midlevel evolutionary account of systematic differences in evolved reproductive strategies. A total of 428 families with mother, father, and two children (range 14–16 years) participated. Parents filled out personality questionnaires (Big Five) and their level of parental support. The children also independently rated the amount of support they perceived from their parents. In the present sample, parents' GFPs were found to explain 33% of the variance in the Big Five. Moreover, the parents' GFPs showed significant relationships with the parents' self-rated parental support, but also with the child-rated parental support. The monoinformant (parents ratings) and multi-informant (parent and child ratings) data support the notion of a substantive GFP that is related to the investment of parents into their offspring.

Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D:4D digit ratio - Corrected Proof

Elizabeth Hampson, Janani S. Sankar — 2012-04-16

Abstract: The ‘Manning hypothesis,’ the idea that small differences in the ratio of the lengths of the human second to fourth digits—the 2D:4D ratio—reflect differences in the level of fetal androgen exposure, has been highly influential in the biological and biobehavioral sciences. The ratio is widely used to investigate the involvement of fetal androgens in the differentiation of sexually dimorphic traits. The validity of such studies is based on the premise that individual differences in the size of the 2D:4D ratio mirror differences across individuals in developmental levels of androgen exposure in a dose-dependent manner. Despite its widespread adoption by researchers, clinical evidence has yet to confirm that individual gradation in the ratio denotes differences in testosterone action. Key support for the view that 2D:4D does, in fact, reflect fetal testosterone in a graded fashion is the finding, based on a single small-sample study, that the magnitude of 2D:4D covaries with a polymorphic repeat (CAG) sequence in exon 1 of the gene coding the androgen receptor, AR. In a larger independent sample, we reexamine this genetic association and fail to substantiate a correlation between AR CAG length and 2D:4D. Combined with other recent reports, these data question one of the fundamental pieces of evidence on which the Manning hypothesis rests and raise new issues regarding the extent to which 2D:4D is a valid reflection of differences in fetal testosterone action in normally developing individuals.

Homeliness is in the disgust sensitivity of the beholder: relatively unattractive faces appear especially unattractive to individuals higher in pathogen disgust - Corrected Proof

Justin H. Park, Florian van Leeuwen, Ian D. Stephen — 2012-04-16

Abstract: Pathogen-relevant variables (e.g., regional variation in pathogen prevalence, individual differences in sensitivity to pathogen disgust) have been found to be associated with judgments and preferences surrounding physical attractiveness, in line with the view that certain morphological features and configurations indicate health and/or immunocompetence. In three studies, we administered the three-domain disgust scale and obtained ratings of attractiveness of faces to examine whether associations emerged between perceivers' disgust sensitivity and their ratings of attractive and/or unattractive targets. The results across the three studies showed that for unattractive targets, perceivers higher in pathogen disgust tended to assign lower attractiveness ratings; for attractive targets, pathogen disgust was uncorrelated with attractiveness ratings. Sexual disgust and moral disgust were not associated with perceptions of unattractive or attractive target faces. These results indicate that disgust-dependent attractiveness perceptions may motivate avoidance of potentially unfit interaction partners.

Parasite primes make foreign-accented English sound more distant to people who are disgusted by pathogens (but not by sex or morality) - Corrected Proof

2012-04-13

Abstract: To avoid disease, people should maintain close ties with ingroup members but maintain distance from outgroup members who possess novel pathogens. Consistent with this disease-avoidance hypothesis, pathogenic stimuli, as well as increased personal vulnerability to disease, are associated with xenophobic and ethnocentric attitudes. Researchers assume that this disease-avoidance process is an automatic emotional response that compels negative attitudes and behavioral avoidance. However, when outgroup contact can represent fitness costs or benefits, and when group membership is an uncertain cue to infection risk, it becomes a fitness advantage for a social perceiver to track group membership and thus infection risk. Given that accents can be a cue to group membership, we predicted that the perception of linguistic similarity to ingroup speakers and dissimilarity from outgroup speakers would increase with individual differences in pathogen disgust, and that this association would be most apparent when threat of disease was salient. This hypothesis was confirmed in two experiments. Further, the mechanism was domain specific—disgust due to sexual acts and moral violations did not moderate perceived linguistic distance. The disease-avoidance mechanism is not just an automatic disgust-based reaction; it also operates through the cognitive appraisal of social distance.

Sociosexuality as predictor of sexual harassment and coercion in female and male high school students - Corrected Proof

Leif Edward Ottesen Kennair, Mons Bendixen — 2012-04-13

Abstract: Sexual harassment and coercion have mainly been considered from a sex difference perspective. While traditional social science theories have explained harassment as male dominance of females, the evolutionary perspective has suggested that sex differences in the desire for sex are a better explanation. This study attempts to address individual differences associated with harassment from an evolutionary perspective. Considering previous research that has found links between sociosexual orientation inventory (SOI) and harassment, we consider whether this association can be replicated in a large, representative sample of high school students (N=1199) from a highly egalitarian culture. Expanding the previous studies which mainly focused on male perpetrators and female victims, we also examine females and males as both perpetrators and as victims. We believe that unrestricted sociosexuality motivates people to test whether others are interested in short-term sexual relations in ways that sometimes might be defined as harassment. Furthermore, unrestricted individuals signal their sociosexual orientation, and while they do not desire all individuals that react to these signals with sexual advances, they attract much more sexual advances than individuals with restricted sociosexual orientations, especially from other unrestricted members of the opposite sex. This more or less unconscious signaling thus makes them exploitable, i.e., harassable. We find that SOI is a predictor for sexual harassment and coercion among high school students. The paper concludes that, as expected, unrestricted sociosexuality predicts being both a perpetrator and a victim of both same-sex and opposite-sex harassment.

Social learning across the life cycle: cultural knowledge acquisition for honey collection among the Jenu Kuruba, India - Corrected Proof

Kathryn Demps, Francisco Zorondo-Rodríguez, Claude García, Victoria Reyes-García — 2012-04-09

Abstract: Accounting for age-dependent patterns of knowledge transmission is critical for understanding cultural evolution in age-structured populations. Cultural evolution theory predicts which social learning biases we expect people to use, but much less often when—during a person's life cycle— different social learning biases will be used. By measuring knowledge and skill variation among age cohorts, it is possible to infer how people socially acquire different types of knowledge at different ages. We use this strategy among the Jenu Kuruba, a tribal community in South India. We document the accumulation of local knowledge required for collecting wild honey among 71 children and 125 adults from five communities. Combining quantitative measurements of knowledge with measures of four honey-collecting skills and self-reported data on learning age, we infer patterns of social learning across the lifecycle. We find that (1) most knowledge related to honey collecting is acquired by the early 20s, and later social learning mainly functions to update information; (2) the eldest cohort has the highest average explicit knowledge, although the most knowledgeable or skilled individuals do not always belong to the most elderly cohort; (3) length of learning can be affected by age-dependent trade-offs of costs and benefits to learners; and (4) children tend to learn from parents, but individuals use other demonstrators later in life. These results have implications for current models of cultural evolution.

Evolution and the expression of biases: situational value changes the endowment effect in chimpanzees - Corrected Proof

2012-03-14

Abstract: Cognitive and behavioral biases, which are widespread among humans, have recently been demonstrated in other primates, suggesting a common origin. Here we examine whether the expression of one shared bias, the endowment effect, varies as a function of context. We tested whether objects lacking inherent value elicited a stronger endowment effect (or preference for keeping the object) in a context in which the objects had immediate instrumental value for obtaining valuable resources (food). Chimpanzee subjects had opportunities to trade tools when food was not present, visible but unobtainable, and obtainable using the tools. We found that the endowment effect for these tools existed only when they were immediately useful, showing that the effect varies as a function of context-specific utility. Such context-specific variation suggests that the variation seen in some human biases may trace predictably to behaviors that evolved to maximize gains in specific circumstances.

Darwin the detective: Observable facial muscle contractions reveal emotional high-stakes lies - Corrected Proof

Leanne ten Brinke, Stephen Porter, Alysha Baker — 2012-03-14

Abstract: Deception—a fundamental aspect of human communication—often is accompanied by the simulation of unfelt emotions or the concealment of genuine emotions to correspond to the false message. We investigated the consequences of extremely high-stakes emotional deception on the engagement of particular facial muscles, posited by Darwin [Darwin, C. (1872/2005). The expression of the emotions in man and animals. In J. D. Watson (Ed.), Darwin: The indelible stamp (pp. 1066–1257) Philadelphia: Running Press] to reveal the false face. The videotaped facial actions of a sample of individuals (N=52) emotionally pleading to the public for the return of a missing relative—half of whom eventually were convicted of murdering that person—were coded frame by frame (30 frames/s for a total of 23,622 frames). Findings support the view that emotional “leakage,” particularly via those facial muscles under less cortical control, is a byproduct of the overextended cognitive resources available to convey elaborate lies. Specifically, the “grief” muscles (corrugator supercilii, depressor anguli oris) were more often contracted in the faces of genuine than deceptive pleaders. Subtle contraction of the zygomatic major (masking smiles) and full contraction of the frontalis (failed attempts to appear sad) muscles were more commonly identified in the faces of deceptive pleaders. Thus, while interpersonal deception often is highly successful, signs of covert emotional states are communicated clearly to the informed observer.

Costly signaling, ritual and cooperation: evidence from Candomblé, an Afro-Brazilian religion - Corrected Proof

Montserrat Soler — 2012-03-09

Abstract: The apparent wastefulness of religious ritual represents a puzzle for rational choice theorists and evolutionary scholars. In recent years, it has been proposed that such rituals represent costly signals that promote intragroup cooperation precisely because of the effort and resources they require. This hypothesis was tested over the course of a 14-month long ethnographic study in the northeast of Brazil. The research focused on adherents of Candomblé, an African diasporic religion organized in autonomous congregations primarily located in low-income urban areas. Individuals who reported higher levels of religious commitment behaved more generously in a public goods economic game and revealed more instances of provided and received cooperation within their religious community. This suggests that ritual as a costly signaling may effectively predict willingness to cooperate with other group members and that the signaler may accrue benefits in the form of received cooperation. Socioeconomic variables are also shown to mediate religious signaling. This raises the possibility that signalers strategically alter their expressions of commitment as their needs and circumstances change.

Low serum vitamin A mothers breastfeed daughters more often than sons in drought-ridden northern Kenya: a test of the Trivers–Willard hypothesis - Corrected Proof

2012-03-09

Abstract: The Trivers–Willard hypothesis predicts that natural selection should favor unequal parental investment between daughters and sons based upon maternal condition and offspring reproductive potential. Specifically, it predicts that mothers in good condition should increase investment toward sons, while mothers in poor condition should favor daughters. Previous tests of the hypothesis in human populations overwhelmingly focused on economic resources as maternal condition indicators. We test the Trivers–Willard hypothesis using maternal nutrition—energy and vitamin A status representing macro- and micronutrition, respectively—as the indicator for maternal condition, with breastfeeding frequency recalls serving as the indicator for parental investment. Data from exclusively breastfeeding mothers (n=83) in drought-ridden Ariaal agropastoral villages of northern Kenya were used to test the hypothesis that mothers in poor condition will breastfeed daughters more frequently than sons. Poor condition was defined as having a body mass index <18.5 or serum retinol (vitamin A) concentration <1.05 µmol/l. A linear regression model was applied using breastfeeding frequency as the dependent variable and respective maternal condition, infant's sex, and the maternal condition–infant's sex interaction as the predictors, controlling for covariates. Results supported the hypothesis only in the vitamin A model which predicts that low-vitamin-A mothers breastfeed daughters significantly more frequently than sons (11 vs. 6 times/day), while vitamin-A-sufficient mothers breastfeed daughters and sons equivalently (9 times). These results indicate that maternal nutritional status, particularly micronutrient status, can contribute to the investigation of the evolutionary hypothesis of sex-biased parental investment.

Venom, speed, and caution: effects on performance in a visual search task - Corrected Proof

Danielle Sulikowski — 2012-03-09

Abstract: Previous reports of faster responses to threatening compared to benign stimuli in visual search tasks have argued that threatening targets are faster to engage and slower to disengage attention than benign targets. This study reinterprets previous findings and resolves inconsistencies in the literature by replacing the theory of differential disengagement of attention with one of differential caution. It also examines whether visual attentional mechanisms are sensitive to more than just the threatening versus benign categorical status of the targets and introduces a novel measure (a caution score) that appears to be sensitive to the level of threat implied by the target image, but immune to other stimulus features (target-distracter similarity and threat status of distracters) known to affect reaction time. As well as locating threatening targets faster than benign targets, participants were also faster, more accurate, and more cautious to detect lethal spiders compared to nonlethal spiders and even more cautious again if the spiders were presented on a person's hand. These results suggest that mechanisms of attention and threat evaluation interact during visual search tasks, producing behaviour that is sensitive to the target's implied threat level and the context in which that target is presented.

Context-dependent model-based biases in cultural transmission: children's imitation is affected by model age over model knowledge state - Corrected Proof

Lara A. Wood, Rachel L. Kendal, Emma G. Flynn — 2012-03-09

Abstract: Many animals, including humans, acquire information through social learning. Although such information can be acquired easily, its potential unreliability means it should not be used indiscriminately. Cultural ‘transmission biases’ may allow individuals to weigh their reliance on social information according to a model's characteristics. In one of the first studies to juxtapose two model-based biases, we investigated whether the age and knowledge state of a model affected the fidelity of children's copying. Eighty-five 5-year-old children watched a video demonstration of either an adult or child, who had professed either knowledge or ignorance regarding a tool-use task, extracting a reward from that task using both causally relevant and irrelevant actions. Relevant actions were imitated faithfully by children regardless of the model's characteristics, but children who observed an adult reproduced more irrelevant actions than those who observed a child. The professed knowledge state of the model showed a weaker effect on imitation of irrelevant actions. Overall, children favored the use of a ‘copy adults’ bias over a ‘copy task-knowledgeable individual’ bias, even though the latter could potentially have provided more reliable information. The use of such social learning strategies has significant implications for understanding the phenomenon of imitation of irrelevant actions (overimitation), instances of maladaptive information cascades, and cumulative culture.

Reported jealousy differs as a function of menstrual cycle stage and contraceptive pill use: a within-subjects investigation - Corrected Proof

2012-03-09

Abstract: Previous research suggests that female jealousy is sensitive to hormonal variation and, more specifically, potentially moderated by estrogen levels. Here, we tracked self-reported jealousy using a within-subjects design, comparing jealousy when the same women were regularly cycling and using hormonal contraceptives. Results show that fertile cycle phases are associated with higher levels of jealousy than nonfertile cycle phases in both single and partnered women. However, patterns of jealousy reported when using hormonal contraceptives, as compared to when regularly cycling, differed between single and partnered women. In single women, levels of jealousy while on the pill fell between those reported when fertile and nonfertile but were not significantly different from either. In partnered women, levels of jealousy while using the pill were significantly higher than those reported during the nonfertile cycle phase and similar to those during the brief period of fertility. We discuss possible reasons for differences between single and partnered women in reported jealousy while using the pill. This research is the first to definitively show that a psychological characteristic, for example, jealousy, may be influenced differentially by endogenous hormones vs. exogenous hormones administered via hormonal contraceptives.

Testing the imprinted brain: parent-of-origin effects on empathy and systemizing - Corrected Proof

Gillian Ragsdale, Robert A. Foley — 2012-03-09

Abstract: Genomic imprinting is a violation of Mendel's laws that enables selection to act on genes, depending on parent of origin, but, even more controversially, on the sex of the offspring. This study tested whether there are parent-of-origin effects on the heritability of empathy and systemizing in the general population as part of a larger question concerning the role of imprinted genes in the evolution of human cognition and behaviour. The measures tested were the Empathy and Systemizing Quotients as proxies for the related terms mentalistic and mechanistic cognition in the imprinted brain theory.To test genomic imprinting hypotheses, correlations in behavioural scores between pairs of full, maternal and paternal siblings were compared. Where scores are influenced by imprinted genes, the actual correlations between pairs of siblings will differ from those expected following classical Mendelian inheritance in a predictable way depending on what kind of imprinting is influencing the trait. These theoretical predictions were used to test the fit of the data against Mendelian and imprinting models using structural equation modeling. The imprinted brain theory proposes a trade-off between maternally influenced mentalistic cognition and paternally influenced mechanistic cognition. However, the results of this study support a model of contrasting maternal and paternal influences on strong and weak empathizing and a maternal influence on systemizing. Although the sample size was insufficient to comprehensively analyse sex-limitation models, there is some evidence that heritability of systemizing is stronger in females than in males.

Sexual exploitability: observable cues and their link to sexual attraction - Corrected Proof

Cari D. Goetz, Judith A. Easton, David M.G. Lewis, David M. Buss — 2012-03-09

Abstract: Although antiexploitation adaptations, such as cheater-detection mechanisms, have been well explored, comparatively little research has focused on identifying adaptations for exploitation. The present study had two purposes: (1) to identify observable cues that afford information about which women are sexually exploitable and (2) to test the hypothesis that men find cues to sexual exploitability sexually attractive, an adaptation that functions to motivate pursuit of accessible women. Male participants rated photographs of women who displayed varying levels of hypothesized cues to exploitability. We identified 22 cues indicative of sexual exploitability. Nineteen of these cues were correlated significantly with sexual attractiveness, supporting the central hypothesis. Results suggest that sexual attraction to exploitability cues functions to motivate men to employ exploitative strategies towards accessible targets, and contribute foundational knowledge to the diverse classes of cues that afford information about which women are and are not sexually exploitable.

Regional variation in pathogen prevalence predicts endorsement of group-focused moral concerns - Corrected Proof

Florian van Leeuwen, Justin H. Park, Bryan L. Koenig, Jesse Graham — 2012-03-09

Abstract: According to Moral Foundations Theory, people endorse “individualizing” foundations (Harm/care, Fairness/reciprocity) or “binding” foundations (Ingroup/loyalty, Authority/respect, Purity/sanctity) to varying degrees. As societies with higher pathogen prevalence have been found to exhibit more pronounced antipathogen psychological tendencies and cultural practices (e.g., conformity, collectivism), we hypothesized that pathogen prevalence may predict endorsement of the binding moral foundations, which may also serve to minimize pathogen transmission. We examined associations between historical and contemporary pathogen prevalence and endorsement of the moral foundations via multilevel analyses. Country-level analyses showed that even when controlling for gross domestic product per capita, historical (but not contemporary) pathogen prevalence significantly predicted endorsement of the binding foundations, but not individualizing foundations. Multilevel analyses showed that this pattern held even when controlling for individual-level variation in political orientation, gender, education, and age. These results highlight the utility of a functional–evolutionary approach to understanding patterns of morals across societies and individuals.

Infidelity, jealousy, and wife abuse among Tsimane forager–farmers: testing evolutionary hypotheses of marital conflict - Corrected Proof

Jonathan Stieglitz, Michael Gurven, Hillard Kaplan, Jeffrey Winking — 2012-03-09

Abstract: The role of men's jealousy over a wife's infidelity in precipitating marital conflict and wife abuse is well documented. The role of women's jealousy over a husband's infidelity has received little attention, which is puzzling given high potential costs to women of withdrawal of paternal investment. We address this gap by investigating marital conflict and wife abuse among Tsimane forager–farmers of Bolivia. We test predictions derived from male jealousy and paternal disinvestment hypotheses, which consider threats and consequences of infidelity by women (male jealousy hypothesis) and men (paternal disinvestment hypothesis). The paternal disinvestment hypothesis proposes that wife abuse is employed by husbands to limit wives' mate retention effort and maintain men's opportunities to pursue extrapair sexual relationships. Interviews were conducted among husbands and wives in the same marriages using a combination of open-ended and structured items. Spouses agree that the most frequently reported type of marital argument is women's jealousy over a husband's infidelity (N=266 arguments). Roughly 60% of abusive events occurred during arguments over men's diversion of household resources (N=124 abusive events). In multivariate analyses, likelihood of wife abuse is greater in marriages where husbands have affairs, where wives are younger, and where spouses spend more time apart (N=60 husbands, 71 wives). While we find strong support for both male jealousy and paternal disinvestment hypotheses, it is men's infidelity, not women's, that precipitates most instances of marital conflict and wife abuse. We conclude that men's aggression towards their wives facilitates men's diversion of family resources for their selfish interests.

Women's behavioural engagement with a masculine male heightens during the fertile window: evidence for the cycle shift hypothesis - Corrected Proof

Heather D. Flowe, Elizabeth Swords, James C. Rockey — 2012-03-07

Abstract: Previous research suggests that women may alter their behaviour during the fertile window of the menstrual cycle to attract a mate who has traits that indicate high-quality genes. We tested whether fertile women demonstrate greater behavioural engagement with a masculine compared to a less masculine male. The test was performed using a quiz show paradigm, in which a male host asked female participants general knowledge questions. The masculinity of the host was varied between participants. Women's performance on the quiz, as well as their romantic attraction to the host, was examined in relation to women's estimated cycle phase and host masculinity. Fertile compared to nonfertile women were more romantically attracted to the host and were faster to answer his questions, but only when he was portrayed as masculine. The results of the study are interpreted as being in keeping with Gangestad and Thornhill's cycle shift hypothesis (Menstrual cycle variation in women's preferences for the scent of symmetrical men. Proceedings of the Royal Society of London B: Biological Sciences, 1998;265:727–733. doi:10.1098/rspb.1998.03801998).

The ontogeny of human prosociality: behavioral experiments with children aged 3 to 8 - Corrected Proof

Bailey R. House, Joseph Henrich, Sarah F. Brosnan, Joan B. Silk — 2012-03-07

Abstract: Humans regularly engage in prosocial behavior that differs strikingly from that of even our closest living relatives, the chimpanzees (Pan troglodytes). In laboratory settings, chimpanzees are indifferent when given the opportunity to deliver valued rewards to conspecifics, while even very young human children have repeatedly been shown to behave prosocially. Although this broadly suggests that prosocial behavior in chimpanzees differs from that of young human children, the methods used in prior work with children have also differed from the methods used in studies of chimpanzees in potentially crucial ways. Here we test 92 pairs of 3–8-year-old children from urban American (Los Angeles, CA, USA) schools in a face-to-face task that closely parallels tasks used previously with chimpanzees. We found that children were more prosocial than chimpanzees have previously been in similar tasks, and our results suggest that this was driven more by a desire to provide benefits to others than a preference for egalitarian outcomes. We did not find consistent evidence that older children were more prosocial than younger children, implying that younger children behaved more prosocially in the current study than in previous studies in which participants were fully anonymous. These findings strongly suggest that humans are more prosocial than chimpanzees from an early age and that anonymity influences children's prosocial behavior, particularly at the youngest ages.

Food-sharing networks in Lamalera, Indonesia: status, sharing, and signaling - Corrected Proof

David A. Nolin — 2012-03-07

Abstract: Costly signaling has been proposed as a possible mechanism to explain food sharing in foraging populations. This sharing-as-signaling hypothesis predicts an association between sharing and status. Using exponential random graph modeling, this prediction is tested on a social network of between-household food-sharing relationships in the fishing and sea-hunting village of Lamalera, Indonesia. Previous analyses (Nolin, 2010. Food-sharing networks in Lamalera, Indonesia: reciprocity, kinship, and distance. Human Nature 21, 243–268) have shown that most sharing in Lamalera is consistent with reciprocal altruism. The question addressed here is whether any additional variation may be explained as sharing-as-signaling by high-status households. The results show that high-status households both give and receive more than other households, a pattern more consistent with reciprocal altruism than costly signaling. However, once the propensity to reciprocate and household productivity are controlled, households of men holding leadership positions show greater odds of unreciprocated giving when compared to households of nonleaders. This pattern of excessive giving by leaders is consistent with the sharing-as-signaling hypothesis. Wealthy households show the opposite pattern, giving less and receiving more than other households. These households may reciprocate in a currency other than food, or their wealth may attract favor-seeking behavior from others. Overall, status covariates explain little variation in the sharing network as a whole, and much of the sharing observed by high-status households is best explained by the same factors that explain sharing by other households. This pattern suggests that multiple mechanisms may operate simultaneously to promote sharing in Lamalera and that signaling may motivate some sharing by some individuals even within sharing regimes primarily maintained by other mechanisms.

Strength and physical fitness predict the perception of looming sounds - Corrected Proof

John G. Neuhoff, Katherine L. Long, Rebecca C. Worthington — 2012-03-05

Abstract: Listeners consistently perceive approaching sounds to be closer than they actually are and perceptually underestimate the time to arrival of looming sound sources. In a natural environment, this underestimation results in more time than expected to evade or engage the source and affords a “margin of safety” that may provide a selective advantage. However, a key component in the proposed evolutionary origins of the perceptual bias is the appropriate timing of anticipatory motor behaviors. Here we show that listeners with poorer physical fitness respond sooner to looming sounds and with a larger margin of safety than listeners with better physical fitness. The anticipatory perceptual bias for looming sounds is negatively correlated with physical strength and positively correlated with recovery heart rate (a measure of aerobic fitness). The results suggest that the auditory perception of looming sounds may be modulated by the response capacity of the motor system.

Sex differences in spatial cognition among Hadza foragers - Corrected Proof

2012-02-17

Abstract: This paper describes sex differences in spatial competencies among the Hadza, a mobile hunter–gatherer population in Tanzania. It addresses the following questions: (a) Is the usual male advantage in Euclidean spatial abilities found in this population, where both women and men are highly mobile? (b) Do Hadza women have better object location memory than men, as the gathering hypothesis predicts? (c) Do women who are nominated by others as being good at finding bushfoods excel at the object location memory task? We tested object location memory with a version of the memory game using cards of local plants and animals. This allowed us to also ask whether women and men would have better spatial memory for the plant and animal cards, respectively. We found that Hadza men were significantly better than women in three tests of spatial ability: the water-level test, targeting, and the ability to point accurately to distant locations (the latter only in the less mobile groups). There was a trend toward a male advantage at the object location memory task, in contrast to results found previously in nonforaging populations, and women's performance at the task deteriorated with age, while that of men did not. The women who were nominated by peers as being good at finding bushfoods were consistently older women. We discuss the probable hormonal causes and functional consequences of age changes in the spatial competencies of female foragers.

Means, variances, and ranges in reproductive success: comparative evidence - Corrected Proof

Laura Betzig — 2012-02-16

Abstract: Data on reproductive success in traditional cultures suggest that for men, but not for women, range and variance rise as subsistence intensifies. For hunter–gatherers, ranges and variances tend to cluster in single digits: they reach 15 or 16, at the high end. For herder-gardeners, ranges and variances are more consistently in double digits: they get as high as 80 or 85. And for full-time agriculturalists in the first civilizations, ranges consistently ran to triple digits: emperors from Mesopotamia to Peru were the fathers of hundreds of children. In human societies, as in other animal societies, reproductive skew goes up with a more sedentary life.

Hamilton vs. Kant: pitting adaptations for altruism against adaptations for moral judgment - Corrected Proof

Robert Kurzban, Peter DeScioli, Daniel Fein — 2012-01-27

Abstract: Prominent evolutionary theories of morality maintain that the adaptations that underlie moral judgment and behavior function, at least in part, to deliver benefits (or prevent harm) to others. These explanations are based on the theories of kin selection and reciprocal altruism, and they predict that moral systems are designed to maximize Hamiltonian inclusive fitness. In sharp contrast, however, moral judgment often appears Kantian and rule-based. To reconcile this apparent discrepancy, some theorists have claimed that Kantian moral rules result from mechanisms that implement simple heuristics for maximizing welfare. To test this idea, we conducted a set of studies in which subjects (N=1290) decided whether they would kill one person to save five others, varying the relationship of the subject with the others involved (strangers, friends, brothers). Are participants more likely to observe the Kantian rule against killing in decisions about brothers and friends, rather than strangers? We found the reverse. Subjects reported greater willingness to kill a brother or friend than a stranger (in order to save five others of the same type). These results suggest that the rule-based structure of moral cognition is not explained by kin selection, reciprocity, or other altruism theories.

Cross-cultural effects of color, but not morphological masculinity, on perceived attractiveness of men's faces - Corrected Proof

2012-01-09

Abstract: Much attractiveness research has focused on face shape. The role of masculinity (which for adults is thought to be a relatively stable shape cue to developmental testosterone levels) in male facial attractiveness has been examined, with mixed results. Recent work on the perception of skin color (a more variable cue to current health status) indicates that increased skin redness, yellowness, and lightness enhance apparent health. It has been suggested that stable cues such as masculinity may be less important to attractiveness judgments than short-term, more variable health cues. We examined associations between male facial attractiveness, masculinity, and skin color in African and Caucasian populations. Masculinity was not found to be associated with attractiveness in either ethnic group. However, skin color was found to be an important predictor of attractiveness judgments, particularly for own-ethnicity faces. Our results suggest that more plastic health cues, such as skin color, are more important than developmental cues such as masculinity. Further, unfamiliarity with natural skin color variation in other ethnic groups may limit observers' ability to utilize these color cues.

Low nonpaternity rate in an old Afrikaner family - Corrected Proof

2012-01-09

Abstract: Extrapair paternity is a crucial parameter for evolutionary explanations of reproductive behavior. Early studies and human testis size suggest that human males secure/suffer frequent extrapair paternity. If these high rates are indeed true, it brings into question studies that use genealogies to infer human life history and the history of diseases since the recorded genealogies do not reflect paths of genetic inheritance. We measure the rate of nonpaternity in an old Afrikaner family in South Africa by comparing Y-chromosome short tandem repeats to the genealogy of males. In this population, the nonpaternity rate was 0.73%. This low rate is observed in other studies that matched genealogies to genetic markers and more recent studies that also find estimates below 1%. It may be that imposed religious morals have led to reduced extrapair activities in some historic populations. We also found that the mutation rate is high for this family, but is unrelated to age at conception.

The advantage of multiple cultural parents in the cultural transmission of stories - Corrected Proof

Kimmo Eriksson, Julie C. Coultas — 2011-12-26

Abstract: Recent mathematical modeling of repeated cultural transmission has shown that the rate at which culture is lost (due to imperfect transmission) will crucially depend on whether individuals receive transmissions from many cultural parents or only from one. However, the modeling assumptions leading up to this conclusion have so far not been empirically assessed. Here we do this for the special case of transmission chains where each individual either receives the same story twice from one cultural parent (and retransmits twice to a cultural child) or receives possibly different versions of the story from two cultural parents (and then retransmits to two cultural children). For this case, we first developed a more general mathematical model of cultural retention that takes into account the possibility of dependence of error rates between transmissions. In this model, under quite plausible assumptions, chains with two cultural parents will have superior retention of culture. This prediction was then tested in two experiments using both written and oral modes of transmission. In both cases, superior retention of culture was found in chains with two cultural parents. Estimation of model parameters indicated that error rates were not identical and independent between transmissions; instead, a primacy effect was suggested, such that the first transmission tends to have higher fidelity than the second transmission.