Adaptive Human Behavior and Physiology

Body Mass Index Predicts Fighting Ability in Female UFC Fighters, but Facial Width-to-Height Ratio May Not Abstract Several lines of evidence suggest that facial-width-to-height ratio (fWHR) provides an accurate cue to men’s formidability, thus providing observers with a reliable estimate of their potential success in an interpersonal physical conflict. The degree to which fWHR provides the same information in women’s faces, however, remains unclear. In fact, morphological characteristics, such as body size, may be more effective indicators of formidability in women. In the present study, we analyzed the effects of both fWHR and body mass index (BMI) on total number of fights and total number of wins in female competitive Ultimate Fighting Challenge (UFC) fighters. After controlling for number of active years fighting, we found that BMI significantly predicted number of wins in female fighters. The effects of fWHR were not significant, but warrant replication with a larger sample. These results suggest that in women, body size, rather than fWHR, may be a more accurate indicator of success in aggressive interpersonal conflicts. Our findings contribute to a growing body of literature that shows distinct differences in men and women’s interpersonal aggressive and dominance behavior, as well as their morphological indicators.

Trait Anxiety Moderates the Relationship Between Testosterone Responses to Competition and Aggressive Behavior Abstract Testosterone (T) concentrations change rapidly in the context of human competition, and these changes in neuroendocrine function may serve to modulate future aggressive behavior. However, an increase in T during competition does not translate into aggressive behavior among all individuals. Here, we examined the extent to which individual differences in trait anxiety moderate the relationship between T responses to competition and aggressive behavior. Across two studies, we found that T responses to competition were positively correlated with subsequent aggression, but only among men scoring relatively low in trait anxiety. Trait anxiety did not moderate the relationship between T reactivity and aggression in women. These findings highlight the importance of considering individual difference in trait anxiety when examining the neuroendocrine correlates of human aggression.

The Impact of Authority on Cooperation: A Cross-Cultural Examination of Systemic Trust Abstract In this article, we examine the effects of authority on systemic trust in four different countries (the Czech Republic, Denmark, Mauritius, and the USA). We used a modified Trust Game to assess whether information about salient authority either in a religious or in a secular domain has the effect of enhancing trust in situations where social information is limited. We found that patterns of behavior differ by country, with the USA and the Czech Republic behaving similarly with relatively high trust for secular authorities, medium trust for religious authorities, and low trust for non-authorities, and that Denmark and Mauritius behave similarly, with medium trust for secular authorities, high trust for religious authorities and low trust for non-authorities. We discuss possible explanations involving how people use social information to make decisions in situations of uncertainty.

Close Versus Decisive Victory Moderates the Effect of Testosterone Change on Competitive Decisions and Task Enjoyment Abstract Prior research found that testosterone change after defeat predicted the decision to compete against the same opponent, but testosterone change after victory was unrelated to competitive behavior. The present research tested whether testosterone responses have differential effects on competitive decision-making depending on whether an individual either barely or decisively won a competition. Seventy-one undergraduate males provided an afternoon saliva sample and then participated in a laboratory cognitive contest in which they were randomly assigned to experience a relatively close or decisive victory against a male confederate. Participants provided a second saliva sample after the competition and then chose whether to: (a) compete against the same opponent, (b) compete against a new opponent, or (c) complete an alternative non-competitive task. Participants also reported how much they enjoyed the competitive task. Testosterone change and the propensity to compete were positively related after a decisive victory, but were negatively related after a close victory. These effects were driven by the decision to compete against a new opponent. In fact, very few participants chose to compete against the same opponent. Testosterone change after a decisive victory was also positively associated with participants’ self-reported enjoyment of the competitive task. Together, these results provide new evidence that a close versus decisive victory moderates the effect of testosterone change on future competitive behavior, an effect that may be linked to changes in reward processing systems.

A Model of Smiling as a Costly Signal of Cooperation Opportunities Abstract We develop a theoretical model under which “genuine” or “convincing” smiling is a costly signal that has evolved to induce cooperation in situations requiring mutual trust. Prior to a trust interaction involving a decision by a sender to send money to a recipient, the recipient can emit a signal to induce the sender to trust them. The signal takes the form of a smile that may be perceived as more or less convincing, and that can be made more convincing with the investment of greater effort. Individuals differ in their degree of altruism and in their tendency to display reciprocity. The model generates three testable predictions. First, the perceived quality of the recipient’s smile is increasing in the size of the stake. Secondly, the amount sent by the sender is increasing in the perceived quality of the recipient’s smile. Thirdly, the expected gain to senders from sending money to the recipient is increasing in the perceived quality of the recipient’s smile.

Ancient DNA Reveals That the Variability of the DRD4 -521 C/T SNP Associated with Novelty Seeking Behavior is Influenced by Selection in Western South American Populations Abstract Certain Polymorphisms in the DRD4 gene have been associated with behavioral traits such as novelty seeking (NS), impulsivity, and ADHD. Research has suggested that these personality traits may affect individual fitness differently depending on social structure, subsistence patterns, and mobility. To test this hypothesis this study examines the diachronic change in the frequency of the C allele of the −521 C/T SNP in the DRD4 promoter region, thought to contribute to the NS phenotype, analyzing ancient DNA from pre-Columbian South America. We used a SBE PCR based approach to analyze the DRD4 SNP in 125 prehistoric samples dating from ~8000 BP to the end of the pre-Columbian period and deriving from a diversity of socioeconomic environments. We performed forward simulations of genetic drift with selection to test if difference in allele frequencies over time can solely be explained by random evolutionary processes or if selection has to be considered. Further, we performed statistical tests to explore possible association of observed differences with socioeconomic or environmental factors. DRD4 genotypes for 79 individuals were obtained. We find signals of slight selection acting on the DRD4-521 T allele. While the postulated association between the −521 C/T DRD4 SNP with modes of subsistence was not confirmed the results suggest a possible impact of socioeconomic complexity and political organization on the variability of the SNP in in pre-Columbian western South American populations.

Sex Differences in Competitiveness: Hunter-Gatherer Women and Girls Compete Less in Gender-Neutral and Male-Centric Tasks Abstract Despite numerous attempts to increase workplace equality, the near universal gender wage gap and underrepresentation of women in high status jobs persists in societies around the world. This persistence has led some researchers to speculate that psychological sex differences may be partly to blame. In particular, economists have begun to focus on sex differences in competitiveness as a possible cause. Here we test whether sex differences in competitiveness exist in a relatively isolated and evolutionarily relevant population of hunter-gatherers in Tanzania. In study 1 we examine sex differences in willingness to compete in a gender-neutral task in Hadza adults and children (N = 191). We find that when choosing between an individualistic payment scheme and a competitive payment scheme, boys and men are significantly more likely to compete than girls and women. We find no evidence that this sex difference varies with age. In study 2 (N = 88 and N = 70) we use both a female-centric and a male-centric task to explore sex differences in competitiveness in adults. While we find no sex difference in willingness to compete in the female-centric task, we find that men are more likely to compete in the male-centric task. While further work is needed, this study lends some support to the idea of a sex difference in willingness to compete among hunter-gatherers, but it also highlights the importance of the task type. The observation that a sizable proportion of male Hadza choose to compete in each of the tasks is discussed in light of the fact that hunter-gatherers are largely egalitarian and non-hierarchical.

Assets at Risk: Menstrual Cycle Variation in the Envisioned Formidability of a Potential Sexual Assailant Reveals a Component of Threat Assessment Abstract Situations of potential agonistic conflict demand rapid and effective decision-making. The process of threat assessment includes assessments of relative fighting capacity, assessments of the likelihood of attack, and assessments of the extent to which one′s assets are at risk. The dimensions of physical size and strength appear to serve as key parameters in a cognitive representation summarizing multiple constituents of threat assessment. Here, we examine the thesis that this same representation summarizes asset risk. The fitness costs of sexual assault are in part a function of conception risk, as pregnancy due to assault compromises female choice and imperils existing and subsequent male investment. Prior research indicates that women′s attitudes and behaviors vary systematically across the menstrual cycle in a manner that would have reduced the likelihood of sexual assault during periods of greatest fertility in ancestral women. If the envisioned size and strength of a potential antagonist is used to represent asset risk, and if the threat that sexual assault poses to a woman′s reproductive assets is in part a product of her fertility, then the conceptualized size and strength of a potential sexual assailant should be a function of conception risk. We find support for this prediction in a large sample of naturally-cycling women in urban Southern California, indicating that asset risk is summarized using the same representation as relative fighting capacity and likelihood of attack. Presumably, this elegant use of a single representation for multiple aspects of threat assessment facilitates rapid decision-making in agonistic contexts.

Testosterone and Economic Risk Taking: A Review Abstract Since precise forecasting of the future is not possible, most of life’s decisions are made with uncertain outcomes. One important facet of uncertainty that is of particular interest to decision scientists is risk—the choice between an option that is less rewarding but more certain and an option that is less certain, but potentially more rewarding. Recent developments in both neuroscience and behavioral endocrinology have helped to reveal the biological mechanisms that support decision-making involving economic risk, and consequently, potential factors associated with individual differences in risk taking. This review is dedicated to surveying recent developments that link the hormone testosterone to economic risk taking. Like neuroeconomics, endocrinological approaches may provide a potentially powerful framework from which to understand decision-making and may help to make sense of a number of well-documented behavioral anomalies involving economic risk. Specifically, we suggest that testosterone functions to modulate risky behaviors in ways that appear to be adaptive. Still, more work is needed to understand the nature of the relationship between testosterone and risk in both sexes.

Effects of Mother and Father Dominance on Offspring Sex in Contemporary Humans Abstract Variation in human sex ratios has been the focus of empirical investigation for decades, but no studies have examined the relationship between parental mating behavior and future offspring sex. Here we tested whether dominance cues in the faces and behavior of mothers and fathers were associated with the sex of their future offspring. Our results show that maternal facial cues indicating lower dominance status may be associated with increased probability of a daughter as a first-born child, but only when moderated by high paternal facial dominance. We also show that both mother and father’s dominance behavior increases the probability of a first-born son. These results suggest that the effect of maternal dominance on the sex of offspring depends on the dominance of their male partner; however, future research should investigate additional characteristics that influence long-term mating decisions in women according to future offspring sex.