Sex differences in indirect aggression: Psychological evidence from young adults

Many studies have found differences in the types of aggression used by males and females, at least in children and adolescents. Boys tend to use direct physical or verbal aggression, whereas girls tend to use more indirect forms of aggression that prominently feature gossip. Evolutionary theories of sex differences in indirect aggression propose selection pressures that would have acted on older teenagers and adults. Evidence for sex differences in indirect aggression in adults, however, is equivocal. Virtually all studies of adults have found a sex difference in physical aggression, but most have failed to find sex differences in the use of the more indirect forms of aggression. Almost all of these studies have measured indirect aggression using self-reports of aggressive behavior. We investigated sex differences in the psychology of indirect aggression by exposing young adult women and men to the same aggression-evoking stimulus. As evolutionary models predict, we found that women had a stronger desire than men to aggress indirectly, even after controlling for perceptions of social norms and approval. Future work on both evolutionary and social norm models of indirect aggression is warranted.     

Weaponry,size, and sex ratio affect spatial distribution within small and large groups of the maritime earwig (Anisolabis maritima)

Dispersion patterns within a group can reveal important aspects about social interactions and sexual selection within a species. We examined the distribution patterns of the maritime earwig (Anisolabis maritima), an insect well suited for studies of aggression, sociality, and sexual selection since both sexes live in close proximity and possess weaponry in the form of sexually dimorphic pincers. To examine intra‐ and intersexual interactions within small groups, we conducted trials with three earwigs with limited access to shelters. In single‐sex trios, we found that both males and females exhibited strong size‐based intrasexual aggression, as larger individuals were less likely to be excluded from shelters; however, males were more likely to cohabitate than females. In mixed‐sex trios, we found that both males and females preferred smaller opposite‐sex partners, and cohabitation patterns indicate that both sex‐ and size‐based differences in aggression can influence overall spatial distribution. We also examined larger single‐sex and mixed‐sex groups of 18 earwigs to determine whether they had random, uniform, or clumped distributions. Similar to previous field observations, males tended to form aggregations, whereas females were distributed uniformly, a pattern indicative of territoriality. Mixed‐sex groups, on the other hand, were uniform during nocturnal periods of high activity but then become clumped after settling into more stable daytime positions. Overall, our results suggest that females have high levels of aggression regardless of the social context, whereas males alter their aggressive behavior in the presence of females.     

Relative health effects of education, socioeconomic status and domestic gender inequity in Sweden: a cohort study

Introduction

Limited existing research on gender inequities suggests that for men workplace atmosphere shapes wellbeing while women are less susceptible to socioeconomic or work status but vulnerable to home inequities.

Methods

Using the 2007 Northern Swedish Cohort (n = 773) we identified relative contributions of perceived gender inequities in relationships, financial strain, and education to self-reported health to determine whether controlling for sex, examining interactions between sex and other social variables, or sex-disaggregating data yielded most information about sex differences.

Results and Discussion

Men had lower education but also less financial strain, and experienced less gender inequity. Overall, low education and financial strain detracted from health. However, sex-disaggregated data showed this to be true for women, whereas for men only gender inequity at home affected health. In the relatively egalitarian Swedish environment where women more readily enter all work arenas and men often provide parenting, traditional primacy of the home environment (for women) and the work environment (for men) in shaping health is reversing such that perceived domestic gender inequity has a significant health impact on men, while for women only education and financial strain are contributory. These outcomes were identified only when data were sex-disaggregated.     

Testing Functional Hypotheses for the Behavior of Resident Pine Voles, Microtus pinetorum, Toward Non-Residents

Members of a social group should attempt to maximize their fitness by maintaining an optimal group composition. Allowing an immigrant into the group may be beneficial or costly depending on the characteristics of the immigrant as well as the composition of the group. Therefore, we examined behavioral interactions between pine voles to test three functional hypotheses proposed to explain behavior of residents toward non‐residents: the resource defense, mate defense, and benefit of extra‐pair copulation hypotheses. To test these, we examined the effects of age, sexual experience and sex of non‐residents as well as the effects of sex of residents on the behavior of residents. Neither male nor female residents showed affiliative behavior toward non‐residents. Residents were more aggressive toward non‐residents than vice versa. The frequency of same‐sex aggression was greater than opposite‐sex aggression for male residents and this aggression was directed at adult male non‐residents to a greater degree than at subadult males. Resident males were least aggressive toward adult females. We found no differences in the behavior of females toward subadults, sexually naive adult non‐residents or sexually experienced adult non‐residents. Females also displayed similar amounts of aggression toward male and female non‐residents. Therefore, for males, aggression may function in defense of a mate while for females, aggression functions in resource defense. For both sexes, aggression is likely to play a role in the regulation of group composition.     

In your face: facial metrics predict aggressive behaviour in the laboratory and in varsity and professional hockey players

Facial characteristics are an important basis for judgements about gender, emotion, personality, motivational states and behavioural dispositions. Based on a recent finding of a sexual dimorphism in facial metrics that is independent of body size, we conducted three studies to examine the extent to which individual differences in the facial width-to-height ratio were associated with trait dominance (using a questionnaire) and aggression during a behavioural task and in a naturalistic setting (varsity and professional ice hockey). In study 1, men had a larger facial width-to-height ratio, higher scores of trait dominance, and were more reactively aggressive compared with women. Individual differences in the facial width-to-height ratio predicted reactive aggression in men, but not in women (predicted 15% of variance). In studies 2 (male varsity hockey players) and 3 (male professional hockey players), individual differences in the facial width-to-height ratio were positively related to aggressive behaviour as measured by the number of penalty minutes per game obtained over a season (predicted 29 and 9% of the variance, respectively). Together, these findings suggest that the sexually dimorphic facial width-to-height ratio may be an 'honest signal' of propensity for aggressive behaviour.     

Sex, Lies and fMRI-Gender Differences in Neural Basis of Deception

Deception has always been a part of human communication as it helps to promote self-presentation. Although both men and women are equally prone to try to manage their appearance, their strategies, motivation and eagerness may be different. Here, we asked if lying could be influenced by gender on both the behavioral and neural levels. To test whether the hypothesized gender differences in brain activity related to deceptive responses were caused by differential socialization in men and women, we administered the Gender Identity Inventory probing the participants' subjective social sex role. In an fMRI session, participants were instructed either to lie or to tell the truth while answering a questionnaire focusing on general and personal information. Only for personal information, we found differences in neural responses during instructed deception in men and women. The women vs. men direct contrast revealed no significant differences in areas of activation, but men showed higher BOLD signal compared to women in the left middle frontal gyrus (MFG). Moreover, this effect remained unchanged when self-reported psychological gender was controlled for. Thus, our study showed that gender differences in the neural processes engaged during falsifying personal information might be independent from socialization.     

Facial width-to-height ratio in a Turkish population is not sexually dimorphic and is unrelated to aggressive behavior

Recently, Weston et al. (2004; Wide faces or large canines? The attractive versus the aggressive primate. Proceedings of the Royal Society of London, Series B, 271, 416–419) found that facial width-to-height ratio (WHR) is a sexually dimorphic characteristic in humans; males have higher facial WHR than females. Following this study, Carré et al. (2008; In your face: facial metrics predict aggressive behavior in the laboratory and in varsity and professional hockey players. Proceedings of the Royal Society of London, Series B, 275, 2651–2656) found that individual differences in facial WHR accounted for a significant proportion of the variance in aggressive behavior of men, but not women. I tested these two hypotheses in a sample of 470 Turkish university students. Facial WHR was measured from frontal photographs. I also measured the aggressiveness level of 212 individuals using the Buss and Perry aggressiveness questionnaire. The mean facial WHR (and standard deviation) was 1.89±0.12 for males and 1.91±0.11 for females. There was no relationship between facial WHR and the self-reported aggressive behavior for either sex. The facial WHR is not a sexually dimorphic characteristic (at least) for Turkish people, and it does not appear to be associated with self-reported trait aggression.     

Sex-role reversal is reflected in the brain of African black coucals (Centropus grillii)

In most bird species males compete over access to females and have elevated circulating androgen levels when they establish and defend a breeding territory or guard a mate. Testosterone is involved in the regulation of territorial aggression and sexual display in males. In few bird species the traditional sex-roles are reversed and females are highly aggressive and compete over access to males. Such species represent excellent models to study the hormonal modulation of aggressive behavior in females. Plasma sex steroid concentrations in sex-role reversed species follow the patterns of birds with "traditional" sex-roles. The neural mechanisms modulating endocrine secretion and hormone-behavior interactions in sex-role reversed birds are currently unknown. We investigated the sex differences in the mRNA expression of androgen receptors, estrogen receptor alpha, and aromatase in two brain nuclei involved in reproductive and aggressive behavior in the black coucal, the nucleus taeniae and the bed nucleus of the stria terminalis. In the bed nucleus there were no sex differences in the receptor or aromatase expression. In the nucleus taeniae, however, we show for the first time, that females have a higher mRNA expression of androgen receptors than males. These results suggest that the expression of agonistic and courtship behavior in females does not depend on elevated blood hormone levels, but may be regulated via increased steroid hormone sensitivity in particular target areas in the brain. Hence, aggression in females and males may indeed be modulated by the same hormones, but regulated at different levels of the neuroendocrine cascade.     

Sex differences in intrasexual aggression among sex-role-reversed,cooperatively breeding cichlid fish <Emphasis Type="Italic">Julidochromis regani</Emphasis>

In sex-role-reversed species, females compete for resources (e.g., mates) more intensively than do males. However, it remains unclear whether these species exhibit sex differences in the intensity of aggressive behavior in the context of within-sex contests. Cichlid fish in the genus Julidochromis exhibit intraspecific variation in mating systems, ranging from monogamy to cooperative polyandry with sex-role reversal. In the study reported here, we observed aggressive interactions among three same-sex individuals in Julidochromis regani in the laboratory and tested whether inter-female aggression was more intense than inter-male aggression. Although difference in body size strongly determined the direction of aggression in fish, aggression by a smaller-sized individuals toward larger ones was occasionally observed. This type of aggression was common between individuals of a similar body size (≤5 mm) and occurred more frequently among females than males. In contrast, differences in body size and sex did not affect the frequency of aggression by larger-sized individuals against smaller ones. Bidirectional aggression (i.e., mouth fighting) occurred frequently when two individuals had similar body size, and there was no difference in its frequency between sexes. However, temporal analysis showed that females performed bidirectional aggression more persistently than males. These sex differences in the intensity of intrasexual aggression could be the behavioral mechanisms underpinning cooperative polyandry.     

Digit length ratios predict reactive aggression in women, but not in men

Considerable evidence suggests that digit length and dermatoglyphic asymmetry patterns in the hand are affected by early exposure to androgens. Because androgens play an important role in sexual differentiation of morphological and behavioral traits, digit length patterns often display sex differences. When present, sex differences in digit lengths are more pronounced on the right side as compared to the left side. Moreover, the ratio of the second to fourth digit length (2D:4D) in the right hand is inversely correlated with testosterone (T) in men. Because T is implicated in agonistic behavior, 2D:4D may be used as a marker of androgen exposure and subsequent behavioral variation in adulthood. Consequently, we investigated the relationships among 2D:4D, directional asymmetry of 2D:4D (left hand 2D:4D minus right hand 2D:4D) as a variant of T, and human reactive aggression. One hundred young men (n = 51) and women (n = 49) participated in our experiments (mean age = 20.1 years). Participants called two noncompliant confederates to solicit donations for a fictitious charity organization and selected follow-up letters after the calls. The force exerted when hanging up the phone and the "tone" of the follow-up letters were used to assess reactive aggression. High aggression scores were associated with high directional asymmetry of 2D:4D and masculinized (low) right hand 2D:4D, only in females and under high provocation. Directional asymmetry of 2D:4D was positively correlated with T in males (pooled data, n = 97). Taken together, these data confirm the predominantly right-sided influence of androgens on digit length and suggest that digit length ratios may be associated with female reactive aggression when sufficient provocation is present.     

Sex,gender, and pharmaceutical politics: From drug development to marketing

Background: Biological sex differences and sociocultural gender norms affect the provision of health care products and services, but there has been little explicit analysis of the impact of sex differences and gender norms on the regulation of pharmaceutical development and marketing.Objectives: This article provides an overview of the regulation of pharmaceuticals and examines the ways that regulatory agencies account for sex and gender in their review of scientific data and marketing materials.Methods: The primary focus is on the US context, but information is also included about regulatory models in Europe, Canada, and Japan for comparative purposes. Specific examples show how sex differences and gender norms influence scientific and policy decisions about pharmaceuticals.Results: The United States and Canada were found to be the only countries that have explicit requirements to include women in clinical trials and to perform sex-based subgroup analysis on study results. The potential influence of politics on regulatory decisions may have led to an uneven application of standards, as seen through the examples of mifepristone (for abortion) and sildenafil citrate (for erectile dysfunction). Three detailed case studies illustrate the importance of considering sex and gender in pharmaceutical development and marketing: Phase I clinical trials; human papillomavirus quadrivalent vaccine; and tegaserod, a drug for irritable bowel syndrome.Conclusions: Sex and gender play important roles in pharmaceutical regulation, from the design of clinical trials and the approval of new drugs to advertising and postmarketing surveillance. However, regulatory agencies pay insufficient attention to both biological sex differences and sociocultural gender norms. This disregard perpetuates inequalities by ignoring drug safety problems that predominate in women and by allowing misleading drug marketing that reinforces gender stereotypes. Recommendations have been made to improve the regulation of pharmaceuticals in regard to sex and gender.     

Social Network Analysis of Male Dominance in the Paper Wasp Mischocyttarus mastigophorus (Hymenoptera: Vespidae)

Social aggression is a pervasive feature of insect societies. In eusocial Hymenoptera, aggression among females can affect task performance and competition over direct reproduction (egg laying); in most species males participate in social interactions relatively rarely. Males of the independent-founding paper wasp Mischocyttarus mastigophorus are exceptional: they are aggressive toward female nestmates, leading us to explore the function of this unusual behavior. We applied social network analyses to data on M. mastigophorus social aggression to quantify sex differences in giving and receiving social aggression. The network analyses supported the pattern of biased male aggression toward female nestmates; females are relatively rarely aggressive to males. We then asked whether male aggression toward females was biased by females’ relative ovary development. Males were more aggressive toward females with better-developed ovaries, opposite to patterns of aggression among females. Because food brought to the colonies is often monopolized by dominant females, we suggest that males direct aggression toward socially dominant females with better-developed ovaries to obtain food. The implications of biased male aggression for female task performance and physiology are unknown.

     

An fMRI study on the role of serotonin in reactive aggression

Reactive aggression after interpersonal provocation is a common behavior in humans. Little is known, however, about brain regions and neurotransmitters critical for the decision-making and affective processes involved in aggressive interactions. With the present fMRI study, we wanted to examine the role of serotonin in reactive aggression by means of an acute tryptophan depletion (ATD). Participants performed in a competitive reaction time task (Taylor Aggression Paradigm, TAP) which entitled the winner to punish the loser. The TAP seeks to elicit aggression by provocation. The study followed a double-blind between-subject design including only male participants. Behavioral data showed an aggression diminishing effect of ATD in low trait-aggressive participants, whereas no ATD effect was detected in high trait-aggressive participants. ATD also led to reduced insula activity during the decision phase, independently of the level of provocation. Whereas previous reports have suggested an inverse relationship between serotonin level and aggressive behavior with low levels of serotonin leading to higher aggression and vice versa, such a simple relationship is inconsistent with the current data.     

Increased aggression and activity level in 3- to 11-year-old girls with congenital adrenal hyperplasia (CAH)

Experimental research in a wide range of mammals has documented powerful influences of androgen during early development on brain systems and behaviors that show sex differences. Clinical research in humans suggests similar influences of early androgen concentrations on some behaviors, including childhood play behavior and adult sexual orientation. However, findings have been inconsistent for some other behaviors that show sex differences, including aggression and activity level in children. This inconsistency may reflect small sample sizes and assessment limitations. In the present study, we assessed aggression and activity level in 3- to 11-year-old children with CAH (38 girls, 29 boys) and in their unaffected siblings (25 girls, 21 boys) using a questionnaire that mothers completed to indicate current aggressive behavior and activity level in their children. Data supported the hypotheses that: (1) unaffected boys are more aggressive and active than unaffected girls; (2) girls with CAH are more aggressive and active than their unaffected sisters; and (3) boys with and without CAH are similar to one another in aggression and activity level. These data suggest that early androgens have a masculinizing effect on both aggressive behavior and activity level in girls.     

Behavioral Interactions of Two Solitary,Halictine Bees with Comparisons among Solitary,Communal and Eusocial Species

Laboratory behavioral experiments were conducted with two solitary, halictine bee species, in the genus Lasioglossum, to examine the nature of behavioral interactions between non-nestmate females. The experimental design of this study was identical to previous work on both a communal (Kukuk 1992) and a eusocial Lasioglossum species (Breed et al. 1978), thereby providing data for comparison of female-female interactions in species within a single, very large genus that exhibit different sociality. Both solitary species exhibited low levels of aggressive behavior yet nearly all females were reproductively active, as determined by subsequent dissections. Neither ovarian width nor size was associated with aggression in either species. Interspecific comparisons reveal large differences in cooperation and aggression among the four congeneric species. The communal species exhibits significantly more cooperative and less aggressive behavior than all other species suggesting that communal social behavior in Lasioglossum is not phenetically intermediate between the behavior of solitary and eusocial species.     

Early trauma and increased risk for physical aggression during adulthood: the moderating role of MAOA genotype

Previous research has reported that a functional polymorphism in the monoamine oxidase A (MAOA) gene promoter can moderate the association between early life adversity and increased risk for violence and antisocial behavior. In this study of a combined population of psychiatric outpatients and healthy volunteers (N = 235), we tested the hypothesis that MAOA genotype moderates the association between early traumatic life events (ETLE) experienced during the first 15 years of life and the display of physical aggression during adulthood, as assessed by the Aggression Questionnaire. An ANOVA model including gender, exposure to early trauma, and MAOA genotype as between-subjects factors showed significant MAOAxETLE (F(1,227) = 8.20, P = 0.005) and genderxMAOAxETLE (F(1,227) = 7.04, P = 0.009) interaction effects. Physical aggression scores were higher in men who had experienced early traumatic life events and who carried the low MAOA activity allele (MAOA-L). We repeated the analysis in the subgroup of healthy volunteers (N = 145) to exclude that the observed GxE interactions were due to the inclusion of psychiatric patients in our sample and were not generalizable to the population at large. The results for the subgroup of healthy volunteers were identical to those for the entire sample. The cumulative variance in the physical aggression score explained by the ANOVA effects involving the MAOA polymorphism was 6.6% in the entire sample and 12.1% in the sub-sample of healthy volunteers. Our results support the hypothesis that, when combined with exposure to early traumatic life events, low MAOA activity is a significant risk factor for aggressive behavior during adulthood and suggest that the use of dimensional measures focusing on behavioral aspects of aggression may increase the likelihood of detecting significant gene-by-environment interactions in studies of MAOA-related aggression.     

Sex differences and developmental effects of oxytocin on aggression and social behavior in prairie voles (Microtus ochrogaster)

Various hormones, including sex steroids and neuropeptides, have been implicated in aggression. In this study we examined (1) sex differences in intrasexual aggression in na?ve prairie voles; (2) the effects of developmental manipulations of oxytocin on intrasexual aggression; and (3) changes in patterns of intrasexual aggression after brief exposure to an animal of the opposite sex. Within 24 h of birth, infants were randomly assigned to receive either an injection of oxytocin (OT) or oxytocin antagonist (OTA) or to one of two control (CTL) groups receiving either isotonic saline or handling without injection. As adults, animals were tested twice in a neutral arena; before (Test 1) and 24 h after (Test 2) a 4-h exposure to an animal of the opposite sex. In Test 1, CTL males were more likely to show aggressive and less likely to show social behavior than CTL females. No significant treatment differences were observed within either sex in Test 1. In Test 2, after brief exposure to a male, females treated with OT became more aggressive and less social than OTA or CTL females. Male aggressive behavior did not change after exposure to a female. An increase in aggression and decline in social behavior toward other females, seen here in OT-treated females, is typically observed only following several days of female-male cohabitation. These findings demonstrate a sex difference in intrasexual aggression and suggest that neonatal exposure to OT may facilitate the onset of the mate-guarding component of pair bonding in female prairie voles.     

Sex-Differences of Face Coding: Evidence from Larger Right Hemispheric M170 in Men and Dipole Source Modelling

The processing of faces relies on a specialized neural system comprising bilateral cortical structures with a dominance of the right hemisphere. However, due to inconsistencies of earlier findings as well as more recent results such functional lateralization has become a topic of discussion. In particular, studies employing behavioural tasks and electrophysiological methods indicate a dominance of the right hemisphere during face perception only in men whereas women exhibit symmetric and bilateral face processing. The aim of this study was to further investigate such sex differences in hemispheric processing of personally familiar and opposite-sex faces using whole-head magnetoencephalography (MEG). We found a right-lateralized M170-component in occipito-temporal sensor clusters in men as opposed to a bilateral response in women. Furthermore, the same pattern was obtained in performing dipole localization and determining dipole strength in the M170-timewindow. These results suggest asymmetric involvement of face-responsive neural structures in men and allow to ascribe this asymmetry to the fusiform gyrus. This specifies findings from previous investigations employing event-related potentials (ERP) and LORETA reconstruction methods yielding rather extended bilateral activations showing left asymmetry in women and right lateralization in men. We discuss our finding of an asymmetric fusiform activation pattern in men in terms of holistic face processing during face evaluation and sex differences with regard to visual strategies in general and interest for opposite faces in special. Taken together the pattern of hemispheric specialization observed here yields new insights into sex differences in face perception and entails further questions about interactions between biological sex, psychological gender and influences that might be stimulus-driven or task dependent.     

Molecular psychogenetics of deviant aggressive behavior in humans

The review considers the known candidate gene loci that are involved in the dopamine, serotonin, and androgen systems and are associated with human deviant aggressive behavior. Both positive and negative correlations with deviant aggressive behavior have been observed for almost all of the candidate gene loci. Many genes of the neurotransmitter and androgen system and intricate interactions among them may influence the propensity to aggression. Further studies should focus not only on individual gene polymorphisms, but also on complex interactions among the alleles of all candidate genes that have functionally important polymorphisms affecting their expression and function. A complex analysis should be performed to study the association of the homozygous genotypes at all candidate gene markers with various forms of human deviant aggressive behavior. The approach will make it possible to assess the individual reactivity to various environmental stimuli that provoke aggression and to develop a means of predicting and preventing deviant aggressive behavior in humans.