Dual functional brain asymmetry refers to the notion that in most individuals the left cerebral hemisphere is specialized for language functions, whereas the right cerebral hemisphere is more important than the left for the perception, construction, and recall of stimuli that are difficult to verbalize. In the last twenty years there have been scattered reports of sex differences in degree of hemispheric specialization. This review provides a critical framework within which two related topics are discussed: Do meaningful sex (...) class='Hi'>differences in verbal or spatial cerebral lateralization exist? and, if so, Is the brain of one sex more symmetrically organized than the other? Data gathered on right-handed adults are examined from clinical studies of patients with unilateral brain lesions; from dichotic listening, tachistoscopic, and sensorimotor studies of functional asymmetries in non-brain-damaged subjects; from anatomical and electrophysiological investigations, as well as from the developmental literature. Retrospective and descriptive findings predominate over prospective and experimental methodologies. Nevertheless, there is an impressive accummulation of evidence suggesting that the male brain may be more asymmetrically organized than the female brain, both for verbal and nonverbal functions. These trends are rarely found in childhood but are often significant in the mature organism. (shrink)

I argue that the magnitude and nature of sex differences in aggression, their development, causation, and variability, can be better explained by sexual selection than by the alternative biosocial version of social role theory. Thus, sex differences in physical aggression increase with the degree of risk, occur early in life, peak in young adulthood, and are likely to be mediated by greater male impulsiveness, and greater female fear of physical danger. Male variability in physical aggression is consistent with (...) an alternative life history perspective, and context-dependent variability with responses to reproductive competition, although some variability follows the internal and external influences of social roles. Other sex differences, in variance in reproductive output, threat displays, size and strength, maturation rates, and mortality and conception rates, all indicate that male aggression is part of a sexually selected adaptive complex. Physical aggression between partners can be explained using different evolutionary principles, arising from the conflicts of interest between males and females entering a reproductive alliance, combined with variability following differences in societal gender roles. In this case, social roles are particularly important since they enable both the relatively equality in physical aggression between partners from Western nations, and the considerable cross-national variability, to be explained. (shrink)

Behavioral ecology, based in the theory of natural selection, predicts that certain behaviors are likely to differ consistently between the sexes in humans as well as other species: aggression, resource striving, information content of sexual signalling. These differences, though of course open to modification by cultural practice, arise because male and female humans, like males and females of other mammal species, typically optimize their reproductive lifetimes through different behaviors: males specializing in mating effort (which has a high fixed cost, (...) and is not offspring-specific), and females in parental effort (which has more linear reproductive returns, and is offspring-specific). The resulting patterns are reviewed. (shrink)

Contemporary mate preferences can provide important clues to human reproductive history. Little is known about which characteristics people value in potential mates. Five predictions were made about sex differences in human mate preferences based on evolutionary conceptions of parental investment, sexual selection, human reproductive capacity, and sexual asymmetries regarding certainty of paternity versus maternity. The predictions centered on how each sex valued earning capacity, ambition— industriousness, youth, physical attractiveness, and chastity. Predictions were tested in data from 37 samples drawn (...) from 33 countries located on six continents and five islands. For 27 countries, demographic data on actual age at marriage provided a validity check on questionnaire data. Females were found to value cues toresource acquisitionin potential mates more highly than males. Characteristics signalingreproductive capacitywere valued more by males than by females. These sex differences may reflect different evolutionary selection pressures on human males and females; they provide powerful cross-cultural evidence of current sex differences in reproductive strategies. Discussion focuses on proximate mechanisms underlying mate preferences, consequences for human intrasexual competition, and the limitations of this study. (shrink)

An evolutionary psychological perspective drawing on sexual selection theory can better explain sex differences in aggression and violence than can social constructionist theories. Moreover, there is accumulating evidence that, in accordance with predictions derived from sexual selection theory, men modulate their willingness to engage in risky and violent confrontations in response to cues to fitness variance and future prospects.

According to I3 Theory, individuals enact aggressive behaviors when (a) instigating triggers are severe, (b) impelling forces are strong, and/or (c) inhibiting forces are weak. Archer's analysis of human sex differences in aggression could be bolstered by a careful analysis of male-female discrepancies in reactivity (or exposure) to instigating triggers, proneness toward impelling forces, and/or proneness toward inhibiting forces.

We agree with Archer that human sex differences in aggression are well explained by sexual selection, but note that explanations of human behaviors are not logically mutually exclusive from explanations and therefore should not be framed as such. We discuss why this type of framing hinders the development of both social learning and evolutionary theories of human behavior.

Benenson et al. postulate that human females evolved unique survival adaptations to facilitate maternal and grandmaternal care. This hypothesis is consistent with the broader hypothesis that female phenotypes are more ecologically optimal, but further evidence is needed to make a compelling case that sex differences in self-protection are not primarily the result of more intense sexual selection on males.

The relation between testosterone levels and aggressive behavior is well established. From an evolutionary viewpoint, testosterone can explain at least part of the sex differences found in aggressive behavior. This explanation, however, is mediated by factors such as prenatal testosterone levels and basal levels of cortisol. Especially regarding sex differences in aggression during adolescence, these mediators have great influence. Based on developmental brain structure research we argue that sex differences in aggression have a pre-pubertal origin and are (...) maintained during adolescence. Evidence of prenatal, adolescent, and adult levels of testosterone in relation to aggression taken together, support Archer's argument for sexual selection as the driver of sex differences in aggression. (shrink)

This study investigated sex differences in interest in infants among children, adolescents, young adults, and older individuals. Interest in infants was assessed with responses to images depicting animal and human infants versus adults, and with verbal responses to questionnaires. Clear sex differences, irrespective of age, emerged in all visual and verbal tests, with females being more interested in infants than males. Male interest in infants remained fairly stable across the four age groups, whereas female interest in infants was (...) highest in childhood and adolescence and declined thereafter, particularly for the responses to visual stimuli. The observed developmental changes in female interest in infants are consistent with the hypothesis that they represent a biological adaptation for parenting. (shrink)

This article considers 3 claims that cognitive sex differ- ences account for the differential representation of men and women in high-level careers in mathematics and sci- ence: (a) males are more focused on objects from the beginning of life and therefore are predisposed to better learning about mechanical systems; (b) males have a pro- file of spatial and numerical abilities producing greater aptitude for mathematics; and (c) males are more variable in their cognitive abilities and therefore predominate at the upper (...) reaches of mathematical talent. Research on cogni- tive development in human infants, preschool children, and students at all levels fails to support these claims. Instead, it provides evidence that mathematical and scientific rea- soning develop from a set of biologically based cognitive capacities that males and females share. These capacities lead men and women to develop equal talent for mathe- matics and science. (shrink)

The COVID-19 pandemic has caused a global societal, economic, and social upheaval unseen in living memory. There have been substantial cross-national differences in the kinds of policies implemented by political decision-makers to prevent the spread of the virus, to test the population, and to manage infected patients. Among other factors, these policies vary with politicians’ sex: early findings indicate that, on average, female leaders seem more focused on minimizing direct human suffering caused by the SARS-CoV-2 virus, while male leaders (...) implement riskier short-term decisions, possibly aiming to minimize economic disruptions. These sex differences are consistent with broader findings in psychology, reflecting women’s stronger empathy, higher pathogen disgust, health concern, care-taking orientation, and dislike for the suffering of other people—as well as men’s higher risk-taking, Machiavellianism, psychopathy, narcissism, and focus on financial indicators of success and status. This review article contextualizes sex differences in pandemic leadership in an evolutionary framework. Evolution by natural selection is the only known process in nature that organizes organisms into higher degrees of functional order, or counteracts the unavoidable disorder that would otherwise ensue, and is therefore essential for explaining the origins of human sex differences. Differential sexual selection and parental investment between males and females, together with the sexual differentiation of the mammalian brain, drive sex differences in cognition and behavioral dispositions, underlying men’s and women’s leadership styles and decision-making during a global pandemic. According to thesexually dimorphic leadership specialization hypothesis, general psychobehavioral sex differences have been exapted during human evolution to create sexually dimorphic leadership styles. They may be facultatively co-opted by societies and/or followers when facing different kinds of ecological and/or sociopolitical threats, such as disease outbreaks or intergroup aggression. Early evidence indicates that against the invisible viral foe that can bring nations to their knees, the strategic circumspection of empathic feminine health “worriers” may bring more effective and humanitarian outcomes than the devil-may-care incaution of masculine risk-taking “warriors”. (shrink)

Sex differences in mortality rates stem from genetic, physiological, behavioral, and social causes that are best understood when integrated in an evolutionary life history framework. This paper investigates the Male-to-Female Mortality Ratio (M:F MR) from external and internal causes and across contexts to illustrate how sex differences shaped by sexual selection interact with the environment to yield a pattern with some consistency, but also with expected variations due to socioeconomic and other factors.

Despite the importance of extrapair copulation (EPC) in human evolution, almost nothing is known about the design features of EPC detection mechanisms. We tested for sex differences in EPC inference-making mechanisms in a sample of 203 young couples. Men made more accurate inferences (φmen = 0.66, φwomen = 0.46), and the ratio of positive errors to negative errors was higher for men than for women (1.22 vs. 0.18). Since some may have been reluctant to admit EPC behavior, we modeled (...) how underreporting could have influenced these results. These analyses indicated that it would take highly sex-differentiated levels of underreporting by subjects with trusting partners for there to be no real sex difference. Further analyses indicated that men may be less willing to harbor unresolved suspicions about their partners’ EPC behavior, which may explain the sex difference in accuracy. Finally, we estimated that women underreported their own EPC behavior (10%) more than men (0%). (shrink)

Venturing into novel terrain poses physical risks to a female and her offspring. Females have a greater tendency to avoid physical harm, while males tend to have larger range sizes and often outperform females in navigation-related tasks. Given this backdrop, we expected that females would explore a novel environment with more caution than males, and that more-cautious exploration would negatively affect navigation performance. Participants explored a novel, large-scale, virtual environment in search of five objects, pointed in the direction of each (...) object from the origin, and then navigated back to the objects. We found that females demonstrated more caution while exploring as reflected in the increased amounts of pausing and revisiting of previously traversed locations. In addition, more pausing and revisiting behaviors led to degradation in navigation performance. Finally, individual levels of trait harm avoidance were positively associated with the amount of revisiting behavior during exploration. These findings support the idea that the fitness costs associated with long-distance travel may encourage females to take a more cautious approach to spatial exploration, and that this caution may partially explain the sex differences in navigation performance. (shrink)

Mower questions some aspects of Geary's proposals regarding the nature of male-male and female-female relationships during human evolution and the implications for understanding the basis for same-sex friendships. The core of this proposal is reviewed and Mower's challenges to the core are addressed.

It is hypothesized from within an evolutionary framework that females should be less invested in peer relations than males. Investment was operationalized as enjoyment in Study 1 and as preference for interaction in Study 2. In the first study, four- and six-year-old children’s enjoyment of peer interaction was observed in 26 groups of same-sex peers. Girls were rated as enjoying their interactions significantly less than boys. In the second study, six- and nine-year-old children were interviewed about the individuals with whom (...) they spend time in their homes and neighborhoods and about the individuals who participate in their favorite activities. The proportion of individuals named by children who were peers was significantly lower for girls than boys both in children’s neighborhoods and in children’s favorite activities. Results strongly support the hypothesis that females and males have evolved differential preferences for interaction with peers. (shrink)

Among hunter-gatherers, the sharing of male and female foods is often assumed to result in virtually the same diet for males and females. Although food sharing is widespread among the hunting and gathering Hadza of Tanzania, women were observed eating significantly more tubers than men. This study investigates the relationship between patterns of dental wear, diet, and extramasticatory use of teeth among the Hadza. Casts of the upper dentitions were made from molds taken from 126 adults and scored according to (...) the Murphy dental attrition scoring system. Females had significantly greater anterior occlusal wear than males when we controlled for age. Males exhibited greater asymmetry in wear, with greater wear on the left side in canines, first premolars, and first molars. We suggest that these sex differences in wear patterns reflect the differences seen in the diet, as well as in the use of teeth as tools. (shrink)

The ultimate causes of sex differences in human aggressive behavior can lie mainly in evolved, inherited mechanisms that differ by sex or mainly in the differing placement of women and men in the social structure. The present commentary contrasts Campbell's evolutionary interpretation of aggression sex differences with a social structural interpretation that encompasses a wider range of phenomena.

The finding that women are attracted to men older than themselves whereas men are attracted to relatively younger women has been explained by social psychologists in terms of economic exchange rooted in traditional sex-role norms. An alternative evolutionary model suggests that males and females follow different reproductive strategies, and predicts a more complex relationship between gender and age preferences. In particular, males' preferences for relatively younger females should be minimal during early mating years, but should become more pronounced as the (...) male gets older. Young females are expected to prefer somewhat older males during their early years and to change less as they age. We briefly review relevant theory and present results of six studies testing this prediction. Study 1 finds support for this gender-differentiated prediction in age preferences expressed in personal advertisements. Study 2 supports the prediction with marriage statistics from two U.S. cities. Study 3 examines the cross-generational robustness of the phenomenon, and finds the same pattern in marriage statistics from 1923. Study 4 replicates Study 1 using matrimonial advertisements from two European countries, and from India. Study 5 finds a consistent pattern in marriages recorded from 1913 through 1939 on a small island in the Philippines. Study 6 reveals the same pattern in singles advertisements placed by financially successful American women and men. We consider the limitations of previous normative and evolutionary explanations of age preferences and discuss the advantages of expanding previous models to include the life history perspective. (shrink)

Anxiety disorders are more predominant in women than men, however there is a lack of understanding as to what neurocognitive mechanisms drive this sex difference. Recent investigation has found a potential moderating role of sex in the relationship between anxiety and the error related negativity —a component of error-monitoring that is prevalent in high anxiety individuals—such that females display a positive relationship between anxiety/worry and ERN amplitude. We strove to further explore the influence of sex on the relationship between trait (...) anxiety and performance monitoring, specifically with ERN, as well as extend this work to include another hallmark of anxiety, attentional bias to threat. To meet this end, participants performed the flanker and dot-probe tasks, respectively. We did not find a significant difference in the relationship between attention bias scores and anxiety for female vs. males participants. Furthermore, ΔERN amplitudes were greater in males compared to females, and males had more positive CRN amplitudes than females. There were no significant associations between ERN or ΔERN with anxiety in both male and female participants. However, there was a significant relationship between CRN amplitudes and trait anxiety in male but not female participants. Given these results, the effect of sex on the relationship between components of performance monitoring—namely the CRN and ERN—and anxiety may be more nuanced than the current understanding. Our study was limited to detecting medium to large sized moderation effects. Our findings may be important for future meta-analysis on sex differences in anxiety. (shrink)

In describing and explaining the sexes, medicine and science participated in the delineation of what was "feminine" and what was "masculine" in the Middle Ages. Hildegard of Bingen and Albertus Magnus, among others, writing about gynecology, the human constitution, fetal development, or the naturalistic dimensions of divine Creation, became increasingly interested in issues surrounding reproduction and sexuality. Did women as well as men produce procreative seed? How did the physiology of the sexes influence their healthy states and their susceptibility to (...) disease? Who derived more pleasure from sexual intercourse, men or women? The answers to such questions created a network of flexible concepts which did not endorse a single model of male-female relations, but did affect views on the health consequences of sexual abstinence for women and men and on the allocation of responsibility for infertility - problems with much social and religious significance in the Middle Ages. Sometimes at odds with, and sometimes in accord with other forces in medieval society, medicine and natural philosophy helped to construct a set of notions that divided significant portions of the world - from the behavior of animals to the operations of astrological signs - into "masculine" and "feminine." Even cases that seemed to exist outside the definitions of this duality, for example, hermaphrodite features or homosexual behavior, were brought under control by the application of gendered labels, such as "masculine women.". (shrink)

This article charts the feminist perspectives that have come out of the author’s thinking on the dance performance text Human Sex and how this has informed her own feminism. In doing so, the author argues that a feminist agenda is shifting and dynamic but also reliant upon prior readings and interpretations that provide the point of reference for a departure to other readings and perspectives. Using autobiographical material, the author highlights the importance of considering the personal histories of subject-hood that (...) influence a feminist consciousness and how these are the condition of possibility for making other readings. To demonstrate the shifting character of identity over time, she engages in different readings of Human Sex through the work of feminist theorists Sara Ahmed, Judith Butler and Peggy Phelan. (shrink)

Evolutionary theory makes further predictions about conflict. It predicts sex differences in the proclivity to attack and defend. It further suggests complementary biases in what we expect of the sexes. Finally, it suggests that the forms of human facial expressions of anger and happiness may have coevolved with the regularity of conflict as a means of signaling, bluffing, and defusing attack.

In this article, we argue that there is an essential difference between social intelligence and creative intelligence, and that they have their foundation in human sexuality. For sex differences, we refer to the vast psychological, neurological, and cognitive science research where problem-solving, verbal skills, logical reasoning, and other topics are dealt with. Intelligence tests suggest that, on average, neither sex has more general intelligence than the other. Though people are equals in general intelligence, they are different in special forms (...) of intelligence such as social intelligence and creative intelligence, the former dominant in women, the latter dominant in men. The dominance of creative intelligence in men needs to be explained. The focus of our research is on the strictly anthropological aspects, and consequently our explanation for this fact is based on the male-female polarity in the mating systems. Sexual dimorphism does not only regard bodily differences but implies different forms of sex life. Sex researchers distinguish between two levels of sexual intercourse: procreative sex and recreational sex, and to these we would add “creative sex.” On all three levels, there is a behavioral difference between men and women, including the subjective experience. These differences are as well attributed to culture as genetically founded in nature. Sexual reproduction is only possible if females cooperate. Their biological inheritance makes females play a decisive role in mate choice. Recreational sex for the purpose of pleasure rather than reproduction results from female extended sexual activity. Creative sex, on the contrary, is a specifically male performance of sexuality. We identify creative sex with eroticism. Eroticism evolved through the transformation of the sexual drive into a mental state of expectation and fantasizing. Hence, sex differences (that nowadays are covered up by cultural egalitarianism) continue to be the evolutionary origin of the difference between social and creative intelligence. (shrink)

ABSTRACTPerception of emotion in the face is a key component of human social cognition and is considered vital for many domains of life; however, little is known about how this ability differs across the lifespan for men and women. We addressed this question with a large community sample of persons ranging from younger than 15 to older than 60 years of age. Participants were viewers of the television show “Tout le Monde Joue”, and the task was presented on television, with (...) participants responding via their mobile devices. Applying latent variable modeling, and establishing measurement invariance between males and females and across age, we found that, for both males and females, emotion perception abilities peak between the ages of 15 and 30, with poorer performance by younger adults and declining performance after the age of 30. In addition, we show a consistent advantage by females across the lifespan, which decreases in magnitude with increasing age. This large scale study with a wide ran... (shrink)

A sex difference in the tendency to outbreed (female exogamy) is a premise for the target article's proposed framework, which receives some support by being shared with chimpanzees but not with more distantly related primates. Further empirical support is provided, and it is suggested that recognition of sex differences might improve effective fairness, taking sexual assault as a case in point.

Background: Losing one’s only child may lead to post-traumatic stress disorder, of which re-experiencing is the core symptom. However, neuroimaging studies of sex differences in re-experiencing in the context of the trauma of losing one’s only child and PTSD are scarce; comparisons of the functional networks from the hippocampal subfields to the thalamus might clarify the neural basis.Methods: Thirty couples without any psychiatric disorder who lost their only child, 55 patients with PTSD, and 50 normal controls underwent resting-state functional (...) magnetic resonance imaging. The functional connectivity from the hippocampal subregions to the thalamus and the correlations of FC with re-experiencing symptoms were analyzed within and between the sexes.Results: Compared with husbands without PTSD, wives without PTSD had higher re-experiencing symptoms and weaker FC between the right hippocampal cornu ammonis 3 and the right thalamus. Moreover, only the correlation between the RCA3-RT FC and re-experiencing in wives without PTSD was significant. Among the three groups, only the RCA3-RT FC in female subjects was markedly different. Additionally, the RCA3-RT FC in wives without PTSD was remarkably lower relative to female patients with PTSD.Conclusion: Wives without PTSD who lost their only child had worse re-experiencing symptoms relative to their husbands, which was associated with the FC alteration between the hippocampal subregions and the thalamus. Importantly, the low level of the RCA3-RT FC may play a potentially protective role against the development of PTSD in wives who have lost their only child. (shrink)

Four issues relevant to sex differences in human aggression and violence are considered. (1) The motivation for play and serious aggression in children and juvenile animals is different. Consequently, the evolutionary explanations for each may be different. (2) Sex differences in intrasexual aggression may be due to effects of the attacker or the target. There is evidence that both males and females are more physically aggressive against males and less physically aggressive against females. The evolutionary explanation for each (...) component of the sex difference in intrasexual aggression may be different. (3) Aggression and violence are defined. The former is the attack, and the latter is the consequent injury or death. The evolutionary explanation for each may not be the same. (4) Most men and women are neither physically aggressive nor criminally violent. The evolutionary explanations of sex differences in aggression and violence should take this polymorphism into account. (shrink)

Gangestad & Simpson make a major contribution by highlighting the importance of mate choice for good genes, the costs of alternative strategies, and tradeoffs inherent in human mating. By downplaying sex differences and ignoring the nongenetic adaptive benefits of short term mating, however, they undermine their goal of “strategic pluralism” by presenting a theory devoid of many documented complexities of human mating.

Evolutionary theory predicts that social status and fertility will be positively related. It also predicts that the relationship between status and fertility will differ for men and women. This is particularly likely in modern societies given evidence that females face greater trade-offs between status and resource acquisition and fertility than males. This paper tests these hypotheses using newly released data from the 2014 wave of the Survey of Income and Program Participation by the US Census, which has the first complete (...) measures of fertility and number of childbearing partners for a large, representative, national probability sample of men and women and also contains comprehensive measures of economic status as measured by personal and family resources, including income from all sources and all assets. Multivariate analyses show that personal income is positively associated with total fertility and number of childbearing unions for men only. For men, personal net worth is positively associated with number of childbearing unions; it is also positively associated with fertility for married men with a spouse present. These findings support evolutionary predictions of a positive relationship between status, access to mates, and reproductive success for males. Whereas personal income and personal net worth are negatively associated with total fertility and number of childbearing unions for women, family income is positively associated with total fertility for women. For married men living with a spouse, family income is negatively associated with total fertility. These findings are consistent with evolutionary theory given the existence of greater trade-offs between production and reproduction for women in an advanced industrial society. For women and men, family net worth is negatively associated with number of childbearing unions and fertility. Implications are discussed. (shrink)

British marriage statistics suggest that women of breeding age choose young men. Women past breeding age who could still be raising children extend choices to include older men. After this, women do not marry. The choices of men over 50 are restricted to women between 40 and 55: past breeding but young enough to be raising children; the few men over 50 that marry choose women in this age range.

Previous research suggests that individuals diagnosed with morbid jealousy have jealousy mechanisms that are activated at lower thresholds than individuals with normal jealousy, but that these mechanisms produce behavior that is similar to individuals with normal jealousy. We extended previous research documenting these similarities by investigating sex differences in partner-directed violence committed by individuals diagnosed with morbid jealousy. The results support some of our predictions. For example, a greater percentage of men than women diagnosed with morbid jealousy used physical (...) violence, attempted to kill, and actually killed their partners, and used their hands rather than an object to kill their partners. These results replicate results generated for individuals with normal jealousy. Discussion addresses implications of the current research and highlights directions for future research on the psychology of morbid jealousy. (shrink)

Schmitt's study provides strong support for sexual strategies theory (Buss & Schmitt 1993) – that men and women both have evolved a complex menu of mating strategies, selectively deployed depending on personal, social, and ecological contexts. It also simultaneously refutes social structural theories founded on the core premise that women and men are sexually monomorphic in their psychology of human mating. Further progress depends on identifying evolved psychological design features sensitive to the costs and benefits of pursuing each strategy from (...) the menu, which vary across mating milieus. These design features, like many well-documented mating adaptations, are likely to be highly sex-differentiated. (shrink)

“Sociosexuality from Argentina to Zimbabwe: A 48-nation study of sex, culture, and strategies of human mating” delivers on its title. By combining empiricism and careful hypothesis testing, it not only contributes to our current knowledge but also points the way to further advances.

Sexual selection traditionally involves male-male competition and female choice, but in some species, including humans, sexual selection can also involve female-female competition and male choice. The degree to which one aspect of sexual selection or another is manifest in human populations will be influenced by a host of social and ecological variables, including the operational sex ratio. These variables are discussed in connection with the relative contribution of sexual selection and the division of labor to the evolution of human sex (...) differences. (shrink)

As Archer argues, recent developmental data on human physical aggression support the sexual selection hypothesis. However, sex differences are largely due to males on a chronic trajectory of aggression. Maternal characteristics of these males suggest that, in societies with low levels of physical violence, females with a history of behavior problems largely contribute to maintenance of physical aggression sex differences.