The official published version can be accessed from the link below - Copyright © Cambridge University Press 2009An 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

Daly

Kimura

Margo Wilson

Martin Daly

Nicholas Pound

Behavioral and Brain Sciences

English

Crossref

There's no contest: Human sex differences are sexually selected

Pound, N

Daly, M

Wilson, M

Brunel University Research Archive

framework has been addressed in a recent and very provocativepaper by Buston and Emlen (2003). They suggest that matechoice, at least among Cornell undergraduates, is not governedby mate “potential” attributes – for example, attractive womenchoosing strong, high-status males, and strong, high status menchoosing attractive women. Instead, they found that individuals’choices were based on their perceptions of the possible durationof the relationships. When there was expectation of a long term,stable relationship, individuals chose mates similar to themselves(on factors such as physical attractiveness, status, and commit-ment to family). Attractive women chose attractive men; theydid not choose strong, aggressive men. Situations in whichcosts associated with mate switching is high, for example,where divorce is economically disruptive, would result in individ-uals choosing mates who are similar to themselves (see Borgerh-off Mulder 2004).By contrast, aggressive men (again, where aggression is intra-sexual) were chosen by physically attractive women in situationsin which the quality of available mates is low. In this latter case,male-male aggression would be reinforced by female choice andfemales would “sample” widely from available males for a strongmate who also protects and provisions her and her offspring.With these limitations stated, Archer’s target article makes animportant contribution toward the theoretical integration of awide and disparate literature. He has done the field an importantservice.There’s no contest: Human sex differencesare sexually selecteddoi:10.1017/S0140525X0999032XNicholas Pound,a Martin Daly,b and Margo WilsonbaDepartment of Psychology, School of Social Sciences, Brunel University,Uxbridge UB8 3PH, United Kingdom; bDepartment of Psychology,Neuroscience, and Behaviour (PNB), McMaster University, Hamilton, OntarioL8S 4K1, Canada.nicholas.pound@brunel.ac.ukdaly@mcmaster.cawilson@mcmaster.caAbstract: An evolutionary psychological perspective drawing on sexualselection theory can better explain sex differences in aggression andviolence than can social constructionist theories. Moreover, there isaccumulating evidence that, in accordance with predictions derivedfrom sexual selection theory, men modulate their willingness to engagein risky and violent confrontations in response to cues to fitnessvariance and future prospects.In the target article, Archer argues persuasively that an evol-utionary psychological perspective drawing on sexual selectiontheory can account for observed sex differences in aggressionand violence more parsimoniously than social constructionist the-ories. In our view, however, the case for sexual selection’s role inthe evolution of these sex differences is even stronger thanArcher’s treatment suggests, and he concedes too much to advo-cates of the discredited null hypothesis that female and malepsyches are undifferentiated.According to Archer (sect. 2.2.1), the “main alternative” to aselectionist explanation of the origins of the sex differences ofinterest is the “social role” theory of Eagly and Wood (1999;Wood & Eagly 2002). At best, these authors can be read as offer-ing a partial account of ontogenetic processes in sexual differen-tiation, which, if upheld, would complement an evolutionaryaccount. At worst, they can be read as proposing that the onlyevolved differences between women and men are “physical”(i.e., non-neural anatomical differences), and if this is indeedtheir meaning, they are simply uninformed (see, e.g., Kimura1999). In neither case have they provided a viable “alternative”to an account that gives centre stage to sexual selection.In explaining why sexual selection should have made menmore intensely competitive than women, Archer (sect. 3.2)aptly cites anatomical, demographic, and behavioural evidencethat Homo sapiens evolved as an effectively polygynous speciesin which male fitness variance exceeded female fitness variance.Recent genetic evidence (Wilder et al. 2004) reinforces this con-clusion: In both our species as a whole and in discrete subpopu-lations thereof, the most recent common ancestor (MRCA) ofmitochondrial DNA, inherited matrilineally, lived about twiceas long ago as the MRCA of Y chromosomes, inherited patriline-ally. These results provide strong evidence that individual menhave consistently faced a higher risk of reproductive failurethan individual women.Archer notes (sect. 2.8) that, consistent with sexual selectiontheory, there is evidence that males with limited access to repro-ductive opportunities, or the resources required to obtain suchopportunities, are more likely to resort to violence. However,there is accumulating evidence to support a more specific predic-tion derived from the same theoretical perspective, namely thatthe prevalence of dangerous confrontations should vary predicta-bly according to variations in the local intensity of intrasexualcompetition and that cues to higher fitness variance shouldlead males to modulate their willingness to engage in risky andviolent interactions with other men (for review, see Wilsonet al. 2002; 2009).In an effectively polygynous species, the intensity of male-malecompetition will in part depend on the extent to which theresources required to obtain reproductive opportunities are dis-tributed equitably. Extreme inequity creates a situation wherethose at the bottom have little to lose if they escalate theirtactics of competition, and much to gain. Consequently, cues toinequity should lead to facultative adjustments in men’s willing-ness to employ violent and risky tactics to gain status andresources, which are the means to fitness. Consistent with this,evidence indicates that relative deprivation (as indexed byincome inequality) is typically a more powerful predictor of vari-ation in male violence than other socioeconomic measures suchas percent below the poverty line or average income (Daly &Wilson 2001). In both cross-national and more local comparisons,the Gini index of income inequality consistently outperformsmost other socioeconomic predictors of homicide rates (e.g.Blau & Blau 1982; Daly et al. 2001).Increased willingness to resort to violence where resources aredistributed inequitably is not uniquely predicted by sexual selec-tion theory. However, in contrast to social constructionistaccounts, an evolutionary psychological perspective treats suchincreased risk-proneness as a facultative adaptive response tosituations where the distribution of resources is such that exces-sive risk-aversion will lead to substantially reduced expectedfitness (Wilson et al. 2002; 2009).An evolutionary psychological approach based on sexual selec-tion and life history theory also predicts that individuals shouldmodulate their willingness to engage in risky and violent confron-tations according to cues of future survival and hence reproduc-tive prospects – in other words, when prospects are poor,organisms may be expected to discount the future steeply inthe pursuit of more immediate goals (Daly & Wilson 2005).Archer notes (sect. 3.1) that greater male than female mortalityis characteristic of the sexually selected “adaptive complex” gen-erated by intense inter-male competition, but the target articlecould perhaps have examined the implications of this in moredetail. It is not just that males are likely to discount the futuremore heavily than females as a consequence of the sex differencein mortality; moreover, future discounting and willingness toengage in risky escalation of social conflicts are expected tovary predictably in relation to future survival prospects. Wherethese are poor, men should become particularly risk prone andwilling to risk death in violent altercations as they compete forCommentary/Archer: Sex differences in aggression286 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:3/4the resources required to obtain reproductive opportunities, anddirectly for the opportunities themselves. Consistent with this,Wilson and Daly (1997) found that across neighbourhoods in amajor U.S. city (Chicago), the best statistical predictor of homi-cide rates was low male life expectancy (even with homicide asa cause of death removed).Finally, we do not understand Archer’s rationale for suggestingthat a sexual selection approach warrants the “prediction” thatthe sexes will not differ in “aggression” or “anger,” but only inhow they manifest these things. Sell’s research and theorizing(e.g., Sell et al. 2005), which Archer cites, clearly suggests thatinsofar as becoming angry entails an elevated risk of violent con-frontation, we may expect people to differ adaptively in theirreadiness to anger. Why should this proposition not apply tosex differences? More fundamentally, what does it even meanto suggest that men and women do not differ in “aggression” or“anger,” but only in the manifestations thereof? We lack bothconsensual definitions and good metrics of these states, andfinding that the sexes give the same mean answer on a self-report scale of “aggression” or “anger” is uninformative. Conse-quently, evidence for the popular claim that men and womenare equally aggressive, but that the former manifest their aggres-sion “directly” (e.g., as violence) and the latter “indirectly” (e.g.,as innuendo) is not convincing.Sex differences in dream aggressiondoi:10.1017/S0140525X09990306Michael SchredlSleep Laboratory, Central Institute of Mental Health, J5, 68159 Mannheim,Germany.Michael.Schredl@zi-mannheim.dewww.dreamresearch.deAbstract:Dream research shows sex differences in dream aggression thatfit very well with the findings for waking-life aggressive behaviour. Dreamstudies are a valuable tool for investigating variables underlying the sexdifference in aggression. One might argue that studying dreamaggression might be even more promising because aggression indreams is not socially labelled, as being aggressive in waking life is.Since dreams reflect waking life experiences (the so-called conti-nuity hypothesis of dreaming; Schredl 2003), dream studies canelucidate sex differences reported for waking behaviour. Forexample, it is a stable finding that men report more sexualdreams than do women (Schredl et al. 1998), which reflects themeta-analytic findings of higher frequency of masturbation andsexual fantasies in males compared to females (Oliver & Hyde1993). Regarding aggression in dreams, the findings are in linewith the meta-analysis reported by Archer: Men’s dreamsincluded more physical aggression than women’s dreams did(Hall & Van de Castle 1966), whereas the amount of verbalaggression did not differ between the sexes (Schredl et al.1998). The gender difference regarding the percentage of phys-ical aggression (50% in men’s dreams vs. 34% in women’sdreams; Hall & Van de Castle 1966) is quite stable over time.The data collection period of Hall and Van de Castle (1966)ranged from the late 1940s to the early ’50s. Subsequentstudies in the ’70s (Hall et al. 1982) and ’90s (Schredl et al.2003), and very recent studies (Schredl & Keller 2008–2009)replicated the higher prevalence of physical aggression inmen’s dreams compared with total aggression. This means thatcultural developments such as the women’s movement did notaffect this sex difference in dreams. And this favours the sexualselection theory over the social role theory.Another aspect of dream aggression fits the theory put forwardby Archer: the higher difference in the aggression per male char-acter compared to the aggression per female character in men(0.23 vs. 0.13 for men, 0.13 vs. 0.10 for women). That is,women experience aggressive interactions with both men andwomen in almost equal frequency in their dreams, whereasmen’s dreams include same-sex aggression more often comparedto opposite-sex aggression (Hall & Domhoff 1963).Also very interesting is the shift in the percentage of physicalaggression over the life-span. Whereas children (younger than11 years old) showed the typical gender difference of more phys-ical aggression in boy’s dreams compared with girl’s dreams, theratio of physical aggression in dreams is the same for both sexesin the age range from 12 to 17 years (Hall & Domhoff 1963;Oberst et al. 2005). This fits with Archer’s argument that malesavoid risky encounters with older males prior to adulthood. Theadults – as reported above – again showed the preponderanceof physical aggression in men’s dreams.Domhoff (1996) reviews cross-cultural dreams studies.Whereas many Western countries showed higher prevalencerates of dream aggression in men compared to women, therewhere several exceptions. The Hopi Indians, for example,showed no gender differences in overall aggression and in thepercentage of physical aggression (Domhoff 1996). The termHopi can be translated into “peaceful ones” reflecting the life-style of these Pueblo Native Americans. But for some industrialcountries, such as Switzerland and Japan, the ratios of physicaldream aggression were not different between the sexes. Thisindicates that cultural factors modulate the amount of aggressionin dream. In females, dream aggression was more often found indreams of non-traditional women, indicating again the culturaleffect on aggression pointed out by Archer. Unfortunately,these studies did not differentiate between same-sex aggressionand opposite-sex aggression to enable us to test Archer’s claimthat cultural factors might be more important in explaining theamount of opposite-sex aggression.Two studies, by Waterman et al. (1988) and Cohen (1973),investigated whether biological sex or feminine versus masculinesex role orientation explains differences in dream aggression.Whereas the finding of the first study was unambiguous (only bio-logical sex was of importance), the second study showed effects ofboth variables on the amount of dream aggression. Again, itwould have been fruitful to differentiate between same-sex andopposite-sex aggression.Another interesting gender difference can be found in the baddreams and nightmares of children (Schredl & Pallmer 1998). InTable 1, the percentages of male and female aggressors arepresented.Male characters threaten the dreamer most often whereaswomen are quite rarely aggressors in dreams. The ratio of maleand female aggressors in dreams is similar for boys and girls,thus reflecting the preponderance of male aggression in massmedia (news, films, etc.). It would be very interesting to studythe gender of the aggressor in cross-cultural dream studies inmore detail, for example, in societies without predominance ofmale aggression. If dream aggressors are still mostly male, oneTable 1 (Schredl). Human aggressors in children’s dreamsAggressorsTotal(n ¼ 111)Boys(n ¼ 35)Girls(n ¼ 76)Male (unfamiliar) 49.7% 51.4% 48.7%Female (unfamiliar) 3.6% 0.0% 5.3%Male (familiar) 27.8% 31.4% 26.2%Female (familiar) 18.9% 17.2% 19.8%Commentary/Archer: Sex differences in aggressionBEHAVIORAL AND BRAIN SCIENCES (2009) 32:3/4 287

There's no contest: Human sex differences are sexually selected

Abstract

Similar works

Full text

Available Versions

Crossref

Brunel University Research Archive