agresividad violencia y conflicto 2010

7/30/2019 Agresividad Violencia y Conflicto 2010

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JOURNAL OF CONFLICOLOGY, Volume 1, Issue 2 (2010) ISSN 2013-8857 1

http://journal-o-conlictology.uoc.edu

E-journal promoted by the Campus or Peace, Universitat Oberta de Catalunya

IntroductIon: genetIc

varIance and herItabIlItyof aggressIveness

Aggressiveness is a prominent behavioural trait in mostanimal species including humans, the expression o whichmust be careully modulated to assure the success oindividuals, small groups and large societies. As it has somany acets, aggressiveness has been studied rom a widevariety o disciplines including sociology, anthropology,biology and pharmacology. When taken together, thevast body o work produced to date indicates that themodulation o aggressiveness is inluenced by a complex

set o biological, psychological and social variables.In the eld o genetics, in order to discuss gene con-

tribution to a particular character or trait, a certain degreeo heterogeneity in the expression o that character within

individuals in the analysed population must rst be demon-strated. Tus, in order to discuss the genetic and biologicalcontributions to human aggressiveness, it has to be acknowl-edged that the human population is not homogeneous withrespect to the development o aggressive behaviours Eleyet al., 1999, 2003. Tat is, some individuals may be moreprone to aggressiveness or may show dierent thresholds toviolent behaviour. Tis does not imply that these complextraits are solely determined by particular gene variants, butthat some gene variants alleles o particular genes havedierent eects on the overall threshold or aggressivenessand predisposal or confict resolution, in conjunction withother genes and multiple cultural and educational actors.

For students o confictology unamiliar with genet-

ics, it should be noted that a gene is a segment o DNAcoding or the synthesis o a particular protein, which inturn has a specic biochemical unction in the organism,thus conerring a particular character. Complex traits,

artIcle

Aggressivity, Violence, Sociability andConfict Resolution: What Genes Can Tell Us

David BuenoSubmitted: July 2010

Accepted: September 2010

Published: November 2010

Abstract

Conficts are inherent to the human condition, as they are or all living beings. Disputes about resources or access tomating partners are among the most common causes o confict. Confict is herein dened as a struggle or contest betweenindividuals or parties, and may involve a variety o aggressive behaviours. In humans, aggressiveness, violence and conficts,including individual predisposal to confict resolution, have traditionally been said to have deep cultural roots, but recentresearch in both neuroscience and genetics has shown the infuence o genes on such complex behavioural traits. In thispaper, recent data on the genetic aspects o these interrelated behaviours will be put together, including the eects oparticular genes, the infuence o stress and gender on gene regulation, and geneenvironment interactions, all o which

may infuence biological predisposal to confict resolution. Other genetically infuenced behavioural aspects involved inconficts and confict resolution, such as sociability, will also be discussed. Te importance o taking into account geneticand biological data to provide strategies or confict resolution will be highlighted.

Keywords

behaviour heritability, aggressiveness, sociability, conficts, neurotransmitters


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David Bueno Aggressivity, Violence, Sociability and Confict

however, such as behavioural traits, are usually infuenced

by a number o dierent genes, each contributing to thenal character, as well as by the environment. Within thisgeneral scheme, each gene may exhibit several dierent al-leles coding slightly dierent genetic messages, leading tovariations in the corresponding trait. Alleles or a particu-lar gene may dier in their coding sequences, producingslightly dierent proteins that in turn coner a particulartrait, and/or they may dier in the non-coding sequencesthat regulate the expression o that gene, conerring slight-ly dierent maniestations o the trait despite exhibit-ing exactly the same corresponding protein. A discus-sion o the particular mutations that generate the sets oalleles o the genes discussed below is beyond the scope

o this paper but can be ound in the cited literature.Coming back to human population heterogeneity,

and in order to demonstrate the genetic contribution tothis heterogeneity, the variance and heritability o aggres-sive behaviours have to be quantied. In genetics, variancemeasures individual deviations rom the mean o the ana-lysed trait, which is mathematically calculated as the meano squares o individual deviations rom the mean. Herit-ability measures the proportion o total phenotypic varia-tion in a particular trait, at the population level, that is at-tributable to variation in the genotype. Phenotype reers tothe visible, or otherwise measurable, physical or biochemi-

cal characteristics o an organism, and the genotype is theprecise genetic constitution o an organism; the phenotyperesults rom the interaction o genotype and environment.

In this regard, a detailed evaluation o a meta-analysiso 24 genetics studies concerning aggression concluded thatheritability accounted overall or about 50% o the varianceo the trait 44% rom genetic eects and 6% rom sharedenvironmental actors. Te remaining 50% o variance canonly be explained by unshared environmental actors Rheeand Waldman, 2002. From these results it can be con-cluded that particular genotypes account or some o theindividual dierences with respect to aggressiveness. It hasalso been demonstrated that this heritability changes withage; whilst genetic actors and a common environment areequally important in childhood, heritability becomes evenmore prominent in adulthood Miles and Carey, 1997.

Another approach that was used to demonstrate theoverall contribution o genes to aggressiveness involvedthe generation o several dierent rat strains by 50 consecu-tive selective reproduction cycles, in which, or each gen-eration, individuals showing higher and lower aggressiveresponses were selectively mated. Comparison o the basalaggressive behaviours between strains under the same envi-ronmental conditions clearly showed the heritability o thistrait Rebollo-Mesa et al., 2010. Moreover, experiments

in which a particular gene named MAOA see below wasknocked out in mice as spontaneously occurs in some hu-mans, causing lack o unction o the corresponding gene,showed the contribution o some o its alleles to impulsive

aggressiveness Scott et al., 2008. Tus, as individual dier-

ences in aggressiveness have an important genetic basis, inorder to analyze its involvement in human conficts as wellas to increase our understanding o confict resolution strat-egies, it is also necessary to consider this trait rom evolu-tionary, physiological and genetic points o view, alongsideits cultural, social and educational roots.

aggressIveness andsocIabIlIty as anevolutIonary adaptatIon

he presence o genes and their variants in populations isthe result o long and complex evolutionary processes.hey include ortuitous mutation events that generatealleles and new genes, as well as natural selection, not onlywithin our current species but also in our ancestors and theancestral species o our lineage rom which we haveinherited our genetic constitution. he various acets ohuman aggression, as well as o sociability, a behaviouraltrait o which the genetic contribution will be discussedlatter, should be considered as a balance between traits thatare evolutionarily advantageous in a competitive worldbased on social cohesion.

Aggression, particularly or males living in communi-ties, as analysed in a wide range o model species includ-ing zebra sh, snakes, dogs, cats, elephants, mice, rats andprimates, including humans, provides a competitive edgein securing resources and in intra-sexual competitionthrough combat. High levels o aggression may also com-pensate or a lack o physical prowess in establishing hier-archy and dominance with respect to reproductive success,one o the key aspects in evolutionary processes.

In emales, aggression can help to protect ospringagainst a range o threats. As this behaviour represents ahigh-risk strategy, due to the associated possibilities o in-

jury or death, there is a high likelihood o both potentiallypositive and negative selective discrimination throughoutevolution, which may explain why human aggression ap-pears to have a strong genetic underpinning MaynardSmith et al., 1988. Interestingly, with respect to aggressionas a response to threat reactive aggression there appearsto be a complex relationship in mammals between anxietyand aggression, and strong evolutionary conservation othe brain regions involved in both, including the amygdalacircuitry, the anterior cingulated cortex and regions o theprerontal cortex on the other hand Lesch, 2005.

aggressIveness and genderIt appears that heritability o aggressiveness is also related togender, although the situation in humans is not completely


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clear and there are conlicting data. In humans, very clear

cut distinctions between sexes have been made on thebasis o crime and a wealth o studies has demonstrateda correlation between testosterone levels and aggressionArcher, 1991, but contradictory observations have alsobeen made urner, 1994.

More recently, a meta-analysis o 45 independent stud-ies, refecting a range o both positive and negative correla-tions, provided support or an overall weak positive eecto testosterone levels on human aggressiveness Book et al.,2001. Dierences between males and emales are likely to bedue to the dierent evolutionary aggression strategies, whichresult in dierent proactive and reactive responses in termso level and requency, based on the interaction o hormones

and neurotransmitters with their receptors and in the subse-quent signal transduction mechanisms, as well as epigeneticmodications see below. Interpretation o the relationshipis, however, complicated in humans, not only because omethodological problems but also because o fuctuations inhormone levels in response to environmental conditions andcircadian rhythm. Age is also a conounder, with regard toboth the changing response and hormone levels.

Despite this, male-associated aggression suggests thepossible involvement o genes or androgen synthesis andunction, possibly via the Y chromosome, as it contains thegene involved in triggering male development and male-

hormone synthesis named Sry. A tendency has been ob-served in healthy Swedish males or a correlation betweenparticular alleles or the androgen receptor gene and mus-cular tension and verbal aggression Jonsson et al., 2001,along with a signicant correlation with respect to violentcriminal activity in Indian males Rajender et al., 2008.

However, it appears that not only the Y chromosome isimportant in contributing to aggression through its role inmale determination, but that other genes are also involved.Tus, i the normal male-determining Srygene is deletedrom the Y chromosome in mice and is subsequently pro-vided as an autosomal copy, the strains created vary in ag-gressiveness Gatewood et al., 2006. Tis indicates thatother candidate genes are required in order to study thistrait rom the genetic point o view.

the serotonInergIc system:some candIdate genes foraggressIveness

Research pointing to the key role o the neurotransmitterserotonin in aggression suggests that the genetic control oall aspects o serotonin metabolism, and particularly o its

synthesis, its release rom neurons, and its action via thevarious receptors, represents a rich ield or the selection ocandidate genes involved in aggressiveness. One aspect othis is the observation that the level o arginine vasopressin

AVP, a peptide hormone in the cerebrospinal luid CSF

within brain ventricles, correlates with the lie historyo aggression, and that a reduced level o the serotoninmetabolite, 5-hydroxy-indole acetic acid 5-HIAA, withinthis luid is associated with violent behaviour Coccaro etal., 1997, 1998. hese molecules are also present in humanand rodent CSF during embryonic and oetal developmentParada et al., 2005 and 2007; Zapaterra et al., 2007; Buenoet al., personal communication, possibly contributing toneural pathways or later behaviours. However, the complexcomposition and the dynamics o CSF make it diicult todetermine whether the levels o these gene products are thecause o aggressive behaviour or the consequence, whichmay be relected in the composition o the CSF.

Other known candidate genes, reported to be involvedin the regulation and/or heterogeneity o aggressiveness,that are also related to serotonin pathways are the seroton-in transporter SLC6A4, the monoamine oxidase enzymesMAOA and MAOB, tryptophan hydroxylases PH1and PH2, and the serotonin receptors 5-H1A and 5-H1B. MAOA and MAOB are two closely related enzymesthat play an important role in the metabolism o biogenicamines in the central nervous system, including serotonin,norepinephrine and epinephrine MAOA and dopamineMAOB. An important watershed o the candidature oMAOA ollowed the above mentioned correlation o null

mutations and aggression in both mice and humans Brun-ner et al., 1993; Cases et al., 1995. More recently, muta-tions that modiy its level o expression have also been as-sociated with aggression and violence DSouza and Craig,2008, although the results show some inconsistencies dueto the complex interactions between unctional alleles, ex-posure to environments and sex hormones.

It has been reported that maltreated males are signi-cantly more likely to develop antisocial behaviour i they hadMAOA alleles exhibiting low activity Caspi et al., 2002,and that the high activity variants conerred some protec-tion against a stressul and abusive childhood Foley et al.,2004; Nilsson et al., 2006; Widon and Kim-Cohen, 2007.Moreover, a signicant correlation between MAOA allelesexhibiting high expression and lower impulsivity has beenound, suggesting a complementary role o this gene in theheterogeneity o aggressiveness: MAOA may act on impul-sivity, and subsequently on aggressive behaviour Huang etal., 2004, Nilsson et al., 2007. More recently, a signicantinteraction between exposure to moderate trauma and thelow activity o MAOA, with respect to aggression scores,has been conrmed, although exposure to extreme levelso trauma results in high aggression scores regardless o thegene variant or MAOA expression Weder et al., 2009.

However, results rom studies relating unctional vari-

ants o MAOA and emale behaviour are intriguing. Forexample, it has been reported that girls with alleles thatshow high rather than low activity appeared to be at risko engaging in criminal behaviour in the presence o psy-


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chosocial risk Sjoberg et al., 2007. Tere is evidence that

MAOA transcription may be regulated by both androgens,including testosterone, and glucocorticoids, linking aggres-siveness with both gender and stress Craig, 2007; Ou et al.,2006; Sjoberg et al., 2008. Tese results make it increas-ingly obvious that it is necessary to consider the impact ogenes not in isolation, but as part o a multiactorial mi-asma including other genetic actors and the environment.

With respect to other genetic actors that are related tothe serotonin pathway mentioned earlier, an interactionbetween the impact o high adversity in childhood on lat-er-lie violence has been reported, though only in peopleexhibiting low activity alleles or the serotonin transporterSLC6A4 Rei et al., 2007. On the other hand, as the action

o serotonin is mediated by a range o receptors, e.g. 5-H1A and 5-H1B, these receptors should also be consid-ered as candidates or generating heterogeneity in humanaggressiveness. It has been demonstrated that up-regula-tion o these receptors in rats is associated with high levelso aggression, behaviour that is urther enhanced ollow-ing victorious aggressive experiences Caramaschi et al.,2007. In humans, more than 20 correlation studies link-ing gene variants o these receptors with aggressivenessand antisocial behaviours have yielded varying resultsSanders et al., 2002, ranging rom antisocial alcohol-ism Hasegawa et al., 2002; Lappalainen et al., 1998;

Soyka et al., 2004 to pervasive aggression in childrenDavidge et al., 2004, and including a range o behaviourssuch as psychosis, agitation, aberrant motor behaviour anddepression Prichard et al., 2008.

Finally, alleles or the tryptophan hydrolase 1 and 2PH1 and PH2 enzymes, involved in serotonin syn-thesis in the brain, have also been associated with lowerlevels o 5-HIAA in the CSF in control males, but not inemales Nielsen et al., 1994, producing higher scores oraggression and a tendency to experience unprovoked angerHenning et al., 2005. In mice, an allele o PH2 has alsobeen associated with inter-male aggressiveness Kulikov etal., 2005; Osipova et al., 2009. Tese results indicate theparticipation o gene variants in aggressiveness and vio-lence regulation and heterogeneity, although as part o themultiactorial miasma, as argued above.

candIdate genes In thestress response pathway

As discussed in terms o its relationship with aggressiveness,the inluence o MAOA also depends on stress levels.wo separate elements exist in the stress pathway: the

neuroendocrine stress response and the autonomic reactionto stressul situations commonly typiied by the ight orlight paradigm. hus, genetic actors interacting with thesesystems are also candidates or regulating aggressiveness

and violence in conlicts. For example, dopamine-beta-

hydroxylase DBH, a key enzyme in the synthesis onorepinephrine, may underpin aspects o antisocialbehaviour, including increased neuroticism scores andimpulsive and/or aggressive behaviour Hess et al., 2009.

Another candidate gene is catechol-O-methyltranserase COM, an enzyme involved in dopamineand epinephrine metabolism. A lack o COM unc-tion in mice leads to increased aggressive behaviour butonly in males Gogos et al., 1998. In humans, some au-thors have ound a correlation between a particular COMallele and increased aggressiveness, particularly or males,and requently associated with schizophrenia. In women,however, this allele correlates with the least aggressive be-

haviour Kulikova et al., 2008. Te dopamine receptorDRD2 has also been associated with stress disorders, and aparticular allele o DRD2 has been recently associated withhigher risk o posttraumatic stress disorder in war veteransVoisey et al., 2009. In rodents and monkeys there is alsoa positive relationship between noradrenergic activity andghting/biting behaviour, and a positive relationship be-tween aggressiveness and the CSF level o norepinephrineor its metabolites is also ound in most human studiesCraig and Halton, 2009; Placidi et al., 2001.

other candIdate genesOther genes have been associated to aggressiveness andviolence. his includes genes encoding the enzyme, nitricoxide synthase NOS1, recently associated with traitsrelating to impulsivity, including hyperactive and aggressivebehaviour Rei et al., 2009, and the arginine vasopressinreceptor AVPR1A, whose ligand, arginine vasopressin, hasbeen mentioned above as being implicated in aggressivenessFerris et al., 1997. In humans, this receptor has beenassociated with social communication and autistic traits,but no direct relationship with aggression has yet been

demonstrated Bachner-Melman et al., 2005.Various physiological changes can also promote in-creased aggressive behaviour. One o the most analysed isthe lowering o sugar levels in the blood, which provokesatigue, dizziness, headaches and irritability. Various stud-ies have attempted to link serotonin mechanisms, insulinlevels and glucose metabolism with aggression and impulsecontrol Linnoila and Virkkunen, 1992; Virkkunen et al.,2007. A variation in the serotonin transporter gene wasound to aect nutritional intervention on asting bloodglucose levels in non-diabetic emales Yamakawa et al.,2005. Furthermore, glucose transporters are also thoughtto have an aect on hypoglycaemia and the resultant be-

haviour. Tis is supported by a amilial study, in which sixout o the eight amily members were ound to have a mu-tation in the glucose transporter SLC2A1 gene, resulting inirritability and impulsive behaviour Weber et al., 2008.


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the role of epIgenetIcs

In aggressIveness andvIolence

New indings in epigenetics have to be taken into account,as these may help to link environmental conditions withgenetic traits and explain the apparently contradictorygender-dependent inluence o some alleles. Epigeneticmodiications o the genetic material consist o bioche-mical modiications o particular DNA nucleotides and/oro some o the proteins involved in chromatin structure,such as histones. hese modiications, which under certainconditions are reversible and are genetically controlled,may aect the regulation o gene expression.

It has been reported that rat pups deprived o maternalcare exhibit an epigenetic modication aecting the glu-cocorticoid receptor promoter, a particular methylation,which subsequently produces behavioural decits andincreased inter-male aggressiveness Weaver et al., 2004;Veenema et al., 2007. In humans, an analogous epige-netic methylation o the glucocorticoid receptor promoterhas been associated with violent suicide victims who hadexperienced adverse events early in lie McGowan et al.,2009. Similarly, rat pups raised by stressed mothers exhib-it increased epigenetic methylation o the gene coding or

the brain-derived neurotrophic actor BDNF, a neuralgrowth actor in the brains prerontal cortex, which tendsto reduce the level o BDNF. It has also been reported thatthis methylation pattern is passed on to the subsequentgeneration Roth et al., 2009.

Interestingly, epigenetic methylation o the MAOA pro-moter region has been reported in emales but not in males;a dierence that may be related to the dierent aggressiveresponses related to gender see above, although the extento this remains to be elucidated Pinsonneault et al., 2006.Epigenetics is a new eld that could contribute to our un-derstanding o the association between genetics and beha

vioural aspects such as aggressiveness and violence, linkingthe environment with the expression o particular genes.

socIabIlIty

Although most o the results discussed in this paperreer to aggression and violence, conlicts may be causedby a variety o aggressive behaviours which, in turn,complicate its resolution, so these aggressive behaviourshave to, at the least, be taken into account. his is just thetip o the iceberg as there are many other behaviouraltraits that may have some genetic basis, as they originate

in the brain, a biological structure that develops andworks under the direction o gene networks. hese genenetworks propitiate neural plasticity, which also dependson a number o environmental actors although these

will not be discussed here. he other behavioural traits

include an inclination to many dierent degrees ooptimism, empathy, sociability, etc.For example, it is known that the level o neurotrans-

mitters such as serotonin and dopamine is related to de-pression, and that some pharmacological treatments ordepression act on these neurotransmitters. Tus, dierentalleles o the relevant genes may result in dierent degreeso optimism in dierent individuals. Moreover, dierentalleles o a gene called VMA2, whose protein product isinvolved in packing neurotransmitters such as serotoninand dopamine within neurons, have been ound to be as-sociated with human spiritual capacity Zimmer, 2004,which in turn infuences many behaviours.

win and amily studies suggest that uniquely humancharacteristics such as empathy, altruism, a sense o equity,love, trust, music, economic behaviour, and even politicsare partially hardwired reviewed by Epstein et al., 2010.Te leap rom twin studies to identiying specic genesengaging the social brain has occurred in the past decade,aided by new insights on social behaviour in lower mam-mals. Remarkably, genes such as the arginine vasopressinreceptor and the oxytocin receptor contribute to social be-haviour in a wide range o species rom voles to humansDonaldson and Young, 2008. Several studies have al-lowed the heritability o social behavioural phenotypes to

be determined or a number o traits able 1. Tis impliesthat although most o the genes and alleles involved in suchtraits may still be unknown, or at least much less knownthan those or the abovementioned serotoninergic path-way that is associated with aggressiveness, research in theseareas is o vital importance to ully understand conficts,and to more eectively apply confict resolution.

Table 1. Heritability o social behaviour traits. Modied rom Ebstein etal. 2010

tigi

si

ui

pi i () 57% 12% 31%

pi i (i) 55% 4% 41%

pi () 57% 12% 31%

Iii () 50% 0% 50%

si i () 50% 25% 25%

si i (i) 39% 42% 19%

e 47% 0% 53%

pii i 42% 23% 35%

ai 40% 17% 43%

li 40% 0% 60%

p 38% 0% 62%

ri 37% 0% 63%

s 37% 0% 63%

di ii 30% 0% 70%


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conclusIon

he impressive amount o data available demonstrates thatgenes play a role in human conlicts: the variety o aggressiveand violent behaviours that derive rom as well as provokethem. For this reason, and to develop strategies or conlictresolution, it is necessary to consider the impact o genes aspart o a multiactorial miasma, including other genetic andepigenetic, as well as environmental actors. In other words,it is necessary to consider not only the cultural, social andeducational environment, but also the genetic and other

biological actors related to evolutionary human traits such

as aggressiveness and violence, sociability and empathy,altruism and optimism.

acknowledgements

Contract grant sponsor: Ministerio de Educacin y Ciencia;Contract grant number: BFU2007-62361 to D.B., includingFEDER sponsorship.

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Recommended citation

BUENO, David 2010. Aggressivity, Violence, Sociability and Conlict Resolution: What Genes Can ell Us[online article].Journal o Conlictology. Vol. 1, Iss. 2. Campus or Peace, UOC. [Consulted: dd/mm/yy].

ISSN 2013-8857

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About the author

David [email protected]

David Bueno, PhD. is a specialist in genetics, developmental biology and neuroscience at the GeneticsDepartment o the Universitat de Barcelona. He is also member o the Conlictology Research Studies CenterCRECUOC.

agresividad violencia y conflicto 2010

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